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Current sensor

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Current Sensors

Current Sensors

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  • 1. APPLICATION NOTE—105 Application Note 105 December 2005Current Sense Circuit CollectionMaking Sense of CurrentTim Regan, EditorINTRODUCTIONSensing and/or controlling current flow is a fundamental This Application Note Will Changerequirement in many electronics systems, and the tech- This Application Note is a growing and changing docu-niques to do so are as diverse as the applications them- ment. Many of the chapters listed below are placeholdersselves. This Application Note compiles solutions to cur- for material that will be filled in soon. As the chapters arerent sensing problems and organizes the solutions by added, their links will be enabled.general application type. These circuits have been culledfrom a variety of Linear Technology documents. Using the Application Note Click the name of a chapter in the “Circuit Collection In-Circuits Organized by General Application dex” below to open the PDF version of that chapter.Each chapter collects together applications that tend tosolve a similar general problem, such as high side cur- Contributorsrent sensing, or negative supply sensing. The chapters Jon Munson, Alexi Sevastopoulos,are titled accordingly (see “Circuit Collection Index” be- Greg Zimmer, Michael Stokowskilow). In this way, the reader has access to many possiblesolutions to a particular problem in one place. , LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, MultimodeIt is unlikely that any particular circuit shown will exactly Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelPro- tect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,meet the requirements for a specific design, but the sug- UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product namesgestion of many circuit techniques and devices should may be trademarks of the companies that manufacture the products.prove useful. Specific circuits may appear in severalchapters if they have broad application. CIRCUIT COLLECTION INDEX Current Sense Basics Level Shifting High Speed High Side High Voltage Fault Sensing Low Side Low Voltage Digitizing Negative Voltage High Current (100mA to Amps) Current Control Unidirectional Low Current (Picoamps to Precision Bidirectional Milliamps) Wide Range AC Motors and Inductive Loads DC Batteries Introduction-1
  • 2. APPLICATION NOTE 105: Current Sense Circuit CollectionCurrent Sense BasicsThis chapter introduces the basic techniques used for HIGH SIDE CURRENT SENSINGsensing current. It serves also as a definition of common Current sensed in the supply path of the power connec-terms. Each technique has advantages and disadvan- tion to the monitored load. Current generally flows in justtages and these are described. The types of amplifiers one direction (uni-directional). Any switching is per-used to implement the circuits are provided. formed on the load-side of monitor.To see other chapters in this Application Note, return to DC VSUPPLYthe Introduction. +LOW SIDE CURRENT SENSING RSENSE ISENSE OUTPUT ∝ ILOAD –Current sensed in the ground return path of the powerconnection to the monitored load. Current generally ILOAD LOADflows in just one direction (uni-directional). Any switch-ing is performed on the load-side of monitor. DC VSUPPLY High Side Advantages Load is grounded ILOAD LOAD VCC Load not activated by accidental short at power con- + nection RSENSE ISENSE OUTPUT ∝ ILOAD High load current caused by short is detected – High Side Disadvantages . High input common mode voltages (often very high) Output needs to be level shifted down to system oper- ating voltage levelsLow Side Advantages Low input common mode voltage Amplifier Types for High Side Implementation Ground referenced output voltage Dedicated current sensing amplifiers: LT6100, Easy single supply design LTC6101, LT1787 Over-the-Top™ op amps: LT1637Low Side Disadvantages Flying capacitor amplifier: LTC6943 Load lifted from direct ground connection Load activated by accidental short at ground end load switch High load current caused by short is not detectedAmplifier Types for Low Side Implementation Precision zero-drift op amps: LTC2050, LTC2054 Instrumentation amplifiers: LTC2053, LT1990, LTC6943 Rail-to-Rail Input op amps: LT1677 Current Sense Basics-1
  • 3. APPLICATION NOTE 105: Current Sense Circuit CollectionFULL-RANGE (HIGH AND LOW SIDE) SUMMARY OF CURRENT SENSE SOLUTIONSCURRENT SENSING The next few pages contain a table that summarizes cur-Bi-directional current sensed in a bridge driven load, or rent sense solutions and applicable devices. Look first inunidirectional high side connection with a supply side the “Type/Circuit” column and the “Gain” column for aswitch. general description of the application. Then scan across the other columns for applicable devices and their speci- DC VSUPPLY fications. VCC RSENSE + LOAD ISENSE OUTPUT ∝ ILOAD ILOAD –Full-Range Advantages Only one current sense resistor needed for bidirec- tional sensing Convenient sensing of load current on/off profiles for inductive loadsFull-Range Disadvantages Wide input common mode voltage swings Common mode rejection may limit high frequency accuracy in PWM applicationsAmplifier Types for Bi-directional Implementation Difference amplifiers-LT1990, LT1991, LT1995, LT1996 Instrumentation amplifiers: LTC2053 Flying capacitor amplifier: LTC6943Current Sense Basics-2
  • 4. APPLICATION NOTE 105: Current Sense Circuit Collection ACCURACY SPEED OFFSET INPUT DIFFERENTIAL GAIN DEVICES AND VSUPPLYTYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE (V/V) PACKAGES RANGE (VS) (VOS) (IBIAS) (SURVIVAL) High Side 10 to 50 LT6100 300µV 5µA 100kHz 0.05V/µs 2.7V to 36V (VS + 1.4V) to 48V ±48V One Direction Voltage Out MSOP-8 RSENSE VIN DFN LOAD (VCC + 1.4V) TO 48V 1 8 VS– VS+ RG1 RG2 5k 5k – + R 25k A1 VCC 2 2.7V TO 36V – VOUT Q1 RE A2 5 10k + VO1 RO R R/3 50k VEE FIL A2 A4 4 3 6 7 6100 F01 High Side Resistor LTC6101 350µV 250nA 200kHz 2.5V/µs 4V to 70V (VS – 1.5V) to 70V ±70V One Direction Ratio LTC6101HV 350µV 250nA 200kHz 2.5V/µs 4V to 105V (VS – 1.5V) to 105V ±105V Current OutILOAD VSENSE SOT23-5 – + VBATTERY RSENSE RIN 5 MSOP-8 10V V+ L O A D IN – 5k – 3 IN + 5k + 4 IOUT 10V OUT ROUT 1 VOUT = VSENSE x RIN LTC6101/LTC6101HV V– 2 ROUT 6101 BD Current Sense Basics-3
  • 5. APPLICATION NOTE 105: Current Sense Circuit Collection ACCURACY SPEED OFFSET INPUT DIFFERENTIAL GAIN DEVICES AND VSUPPLYTYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE (V/V) PACKAGES RANGE (VS) (VOS) (IBIAS) (SURVIVAL) High Side Fixed 8 LT1787 75µV 20µA 300kHz 0.1V/µs 2.5V to 36V 2.5V to 36V ±10V Bi-directional or LT1787HV 75µV 20µA 300kHz 0.1V/µs 2.5V to 60V 2.5V to 60V ±10V Current or Voltage (ROUT = 20k) Scaleable RSENSE ISENSE SO-8 VS– VS+ MSOP-8 RG1A RG2A 1.25k 1.25k FIL– FIL+ RG1B RG2B 1.25k 1.25k – + A1 IOUT VBIAS ROUT Q1 Q2 20k VOUT VEE CURRENT MIRROR 1787 F 01 High Side Resistor LT1494 150µV 250pA 3kHz 0.001V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V One Direction Ratio LT1636 50µV 5nA 200kHz 0.07V/µs 2.6V to 44V 0 to VS + (44V – VS) 44V Voltage Out LT1637 100µV 20nA 1MHz 0.35V/µs 1.8V to 44V 0 to VS + (44V – VS) 44V Over the Top Amplifiers LT1672 150µV 250pA 12kHz 0.005V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V 3V TO 44V LT1782 400µV 8nA 200kHz 0.07V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V R1 200Ω LT1783 400µV 45nA 1.25MHz 0.42V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V 3V RS + LT1784 1500µV 250nA 2.5MHz 2.4V/µs 2V to 18V 0 to VS + (18V – VS) 36V 0.2Ω Q1 LT1637 2N3904 – VOUT ILOAD R2 (0V TO 2.7V) DIP-8 VOUT 2k LOAD ILOAD = (RS)(R2/R1) 1637 TA06 MS-8 SO-8 DFN SOT23-5 SOT23–6Current Sense Basics-4
  • 6. APPLICATION NOTE 105: Current Sense Circuit Collection ACCURACY SPEED OFFSET INPUT DIFFERENTIAL GAIN DEVICES AND VSUPPLYTYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE (V/V) PACKAGES RANGE (VS) (VOS) (IBIAS) (SURVIVAL) High Side Resistor LTC2053 5µV 4nA 200kHz 0.2V/µs 2.7V to 11V 2.7V to 11V 5.5V One Direction Ratio LTC6800 5µV 4nA 200kHz 0.2V/µs 2.7V to 5.5V 2.7V to 5.5V 5.5V Voltage Out Instrumentation Amplifier DFN 5V 0.1µF MS-8 3 + 8 7 LTC2053 VOUT 2 6 VIN – 5 4 1 0.1µF VOUT = –VIN 2053 TA07 –5V High Side or Low Side Unity LTC6943 6pA 90kHz 5V to 18V 5V to 18V 18V One Direction Voltage on a TSSOP – 16 capacitor output Flying Capacitor E I POSITIVE OR NEGATIVE RAIL RSHUNT 1/2 LTC6943 11 12 10 1µF 1µF E I= E RSHUNT 9 6 7 14 15 0.01µF 6943 • TA01b Current Sense Basics-5
  • 7. APPLICATION NOTE 105: Current Sense Circuit Collection ACCURACY SPEED OFFSET INPUT DIFFERENTIAL GAIN DEVICES AND VSUPPLYTYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE (V/V) PACKAGES RANGE (VS) (VOS) (IBIAS) (SURVIVAL) High Side or Low Side 1 and 10 LT1990 900µV 2.5nA 105kHz 0.55V/µs 2.4V to 36V –250V to 250V ±250V Bi-Directional 1 to 13 LT1991 15µV 110kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V Voltage Out 1 to 7 LT1995 1000µV 2.5nA 32MHz 1000V/µs 5V to 36V 0V to 36V VS + 0.3V Difference Amplifiers 9 to 117 LT1996 15µV 38kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V V S+ 8 9 10 M9 M3 7 Pin Strap SO-8 VIN – M1 VIN + 1 LT1991 6 R1 Configurable DFN P1 R2* 2 5 10k 10k 3 P3 P9 4 MS–10 V + – VIN – ILOAD = IN V S– 10kΩ *SHORT R2 FOR LOWEST OUTPUT OFFSET CURRENT. INCLUDE R2 FOR HIGHEST OUTPUT IMPEDANCE. Low Side Resistor LTC2050 0.5µV 75pA 3MHz 2V/µs 2.7V to 7V 0V to (VS – 1.3V) VS + 0.3V One Direction Ratio LTC2054 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 7V 0V to (VS – 0.7V) VS + 0.3V Voltage Out LTC2054HV 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 12V 0V to (VS – 0.7V) VS + 0.3V Zero-Drift Amplifiers 5V SO-8 3 + 5 OUT 3V/AMP SOT23-5 1 4 LTC2050HV LOAD CURRENT IN MEASURED SOT23 – 6 – 2 CIRCUIT, REFERRED TO –5V 10Ω 10k TO 3mΩ MEASURED CIRCUIT LOAD CURRENT 0.1µF – 5V 2050 TA08 Low Side Resistor LT1218 25µV 30nA 300kHz 0.1V/µs 2V to 36V 0V to VS VS + 0.3V One Direction Ratio LT1677 20µV 2nA 7.2MHz 2.5V/µs 2.5V to 44V 0V to VS VS + 0.3V Voltage Out LT1800 75µV 25nA 80MHz 25V/µs 2V to 12.6V 0V to VS VS + 0.3V Rail to Rail I/O Amplifiers LT1806 100µV 1µA 325MHz 125V/µs 1.8V to 12.6V 0V to VS VS + 0.3V IL 3V LT6200 1400µV 10µA 110MHz 50V/µs 2.2V to 12.6V 0V to VS VS + 0.3V 0A TO 1A 52.3Ω + LT6220 70µV 15nA 60MHz 20V/µs 2.2V to 12.6V 0V to VS VS + 0.3V VOUT LT1800 0V TO 2V 0.1Ω – 52.3Ω 1k SO-8 1800 F02 DIP-8 VOUT = 2 • IL f–3dB = 4MHz UNCERTAINTY DUE TO VOS, IB < 4mA SOT23-5 SOT23 – 6Current Sense Basics-6
  • 8. APPLICATION NOTE 105: Current Sense Circuit CollectionHigh SideThis chapter discusses solutions for high side current “Classic” Positive Supply Rail Current Sensesensing. With these circuits the total current supplied to 5Va load is monitored in the positive power supply line. 200ΩTo see other chapters in this Application Note, return tothe Introduction. 0.2Ω +LT6100 Load Current Monitor LT1637 Q1 200Ω 2N3904 TO LOAD RSENSE – 0V TO 4.3V LOAD ILOAD 2k C1 + 1 8 0.1µF 5V VOUT = (2Ω)(ILOAD) 1637 TA02 V S– V S+ 2 VCC A4 7 This circuit uses generic devices to assemble a function + 3V C2 – + similar to an LTC6101. A Rail-to-Rail Input type op amp 0.1µF 3 6 is required since input voltages are right at the upper rail. FIL A2 The circuit shown here is capable of monitoring up to 44V applications. Besides the complication of extra parts, 4 VEE OUT 5 OUTPUT the VOS performance of op amps at the supply is gener- LT6100 ally not factory trimmed, thus less accurate than other 6100 F04 solutions. The finite current gain of the bipolar transistorThis is the basic LT6100 circuit configuration. The inter- is a small source of gain error.nal circuitry, including an output buffer, typically operatesfrom a low voltage supply, such as the 3V shown. The Over-The-Top Current Sensemonitored supply can range anywhere from VCC + 1.4V 3V TO 44Vup to 48V. The A2 and A4 pins can be strapped various R1 200Ωways to provide a wide range of internally fixed gains. 3VThe input leads become very hi-Z when VCC is powered RSdown, so as not to drain batteries for example. Access to 0.2Ω + Q1an internal signal node (pin 3) provides an option to in- LT1637 2N3904clude a filtering function with one added capacitor. Small- – VOUT (0V TO 2.7V) ILOADsignal range is limited by VOL in single-supply operation. R2 VOUT 2k LOAD ILOAD = (RS)(R2/R1) 1637 TA06 This circuit is a variation on the “classic” high-side cir- cuit, but takes advantage of Over-the-Top input capability to separately supply the IC from a low-voltage rail. This provides a measure of fault protection to downstream circuitry by virtue of the limited output swing set by the low-voltage supply. The disadvantage is VOS in the Over- the-Top mode is generally inferior to other modes, thus less accurate. The finite current gain of the bipolar tran- sistor is a source of small gain error. High Side-1
  • 9. APPLICATION NOTE 105: Current Sense Circuit CollectionSelf-Powered High Side Current Sense Precision High Side Power Supply Current Sense 1.5mΩ VREGULATOR 2 – 8 OUT 7 100mV/A LTC6800 OF LOAD 3 + 6 10k CURRENT 5 4 0.1µF ILOAD LOAD 150Ω 6800 TA01 This is a low-voltage, ultra-high-precision monitor featur-This circuit takes advantage of the microampere supply ing a Zero-Drift Instrumentation Amplifier (IA) that pro-current and Rail-to-Rail input of the LT1494. The circuit vides Rail-to-Rail inputs and outputs. Voltage gain is setis simple because the supply draw is essentially equal to by the feedback resistors. Accuracy of this circuit is setthe load current developed through RA. This supply cur- by the quality of resistors selected by the user, small-rent is simply passed through RB to form an output volt- signal range is limited by VOL in single-supply operation.age that is appropriately amplified. The voltage rating of this part restricts this solution to applications of <5.5V. This IA is sampled, so the output isHigh Side Current Sense and Fuse Monitor discontinuous with input changes, thus only suited to RSENSE very low frequency measurements. TO LOAD 2mΩ FUSE BATTERY BUS + Positive Supply Rail Current Sense 1 8 VS– VS+ VCC ADC R1 2 7 200Ω POWER VCC A4 ≥2.7V C2 – + 0.1µF Rs 3 6 0.2Ω – – FIL A2 Q1 1/2 LT1366 1/2 LT1366 TP0610L + + 4 VEE OUT 5 OUTPUT 2.5V = 25A ILOAD R2 VO = ILOAD • RS ( ) R2 R1 LT6100 LOAD 20k = ILOAD • 20Ω DN374 F02 1366 TA01The LT6100 can be used as a combination current sensorand fuse monitor. This part includes on-chip output buff- This is a configuration similar to an LT6100 implementedering and was designed to operate with the low supply with generic components. A Rail-to-Rail or Over-the-Topvoltage (≥2.7V), typical of vehicle data acquisition sys- input op amp type is required (for the first section). Thetems, while the sense inputs monitor signals at the first section is a variation on the classic high-side wherehigher battery bus potential. The LT6100 inputs are toler- the P-MOSFET provides an accurate output current intoant of large input differentials, thus allowing the blown- R2 (compared to a BJT). The second section is a bufferfuse operating condition (this would be detected by an to allow driving ADC ports, etc., and could be configuredoutput full-scale indication). The LT6100 can also be with gain if needed. As shown, this circuit can handle uppowered down while maintaining high impedance sense to 36V operation. Small-signal range is limited by VOL ininputs, drawing less than 1µA max from the battery bus. single-supply operation.High Side-2
  • 10. APPLICATION NOTE 105: Current Sense Circuit CollectionPrecision Current Sensing in Supply Rails Measuring bias current into an Avalanche Photo E Diode (APD) using an instrumentation amplifier. I POSITIVE ORNEGATIVE RAIL 1k RSHUNT 1% VIN BIAS OUTPUT 1/2 LTC6943 10V TO 33V TO APD 35V 11 12 – CURRENT LT1789 MONITOR OUTPUT 10 0mA TO 1mA = 0V TO 1V 1µF 1µF E I= E + A=1 RSHUNT 9 AN92 F02a 1N4684 1k 6 7 3.3V 1% VIN BIAS OUTPUT 10V TO 35V TO APD 14 15 10M 0.01µF – CURRENT LT1789 MONITOR OUTPUT 6943 • TA01b 0mA TO 1mA = 0V TO 1V + A=1This is the same sampling architecture as used in the AN92 F02bfront-end of the LTC2053 and LTC6800, but sans op ampgain stage. This particular switch can handle up to 18V, The upper circuit uses an instrumentation amplifier (IA)so the ultra-high precision concept can be utilized at powered by a separate rail (>1V above VIN) to measurehigher voltages than the fully integrated ICs mentioned. across the 1kΩ current shunt. The lower figure is similarThis circuit simply commutates charge from the flying but derives its power supply from the APD bias line. Thesense capacitor to the ground-referenced output capaci- limitation of these circuits is the 35V maximum APDtor so that under dc input conditions the single-ended voltage, whereas some APDs may require 90V or more.output voltage is exactly the same as the differential In the single-supply configuration shown, there is also aacross the sense resistor. A high precision buffer ampli- dynamic range limitation due to VOL to consider. The ad-fier would typically follow this circuit (such as an vantage of this approach is the high accuracy that isLTC2054). The commutation rate is user-set by the ca- available in an IA.pacitor connected to pin 14. For negative supply monitor-ing, pin 15 would be tied to the negative rail rather thanground. High Side-3
  • 11. APPLICATION NOTE 105: Current Sense Circuit CollectionSimple 500V Current Monitor Bidirectional Battery-Current Monitor TO RSENSE CHARGER/ LOAD C1 15V 1 8 1µF – FIL FIL+ – LT1787 2 VS VS+ 7 3 VBIAS 6 DNC ROUT 4 5 VEE OUTPUT VOUT C2 C3* –5V 1µF 1000pF 1787 F02 *OPTIONAL This circuit provides the capability of monitoring current in either direction through the sense resistor. To allow negative outputs to represent charging current, VEE is connected to a small negative supply. In single-supplyAdding two external Mosfets to hold off the voltage al- operation (VEE at ground), the output range may be offsetlows the LTC6101 to connect to very high potentials and upwards by applying a positive reference level to VBIASmonitor the current flow. The output current from the (1.25V for example). C3 may be used to form a filter inLTC6101, which is proportional to the sensed input volt- conjunction with the output resistance (ROUT) of the part.age, flows through M1 to create a ground referenced This solution offers excellent precision (very low VOS)output voltage. and a fixed nominal gain of 8.High Side-4
  • 12. APPLICATION NOTE 105: Current Sense Circuit CollectionLTC6101 Supply Current Simple High Side Currentincluded as Load in Measurement Sense Using the LTC6101 V+ BATTERY BUS RIN RSENSE RIN RSENSE 100Ω 4 3 0.01Ω 4 3 + – LOAD + – LOAD 2 5 2 5 1 1 VOUT LTC6101 VOUT LT6101 4.99V = 10A ROUT ROUT 4.99k 6101 F06 VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01 This is a basic high side current monitor using theThis is the basic LTC6101 high-side sensing supply- LTC6101. The selection of RIN and ROUT establishes themonitor configuration, where the supply current drawn desired gain of this circuit, powered directly from theby the IC is included in the readout signal. This configu- battery bus. The current output of the LTC6101 allows itration is useful when the IC current may not be negligible to be located remotely to ROUT. Thus, the amplifier canin terms of overall current draw, such as in low-power be placed directly at the shunt, while ROUT is placed nearbattery-powered applications. RSENSE should be selected the monitoring electronics without ground drop errors.to limit voltage-drop to <500mV for best linearity. If it is This circuit has a fast 1µs response time that makes itdesirable not to include the IC current in the readout, as ideal for providing MOSFET load switch protection. Thein load monitoring, pin 5 may be connected directly to V+ switch element may be the high side type connected be-instead of the load. Gain accuracy of this circuit is limited tween the sense resistor and the load, a low side typeonly by the precision of the resistors selected by the user. between the load and ground or an H-bridge. The circuit is programmable to produce up to 1mA of full-scale out- put current into ROUT, yet draws a mere 250µA supply current when the load is off. High Side-5
  • 13. APPLICATION NOTE 105: Current Sense Circuit CollectionHigh-Side Transimpedance Amplifier Intelligent High Side Switch The LT1910 is a dedicated high side MOSFET driver with built in protection features. It provides the gate drive for a power switch from standard logic voltage levels. It pro- vides shorted load protection by monitoring the currentCurrent through a photodiode with a large reverse bias flow to through the switch. Adding an LTC6101 to thepotential is converted to a ground referenced output volt- same circuit, sharing the same current sense resistor,age directly through an LTC6101. The supply rail can be provides a linear voltage signal proportional to the loadas high as 70V. Gain of the I to V conversion, the trans- current for additional intelligent control.impedance, is set by the selection of resistor RL.High Side-6
  • 14. APPLICATION NOTE 105: Current Sense Circuit Collection48V Supply Current Monitor withIsolated Output and 105V SurvivabilityThe HV version of the LTC6101 can operate with a totalsupply voltage of 105V. Current flow in high supply volt-age rails can be monitored directly or in an isolated fash-ion as shown in this circuit. The gain of the circuit andthe level of output current from the LTC6101 depends onthe particular opto-isolator used. High Side-7
  • 15. APPLICATION NOTE 105: Current Sense Circuit CollectionLow SideThis chapter discusses solutions for low side current Precision Current Sensing in Supply Railssensing. With these circuits the current flowing in the E Iground return or negative power supply line is moni- POSITIVE OR NEGATIVE RAILtored. RSHUNT 1/2 LTC6943To see other chapters in this Application Note, return to 11 12the Introduction. 10“Classic” High-Precision Low Side Current Sense 1µF 1µF E I= E RSHUNT 5V 9 3 5 OUT + 3V/AMP 6 7 1 LOAD CURRENT LTC2050HV 4 IN MEASURED – 2 CIRCUIT, REFERRED 14 15 TO –5V 0.01µF 10Ω 10k 6943 • TA01b TO 3mΩMEASURED CIRCUIT LOAD CURRENT 0.1µF This is the same sampling architecture as used in the – 5V 2050 TA08 front-end of the LTC2053 and LTC6800, but sans op amp gain stage. This particular switch can handle up to 18V,This configuration is basically a standard non-inverting so the ultra-high precision concept can be utilized atamplifier. The op amp used must support common-mode higher voltages than the fully integrated ICs mentioned.operation at the lower rail and the use of a Zero-Drift type This circuit simply commutates charge from the flying(as shown) provides excellent precision. The output of sense capacitor to the ground-referenced output capaci-this circuit is referenced to the lower Kelvin contact, tor so that under dc input conditions the single-endedwhich could be ground in a single-supply application. output voltage is exactly the same as the differentialSmall-signal range is limited by VOL for single-supply across the sense resistor. A high precision buffer ampli-designs. Scaling accuracy is set by the quality of the fier would typically follow this circuit (such as anuser-selected resistors. LTC2054). The commutation rate is user-set by the ca- pacitor connected to pin 14. For negative supply monitor- ing, pin 15 would be tied to the negative rail rather than ground. Low Side-1
  • 16. APPLICATION NOTE 105: Current Sense Circuit Collection–48V Hot Swap Controller GND RIN 3× 1.8k IN SERIES + CL 1/4W EACH 100µF CIN LOAD GND 1 1µF R3 (SHORT PIN) 5.1k R1 VIN EN 402k LTC4252-1 VOUT 1% * 8 2 OV PWRGD 9 7 RD 1M UV DRAIN R2 10 6 Q1 32.4k TIMER GATE IRF530S 1% CT 3 4 SS VEE SENSE 0.33µF RC RS C1 CSS 5 10Ω 0.02Ω 10nF 68nF CC 18nF –48V * M0C207This load protecting circuit employs low-side current event of supply or load faults. An internal shunt regulatorsensing. The N-MOSFET is controlled to soft-start the establishes a local operating voltage.load (current ramping) or to disconnect the load in the–48V Low Side Precision Current SenseThe first stage amplifier is basically a complementary and furnishes a positive output voltage for increasingform of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-to operate with telecom negative supply voltage. The plementation due to the different supply voltages forZener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to thesupply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operationThe second op amp is powered from a positive supply of the second stage.Low Side-2
  • 17. APPLICATION NOTE 105: Current Sense Circuit CollectionFast Compact –48V Current Sense VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A ACCURACY ≈ 3% VOUT Q1 R1 1k FMMT493 4.7k 1% VS = 3V 30.1Ω 1% – 3.3k R1 REDUCES Q1 DISSIPATION 0805 LT1797 ×3 + 0.1µF SETTLES TO 1% IN 2µs, BZX84C6V8 1V OUTPUT STEP VZ = 6.8V 0.003Ω 1% 3W –48V SUPPLY –48V LOAD (–42V TO –56V) – + 1797 TA01 ISENSEThis amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-mentary implementation to the classic high-side configu- age is referenced to a positive potential and movesration. The op amp used must support common-mode downward when representing increasing –48V loading.operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors usedlocal supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.sistor provides metered current to an output load resis-–48V Current Monitor Low Side-3
  • 18. APPLICATION NOTE 105: Current Sense Circuit CollectionIn this circuit an economical ADC is used to acquire the and/or higher efficiency operation, the ADC may be pow-sense resistor voltage drop directly. The converter is ered from a small transformer circuit as shown below.powered from a “floating” high-accuracy shunt-regulatedsupply and is configured to perform continuous conver-sions. The ADC digital output drives an opto-isolator,level-shifting the serial data stream to ground. For widersupply voltage applications, the 13k biasing resistor maybe replaced with an active 4mA current source such asshown to the right. For complete dielectric isolation–48V Hot Swap Controller GND RIN 3× 1.8k IN SERIES + CL 1/4W EACH 100µF CIN LOAD GND 1 1µF R3 (SHORT PIN) 5.1k R1 VIN EN 402k LTC4252-1 VOUT 1% * 8 2 OV PWRGD 9 7 RD 1M UV DRAIN R2 10 6 Q1 32.4k TIMER GATE IRF530S 1% CT 3 4 SS VEE SENSE 0.33µF RC RS C1 CSS 5 10Ω 0.02Ω 10nF 68nF CC 18nF –48V * M0C207This load protecting circuit employs low-side current event of supply or load faults. An internal shunt regulatorsensing. The N-MOSFET is controlled to soft-start the establishes a local operating voltage.load (current ramping) or to disconnect the load in theLow Side-4
  • 19. APPLICATION NOTE 105: Current Sense Circuit CollectionSimple Telecom Power Supply Fuse Monitor 47k –48V 5V RETURN FUSE STATUS R1 R2 100k 100k 3 MOC207 SUPPLY A SUPPLY B RTN 47k VA VB STATUS STATUS 1 4 5V VA OUT F OK OK 0 0 SUPPLY A OK UV OR OV 0 1 8 STATUS UV OR OV OK 1 0 VB UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION 2 MOC207 OV: OVERVOLTAGE FUSE A 47k UV: UNDERVOLTAGE 5V 7 5 VFUSE A VFUSE B FUSE STATUS FUSE B OUT A SUPPLY B STATUS = VA = VB 0 = VA ≠ VB 1 ≠ VA = VB 1 MOC207 ≠ VA ≠ VB 1* 6 OUT B 0: LED/PHOTODIODE ON R3 47k 1: LED/PHOTODIODE OFF F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, SUPPLY A 1/4W –48V OUT ALL STATUS OUTPUTS WILL BE HIGH –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48VThe LTC1921 provides an all-in-one telecom fuse and status flags are generated that indicate the condition ofsupply-voltage monitoring function. Three opto-isolated the supplies and the fuses. Low Side-5
  • 20. APPLICATION NOTE 105: Current Sense Circuit CollectionNegative VoltageThis chapter discusses solutions for negative voltage To see other chapters in this Application Note, return tocurrent sensing. the Introduction.Telecom Supply Current Monitor + 5V LOAD IL 48V – 3 + 7 G2 5 6 RS LT1990 VOUT 2 8 – 4 G1 1 –77V ≤ VCM ≤ 8V REF VOUT = VREF – (10 • IL • RS) VREF = 4V 4 5 IN OUT 174k 1nF LT6650 1 GND FB 2 20k 1990 AI01 1µFThe LT1990 is a wide common-mode range difference mately 4V by the LT6650. The output signal movesamplifier used here to amplify the sense resistor drop by downward from the reference potential in this connection10. To provide the desired input range when using a sin- so that a large output swing can be accommodated.gle 5V supply, the reference potential is set to approxi-–48V Hot Swap Controller GND RIN 3× 1.8k IN SERIES + CL 1/4W EACH 100µF CIN LOAD GND 1 1µF R3 (SHORT PIN) 5.1k R1 VIN EN 402k LTC4252-1 VOUT 1% * 8 2 OV PWRGD 9 7 RD 1M UV DRAIN R2 10 6 Q1 32.4k TIMER GATE IRF530S 1% CT 3 4 SS VEE SENSE 0.33µF RC RS C1 CSS 5 10Ω 0.02Ω 10nF 68nF CC 18nF –48V * M0C207This load protecting circuit employs low-side current event of supply or load faults. An internal shunt regulatorsensing. The N-MOSFET is controlled to soft-start the establishes a local operating voltage.load (current ramping) or to disconnect the load in the Negative Voltage-1
  • 21. APPLICATION NOTE 105: Current Sense Circuit Collection–48V Low Side Precision Current SenseThe first stage amplifier is basically a complementary and furnishes a positive output voltage for increasingform of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-to operate with telecom negative supply voltage. The plementation due to the different supply voltages forZener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to thesupply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operationThe second op amp is powered from a positive supply of the second stage.Fast Compact –48V Current Sense VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A ACCURACY ≈ 3% VOUT Q1 R1 1k FMMT493 4.7k 1% VS = 3V 30.1Ω 1% – 3.3k R1 REDUCES Q1 DISSIPATION 0805 LT1797 ×3 + 0.1µF SETTLES TO 1% IN 2µs, BZX84C6V8 1V OUTPUT STEP VZ = 6.8V 0.003Ω 1% 3W –48V SUPPLY –48V LOAD (–42V TO –56V) – + 1797 TA01 ISENSEThis amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-mentary implementation to the classic high-side configu- age is referenced to a positive potential and movesration. The op amp used must support common-mode downward when representing increasing –48V loading.operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors usedlocal supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.sistor provides metered current to an output load resis-Negative Voltage-2
  • 22. APPLICATION NOTE 105: Current Sense Circuit Collection–48V Current MonitorIn this circuit an economical ADC is used to acquire the and/or higher efficiency operation, the ADC may be pow-sense resistor voltage drop directly. The converter is ered from a small transformer circuit as shown below.powered from a “floating” high-accuracy shunt-regulatedsupply and is configured to perform continuous conver-sions. The ADC digital output drives an opto-isolator,level-shifting the serial data stream to ground. For widersupply voltage applications, the 13k biasing resistor maybe replaced with an active 4mA current source such asshown to the right. For complete dielectric isolationSimple Telecom Power Supply Fuse Monitor 47k –48V 5V RETURN FUSE STATUS R1 R2 100k 100k 3 MOC207 SUPPLY A SUPPLY B RTN 47k VA VB STATUS STATUS 1 4 5V VA OUT F OK OK 0 0 SUPPLY A OK UV OR OV 0 1 8 STATUS UV OR OV OK 1 0 VB UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION 2 MOC207 OV: OVERVOLTAGE FUSE A 47k UV: UNDERVOLTAGE 5V 7 5 VFUSE A VFUSE B FUSE STATUS FUSE B OUT A SUPPLY B STATUS = VA = VB 0 = VA ≠ VB 1 ≠ VA = VB 1 MOC207 ≠ VA ≠ VB 1* 6 OUT B 0: LED/PHOTODIODE ON R3 47k 1: LED/PHOTODIODE OFF F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, SUPPLY A 1/4W –48V OUT ALL STATUS OUTPUTS WILL BE HIGH –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48VThe LTC1921 provides an all-in-one telecom fuse and status flags are generated that indicate the condition ofsupply-voltage monitoring function. Three opto-isolated the supplies and the fuses. Negative Voltage-3
  • 23. APPLICATION NOTE 105: Current Sense Circuit CollectionUnidirectionalUnidirectional current sensing monitors the current flow- Unidirectional Current Sensing Modeing only in one direction through a sense resistor. RSENSE TO LOAD 2.5V TOTo see other chapters in this Application Note, return to C 0.1µF 60Vthe Introduction. 1 8Unidirectional Output into FIL– FIL+ LT1787HV +A/D with Fixed Supply at VS+ 2 VS – VS 7 3 VBIAS 6 RSENSE DNC 5V ROUT C1 4 5 5V 1 8 1µF VEE VOUT FIL– FIL+ VOUT – LT1787 2 VS VS+ 7 R1 1787 F08 20k 3 VBIAS 6 IOUT 5% DNC ROUT 4 5 VCC VEE +IN CS VOUT LTC1286 CLK TO µP –IN D R2 VREF GND OUT 5k 1787 F06 5%Here the LT1787 is operating with the LTC1286 A/D con-verter. The –IN pin of the A/D converter is biased at 1V bythe resistor divider R1 and R2. This voltage increases assense current increases, with the amplified sense voltageappearing between the A/D converters –IN and +IN ter- This is just about the simplest connection in which theminals. The LTC1286 converter uses sequential sampling LT1787 may be used. The VBIAS pin is connected toof its –IN and +IN inputs. Accuracy is degraded if the ground, and the VOUT pin swings positive with increasinginputs move between sampling intervals. A filter capaci- sense current. The output can swing as low as 30mV.tor from FIL+ to FIL– as well as a filter capacitor from Accuracy is sacrificed at small output levels, but this isVBIAS to VOUT may be necessary if the sensed current not a limitation in protection circuit applications or where sensed currents do not vary greatly. Increased low levelchanges more than 1LSB within a conversion cycle. accuracy can be obtained by level shifting VBIAS above ground. The level shifting may be done with resistor di- viders, voltage references or a simple diode. Accuracy is ensured if the output signal is sensed differentially be- tween VBIAS and VOUT. Unidirectional-1
  • 24. APPLICATION NOTE 105: Current Sense Circuit Collection16-Bit Resolution Unidirectional 48V Supply Current Monitor withOutput into LTC2433 ADC Isolated Output and 105V SurvivabilityThe LTC2433-1 can accurately digitize signal with sourceimpedances up to 5kΩ. This LTC6101 current sense cir-cuit uses a 4.99kΩ output resistance to meet this re-quirement, thus no additional buffering is necessary.Intelligent High Side Switch The HV version of the LTC6101 can operate with a total supply voltage of 105V. Current flow in high supply volt- age rails can be monitored directly or in an isolated fash- ion as shown in this circuit. The gain of the circuit and the level of output current from the LTC6101 depends on the particular opto-isolator used. 12-Bit Resolution Unidirectional Output into LTC1286 ADC RSENSE TO I = 100A 0.0016Ω LOAD 1 8 2.5V TO 60V FIL– FIL+ LT1787HV +The LT1910 is a dedicated high side MOSFET driver with 2 VS – VS 7 R1 C1 5Vbuilt in protection features. It provides the gate drive for 3 DNC VBIAS 6 15k 1µFa power switch from standard logic voltage levels. It pro- ROUT 20k VREF VCC 4 5 CSvides shorted load protection by monitoring the current VEE VOUT +IN LTC1286 CLK TO µP –INflow to through the switch. Adding an LTC6101 to the C2 D GND OUTsame circuit, sharing the same current sense resistor, VOUT = VBIAS + (8 • ILOAD • RSENSE) 0.1µF LT1634-1.25 1787 TA01provides a linear voltage signal proportional to the loadcurrent for additional intelligent control. While the LT1787 is able to provide a bidirectional out- put, in this application the economical LTC1286 is used to digitize a unidirectional measurement. The LT1787 has a nominal gain of eight, providing a 1.25V full-scale out- put at approximately 100A of load current.Unidirectional-2
  • 25. APPLICATION NOTE 105: Current Sense Circuit CollectionBidirectionalBidirectional current sensing monitors current flow in Practical H-Bridge Current Monitor Offers Faultboth directions through a sense resistor. Detection and Bidirectional Load InformationTo see other chapters in this Application Note, return to – BATTERY BUS DIFF OUTPUTthe Introduction. TO ADC + LTC6101 RIN RIN LTC6101Bidirectional Current Sensing ROUT ROUT RS RSwith Single Ended Output + VS FOR IM RANGE = ±100A, DIFF OUT = ±2.5V RS = 1mΩ B A B A RIN = 200Ω LOAD ROUT = 4.99k IM RS 100Ω 0.1 100Ω I 100Ω DN374 F04 100Ω 4 3 5 5 3 4 This circuit implements a differential load measurement for an ADC using twin unidirectional sense measure- ments. Each LTC6101 performs high side sensing that – – LTC6101 LTC6101 rapidly responds to fault conditions, including load + + shorts and MOSFET failures. Hardware local to the switch 2 1 1 2 module (not shown in the diagram) can provide the pro- tection logic and furnish a status flag to the control sys- 2.5V REF 5V tem. The two LTC6101 outputs taken differentially pro- 2.5k + duce a bidirectional load measurement for the control LT1490 VOUT servo. The ground-referenced signals are compatible 2.5V TO 5V (CONNECTION A) – with most ∆ΣADCs. The ∆ΣADC circuit also provides a 2.5V TO 0V (CONNECTION B) “free” integration function that removes PWM content 0A TO 1A IN EITHER DIRECTION 2.5k from the measurement. This scheme also eliminates the need for analog-to-digital conversions at the rate neededTwo LTC6101’s are used to monitor the current in a load to support switch protection, thus reducing cost andin either direction. Using a separate rail-to-rail op amp to complexity.combine the two outputs provides a single ended output.With zero current flowing the output sits at the referencepotential, one-half the supply voltage for maximum out-put swing or 2.5V as shown. With power supplied to theload through connection A the output will move positivebetween 2.5V and Vcc. With connection B the outputmoves down between 2.5V and 0V. Bidirectional-1
  • 26. APPLICATION NOTE 105: Current Sense Circuit CollectionConventional H-Bridge Current Monitor Single Supply 2.5V Bidirectional Operation with BATTERY BUS External Voltage Reference and I/V Converter + ISENSE TO RSENSE CHARGER/ LOAD C1 2.5V + VSENSE(MAX) 1 8 1µF FIL– FIL+ – LT1787 RS + 2 VS VS+ 7 DIFF 2.5V 3 VBIAS 6 AMP DNC IM – ROUT C3 4 5 1000pF VEE VOUT – A1 VOUT A 2.5V + LT1495 1M DN374 F03 5% LT1389-1.25 1787 F07Many of the newer electric drive functions, such as steer-ing assist, are bidirectional in nature. These functions aregenerally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-railwidth-modulation (PWM) methods to vary the com- op-amp configured as an I/V converter. This configura-manded torque. In these systems, there are two main tion is ideal for monitoring very low voltage supplies. Thepurposes for current monitoring. One is to monitor the LT1787’s VOUT pin is held equal to the reference voltagecurrent in the load, to track its performance against the appearing at the op amp’s non-inverting input. This al-desired command (i.e., closed-loop servo law), and an- lows one to monitor supply voltages as low as 2.5V. Theother is for fault detection and protection features. op-amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op ampA common monitoring approach in these systems is to may drive following circuitry more effectively than theamplify the voltage on a “flying” sense resistor, as high output impedance of the LT1787. The I/V convertershown. Unfortunately, several potentially hazardous fault configuration also works well with split supply voltages.scenarios go undetected, such as a simple short toground at a motor terminal. Another complication is the Battery Current Monitornoise introduced by the PWM activity. While the PWM IL RSENSE CHARGEnoise may be filtered for purposes of the servo law, in- 0.1Ωformation useful for protection becomes obscured. The DISCHARGE 5V 12Vbest solution is to simply provide two circuits that indi- RA RAvidually protect each half-bridge and report the bidirec- – – A2 A1tional load current. In some cases, a smart MOSFET 1/2 LT1495 RA RA 1/2 LT1495 + +bridge driver may already include sense resistors andoffer the protection features needed. In these situations, 2N3904 2N3904the best solution is the one that derives the load informa-tion with the least additional circuitry. DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSE RB RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05 One LT1495 dual op-amp package can be used to estab- lish separate charge and discharge current monitoring outputs. The LT1495 features Over-the-Top operation allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage.Bidirectional-2
  • 27. APPLICATION NOTE 105: Current Sense Circuit CollectionFast Current Sense with Alarm The LT1995 is shown as a simple unity gain difference amplifier. When biased with split supplies the input cur- rent can flow in either direction providing an output volt- age of 100mV per Amp from the voltage across the 100mΩ sense resistor. With 32MHz of bandwidth and 1000V/usec slew rate the response of this sense ampli- fier is fast. Adding a simple comparator with a built in reference voltage circuit such as the LT6700-3 can be used to generate an over-current flag. With the 400mV reference the flag occurs at 4A.Bidirectional Current Sense with Separate Charge/Discharge Output IDISCHARGE RSENSE ICHARGE CHARGER RIN D RIN C 100 100 RIN D RIN C 100 100 4 3 3 4 VBATT L 2 + – 5 5 – + 2 O A D 1 1 LTC6101 LTC6101 + + ROUT D VOUT D VOUT C ROUT C 4.99k 4.99k – – 6101 TA02 DISCHARGING: VOUT D = IDISCHARGE • RSENSE ( ROUT D RIN D ) WHEN IDISCHARGE ≥ 0 CHARGING: VOUT C = ICHARGE • RSENSE ( ROUT C RIN C ) WHEN ICHARGE ≥ 0In this circuit the outputs are enabled by the direction of while the other LT6101, VOUT C, ramps from low to highcurrent flow. The battery current when either charging or in proportion to the charging current. The active outputdischarging enables only one of the outputs. For example reverses when the charger is removed and the batterywhen charging, the VOUT D signal goes low since the discharges into the load.output MOSFET of that LTC6101 turns completely off Bidirectional-3
  • 28. APPLICATION NOTE 105: Current Sense Circuit CollectionBidirectional Absolute Value Current Sense IDISCHARGE RSENSE ICHARGE CHARGER RIN D RIN C RIN D RIN C 4 3 3 4 VBATT L 2 + – 5 5 – + 2 O A D 1 1 LTC6101 LTC6101 + VOUT ROUT – 6101 TA05 DISCHARGING: VOUT = IDISCHARGE • RSENSE ( ROUT RIN D ) WHEN IDISCHARGE ≥ 0 CHARGING: VOUT = ICHARGE • RSENSE ( ROUT RIN C ) WHEN ICHARGE ≥ 0The high impedance current source outputs of two value of the magnitude of the current into or out of theLTC6101’s can be directly tied together. In this circuit the battery. The direction or polarity of the current flow is notvoltage at VOUT continuously represents the absolute discriminated.Full-Bridge Load Current Monitor +VSOURCE 5V LT1990 900k 10k 8 7 1M 100k 2 – RS 6 VOUT – + 3 1M + IL VREF = 1.5V 4 10k 5 IN OUT 54.9k 1nF LT6650 40k 900k GND FB 40k 100k 20k –12V ≤ VCM ≤ 73V VOUT = VREF ± (10 • IL • RS) 1 1990 TA01 1µFThe LT1990 is a difference amplifier that features a very the output away from ground. The output will movewide common mode input voltage range that can far ex- above or below 1.5V as a function of which direction theceed its own supply voltage. This is an advantage to re- current in the load is flowing. As shown, the amplifierject transient voltages when used to monitor the current provides a gain of 10 to the voltage developed acrossin a full bridge driven inductive load such as a motor. The resistor RS.LT6650 provides a voltage reference of 1.5V to bias upBidirectional-4
  • 29. APPLICATION NOTE 105: Current Sense Circuit CollectionLow Power, Bidirectional 60V Precision Hi Side Current SenseUsing a very precise zero-drift amplifier as a pre-amp the 60V limit of the LT1787HV circuit. Overall gain of thisallows for the use of a very small sense resistor in a high circuit is 1000. A 1mA change in current in either direc-voltage supply line. A floating power supply regulates the tion through the 10mΩ sense resistor will produce avoltage across the pre-amplifier on any voltage rail up to 10mV change in the output voltage.Split or Single Supply Operation, Bidirectional Output into A/D 1Ω 1% IS = ±125mA VCC 5V VSRCE 1 8 FIL– FIL+ ≈4.75V LT1787 – 2 VS VS+ 7 10µF 16V 3 VBIAS 6 1 DNC 7 20k CONV VEE 4 5 VOUT (±1V) 2 6 CLOCKING VEE AIN LTC1404 CLK –5V VOUT 3 CIRCUITRY OPTIONAL SINGLE VREF 5 DOUT SUPPLY OPERATION: 10µF GND DISCONNECT VBIAS 16V 4 8 FROM GROUND AND CONNECT IT TO VREF. 10µF DOUT REPLACE –5V SUPPLY 16V WITH GROUND. VEE 1787 TA02 OUTPUT CODE FOR ZERO –5V CURRENT WILL BE ~2430In this circuit, split supply operation is used on both the LT1787 pin 6 is driven by VREF, the bidirectional meas-LT1787 and LT1404 to provide a symmetric bidirectional urement range is slightly asymmetric due to VREF beingmeasurement. In the single-supply case, where the somewhat greater than mid-span of the ADC input range. Bidirectional-5
  • 30. APPLICATION NOTE 105: Current Sense Circuit CollectionACSensing current in ac power lines is quite tricky in thesense that both the current and voltage are continuouslychanging polarity. Transformer coupling of signals todrive ground referenced circuitry is often a good ap-proach.To see other chapters in this Application Note, return tothe Introduction.Single Supply RMS Current MeasurementThe LT1966 is a true RMS-to-DC converter that takes asingle-ended or differential input signal with rail-to-railrange. The output of a pcb mounted current sense trans-former can be connected directly to the converter. Up to75A of AC current is measurable without breaking thesignal path from a power source to a load. The accurateoperating range of the circuit is determined by the selec-tion of the transformer termination resistor. All of themath is built in to the LTC1966 to provide a dc outputvoltage that is proportional to the true rms value of thecurrent. This is valuable in determining the power/energyconsumption of ac powered appliances. AC-1
  • 31. APPLICATION NOTE 105: Current Sense Circuit CollectionDCDC current sensing is for measuring current flow that is The power introduced to the elements, and thereby theirchanging at a very slow rate. temperature, is ascertained from the voltage-current product with the LT6100 measuring the current and theTo see other chapters in this Application Note, return to LT1991 measuring the voltage. The LT6100 senses thethe Introduction. current by measuring the voltage across the 10Ω resistor, applies a gain of 50, and provides a groundMicro-Hotplate Voltage and Current Monitor referenced output. The I to V gain is therefore VDR+ 500mV/mA, which makes sense given the 10mA full scale heater current and the 5V output swing of the 10Ω 1% LT6100. The LT1991’s task is the opposite, applying pre- VS– VS+ IHOTPLATE cision attenuation instead of gain. The full scale voltage + – of the heater is a total of 40V (±20), beyond which the life 5V VCC CURRENT MONITOR of the heater may be reduced in some atmospheres. The LT6100 VEE A2 A4 VOUT = 500mV/mA LT1991 is set up for an attenuation factor of 10, so that MICRO-HOTPLATE BOSTON the 40V full scale differential drive becomes 4V ground MICROSYSTEMS MHP100S-005 referenced at the LT1991 output. In both cases, the volt- 5V 5V M9 ages are easily read by 0V–5V PC I/O cards and the sys- M3 M1 LT1991 VOLTAGE MONITOR tem readily software controlled. P1 V + – VDR– P3 VOUT = DR 10 P9 Battery Current Monitor VDR– 6100 TA06 www.bostonmicrosystems.com IL RSENSE CHARGE 0.1ΩMaterials science research examines the properties andinteractions of materials at various temperatures. Some DISCHARGE 5V 12Vof the more interesting properties can be excited with – RA RA –localized nano-technology heaters and detected using the A2 A1 1/2 LT1495 1/2 LT1495presence of interactive thin films. + RA RA +While the exact methods of detection are highly complex 2N3904 2N3904and relatively proprietary, the method of creating local- DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSEized heat is as old as the light bulb. Shown is the sche- RB RB FOR RA = 1k, RB = 10kmatic of the heater elements of a Micro-hotplate from VO IL = 1V/A 1495 TA05Boston Microsystems (www.bostonmicrosystems.com).The physical dimensions of the elements are tens of mi- One LT1495 dual op-amp package can be used to estab-crons. They are micromachined out of SiC and heated lish separate charge and discharge current monitoringwith simple DC electrical power, being able to reach outputs. The LT1495 features Over-the-Top operation1000°C without damage. allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage. DC-1
  • 32. APPLICATION NOTE 105: Current Sense Circuit CollectionBidirectional Battery-Current Monitor High Side Current Sense and Fuse Monitor TO RSENSE RSENSE TO LOAD 2mΩ FUSE CHARGER/ BATTERY LOAD C1 BUS 15V 1 8 1µF 1 8 + FIL– FIL+ LT1787 VS– VS+ – 2 VS V S+ 7 ADC 2 7 POWER VCC A4 3 VBIAS 6 ≥2.7V DNC C2 – + 0.1µF ROUT 4 5 3 6 VEE OUTPUT FIL A2 VOUT C2 C3* –5V 1µF 1000pF 4 OUT 5 OUTPUT 1787 F02 VEE 2.5V = 25A LT6100 *OPTIONAL DN374 F02This circuit provides the capability of monitoring current The LT6100 can be used as a combination current sensorin either direction through the sense resistor. To allow and fuse monitor. This part includes on-chip output buff-negative outputs to represent charging current, VEE is ering and was designed to operate with the low supplyconnected to a small negative supply. In single-supply voltage (≥2.7V), typical of vehicle data acquisition sys-operation (VEE at ground), the output range may be offset tems, while the sense inputs monitor signals at theupwards by applying a positive reference level to VBIAS higher battery bus potential. The LT6100 inputs are toler-(1.25V for example). C3 may be used to form a filter in ant of large input differentials, thus allowing the blown-conjunction with the output resistance (ROUT) of the part. fuse operating condition (this would be detected by anThis solution offers excellent precision (very low VOS) output full-scale indication). The LT6100 can also beand a fixed nominal gain of 8. powered down while maintaining high impedance sense inputs, drawing less than 1µA max from the battery bus.“Classic” Positive Supply Rail Current Sense 5V Gain of 50 Current Sense ISENSE RSENSE 200Ω VSUPPLY 6.4V TO 48V + LT6100 VS VS– LOAD 0.2Ω + Q1 LT1637 + – 200Ω 2N3904 – 0V TO 4.3V 5V VCC LOAD ILOAD 2k VOUT = (2Ω)(ILOAD) 1637 TA02 FIL VOUT 50 • RSENSE • ISENSEThis circuit uses generic devices to assemble a function VEE A2 A4 6100 TA04similar to an LTC6101. A Rail-to-Rail Input type op ampis required since input voltages are right at the upper rail.The circuit shown here is capable of monitoring up to The LT6100 is configured for a gain of 50 by grounding44V applications. Besides the complication of extra parts, both A2 and A4. This is one of the simplest current sens-the VOS performance of op amps at the supply is gener- ing amplifier circuits where only a sense resistor is re-ally not factory trimmed, thus less accurate than other quired.solutions. The finite current gain of the bipolar transistoris a small source of gain error.DC-2
  • 33. APPLICATION NOTE 105: Current Sense Circuit CollectionDual LTC6101’s Allow High-Low Current Ranging CMPZ4697 VLOGIC (3.3V TO 5V) 10k 7 M1 3 Si4465 + VIN 4 ILOAD RSENSE HI – 10m 8 Q1 5 CMPT5551 VOUT RSENSE LO 40.2k 6 301 100m 301 301 301 4.7k 1.74M LTC1540 4 3 4 3 2 1 HIGH 2 + – 5 2 + – 5 RANGE VIN 619k INDICATOR (ILOAD > 1.2A) 1 1 HIGH CURRENT RANGE OUT LTC6101 LTC6101 250mV/A 7.5k VLOGIC BAT54C LOW CURRENT RANGE OUT 2.5V/A R5 7.5k (VLOGIC +5V) ≤ VIN ≤ 60V 0 ≤ ILOAD ≤ 10A 6101 F03bUsing two current sense amplifiers with two values of rents, less than 1.2 Amps, than with higher currents. Asense resistors is an easy method of sensing current comparator detects higher current flow, up to 10 Amps,over a wide range. In this circuit the sensitivity and reso- and switches sensing over to the high current circuitry.lution of measurement is 10 times greater with low cur-Two Terminal Current Regulator High Side Power Supply Current SenseThe LT1635 combines an op amp with a 200mV refer-ence. Scaling this reference voltage to a potential acrossresistor R3 forces a controlled amount of current to flow The low offset error of the LTC6800 allows for unusuallyfrom the +terminal to the –terminal. Power is taken from low sense resistance while retaining accuracy.the loop. DC-3
  • 34. APPLICATION NOTE 105: Current Sense Circuit Collection0nA to 200nA Current Meter Conventional H-Bridge Current Monitor 100pF BATTERY BUS + R1 10M R4 – 10k 1/2 – 1.5V LT1495 RS + INPUT 1/2CURRENT LT1495 + DIFF R2 AMP + 9k 1.5V IM – R3 IS = 3µA WHEN IIN = 0 2k FULL-SCALE NO ON/OFF SWITCH ADJUST REQUIRED 0µA TO µA 200µA 1495 TA06 DN374 F03A floating amplifier circuit converts a full-scale 200nA Many of the newer electric drive functions, such as steer-flowing in the direction indicated at the inputs to 2V at ing assist, are bidirectional in nature. These functions arethe output of the LT1495. This voltage is converted to a generally driven by H-bridge MOSFET arrays using pulse-current to drive a 200µA meter movement. By floating width-modulation (PWM) methods to vary the com-the power to the circuit with batteries, any voltage poten- manded torque. In these systems, there are two maintial at the inputs are handled. The LT1495 is a micro- purposes for current monitoring. One is to monitor thepower op amp so the quiescent current drain from the current in the load, to track its performance against thebatteries is very low and thus no on/off switch is re- desired command (i.e., closed-loop servo law), and an-quired. other is for fault detection and protection features.Over-The-Top Current Sense A common monitoring approach in these systems is to 3V TO 44V amplify the voltage on a “flying” sense resistor, as R1 shown. Unfortunately, several potentially hazardous fault 200Ω scenarios go undetected, such as a simple short to 3V ground at a motor terminal. Another complication is the RS 0.2Ω + noise introduced by the PWM activity. While the PWM LT1637 Q1 2N3904 noise may be filtered for purposes of the servo law, in- – VOUT formation useful for protection becomes obscured. The (0V TO 2.7V) ILOAD R2 best solution is to simply provide two circuits that indi- LOAD ILOAD = VOUT 2k vidually protect each half-bridge and report the bidirec- (RS)(R2/R1) 1637 TA06 tional load current. In some cases, a smart MOSFET bridge driver may already include sense resistors andThis circuit is a variation on the “classic” high-side cir- offer the protection features needed. In these situations,cuit, but takes advantage of Over-the-Top input capability the best solution is the one that derives the load informa-to separately supply the IC from a low-voltage rail. This tion with the least additional circuitry.provides a measure of fault protection to downstreamcircuitry by virtue of the limited output swing set by thelow-voltage supply. The disadvantage is VOS in the Over-the-Top mode is generally inferior to other modes, thusless accurate. The finite current gain of the bipolar tran-sistor is a source of small gain error.DC-4
  • 35. APPLICATION NOTE 105: Current Sense Circuit CollectionSingle Supply 2.5V Bidirectional Operation with Fast Current Sense with AlarmExternal Voltage Reference and I/V Converter ISENSE TO RSENSECHARGER/ LOAD C1 2.5V + VSENSE(MAX) 1 8 1µF FIL– FIL+ – LT1787 2 VS VS+ 7 2.5V 3 VBIAS 6 DNC C3 ROUT 4 5 1000pF VEE VOUT – A1 VOUT A 2.5V + 1M LT1495 The LT1995 is shown as a simple unity gain difference 5% LT1389-1.25 amplifier. When biased with split supplies the input cur- 1787 F07 rent can flow in either direction providing an output volt- age of 100mV per Amp from the voltage across theThe LT1787’s output is buffered by an LT1495 rail-to-rail 100mΩ sense resistor. With 32MHz of bandwidth andop-amp configured as an I/V converter. This configura- 1000V/usec slew rate the response of this sense ampli-tion is ideal for monitoring very low voltage supplies. The fier is fast. Adding a simple comparator with a built inLT1787’s VOUT pin is held equal to the reference voltage reference voltage circuit such as the LT6700-3 can beappearing at the op amp’s non-inverting input. This al- used to generate an over-current flag. With the 400mVlows one to monitor supply voltages as low as 2.5V. The reference the flag occurs at 4A.op-amp’s output may swing from ground to its positivesupply voltage. The low impedance output of the op amp Positive Supply Rail Current Sensemay drive following circuitry more effectively than the VCC R1high output impedance of the LT1787. The I/V converter 200Ωconfiguration also works well with split supply voltages. Rs 0.2Ω – –Battery Current Monitor 1/2 LT1366 Q1 1/2 LT1366 TP0610L + + ( ) IL RSENSE ILOAD R2 CHARGE VO = ILOAD • RS 0.1Ω R1 R2 20k = ILOAD • 20Ω LOAD DISCHARGE 5V 12V 1366 TA01 – RA RA – A2 1/2 LT1495 RA RA A1 1/2 LT1495 This is a configuration similar to an LT6100 implemented + + with generic components. A Rail-to-Rail or Over-the-Top input op amp type is required (for the first section). The 2N3904 2N3904 first section is a variation on the classic high-side where DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSE the P-MOSFET provides an accurate output current into FOR RA = 1k, RB = 10k RB RB VO R2 (compared to a BJT). The second section is a buffer = 1V/A IL 1495 TA05 to allow driving ADC ports, etc., and could be configuredOne LT1495 dual op-amp package can be used to estab- with gain if needed. As shown, this circuit can handle uplish separate charge and discharge current monitoring to 36V operation. Small-signal range is limited by VOL inoutputs. The LT1495 features Over-the-Top operation single-supply operation.allowing the battery potential to be as high as 36V withonly a 5V amplifier supply voltage. DC-5
  • 36. APPLICATION NOTE 105: Current Sense Circuit CollectionLT6100 Load Current Monitor LTC6101 Supply Current TO LOAD included as Load in Measurement RSENSE + V+ C1 1 8 0.1µF 5V V S– V S+ RIN 2 7 RSENSE VCC A4 4 3 + C2 3V – + 0.1µF + – 3 6 FIL A2 LOAD 2 5 4 OUT 5 VEE OUTPUT LT6100 1 6100 F04 LTC6101 VOUT ROUTThis is the basic LT6100 circuit configuration. The inter- 6101 F06nal circuitry, including an output buffer, typically operatesfrom a low voltage supply, such as the 3V shown. Themonitored supply can range anywhere from VCC + 1.4V This is the basic LTC6101 high-side sensing supply-up to 48V. The A2 and A4 pins can be strapped various monitor configuration, where the supply current drawnways to provide a wide range of internally fixed gains. by the IC is included in the readout signal. This configu-The input leads become very hi-Z when VCC is powered ration is useful when the IC current may not be negligibledown, so as not to drain batteries for example. Access to in terms of overall current draw, such as in low-poweran internal signal node (pin 3) provides an option to in- battery-powered applications. RSENSE should be selectedclude a filtering function with one added capacitor. Small- to limit voltage-drop to <500mV for best linearity. If it issignal range is limited by VOL in single-supply operation. desirable not to include the IC current in the readout, as in load monitoring, pin 5 may be connected directly to V+1A Voltage-Controlled Current Sink instead of the load. Gain accuracy of this circuit is limited only by the precision of the resistors selected by the user. V+ Powered Separately from Load SupplyThis is a simple controlled current sink, where the opamp drives the NMOSFET gate to develop a match be-tween the 1Ω sense resistor drop and the VIN currentcommand. Since the common-mode voltage seen by the The inputs of the LTC6101 can function from 1.4V aboveop amp is near ground potential, a “single-supply” or the device positive supply to 48V DC. In this circuit theRail-to-Rail type is required in this application. current flow in the high voltage rail is directly translated to a 0V to 3V range.DC-6
  • 37. APPLICATION NOTE 105: Current Sense Circuit CollectionSimple High Side Current “Classic” High-Precision Low Side Current SenseSense Using the LTC6101 5V BATTERY BUS 3 5 OUT + 3V/AMP 1 LOAD CURRENT LTC2050HV RIN 4 IN MEASURED RSENSE 0.01Ω 100Ω – 2 CIRCUIT, REFERRED 4 3 TO –5V 10Ω 10k LOAD + – TO 3mΩ 2 5 MEASURED CIRCUIT LOAD CURRENT 0.1µF – 5V 2050 TA08 1 This configuration is basically a standard non-inverting VOUT LT6101 ROUT 4.99V = 10A amplifier. The op amp used must support common-mode 4.99k operation at the lower rail and the use of a Zero-Drift type VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01 (as shown) provides excellent precision. The output of this circuit is referenced to the lower Kelvin contact,This is a basic high side current monitor using the which could be ground in a single-supply application.LTC6101. The selection of RIN and ROUT establishes the Small-signal range is limited by VOL for single-supplydesired gain of this circuit, powered directly from thebattery bus. The current output of the LTC6101 allows it designs. Scaling accuracy is set by the quality of the user-selected resistors.to be located remotely to ROUT. Thus, the amplifier canbe placed directly at the shunt, while ROUT is placed nearthe monitoring electronics without ground drop errors.This circuit has a fast 1µs response time that makes itideal for providing MOSFET load switch protection. Theswitch element may be the high side type connected be-tween the sense resistor and the load, a low side typebetween the load and ground or an H-bridge. The circuitis programmable to produce up to 1mA of full-scale out-put current into ROUT, yet draws a mere 250µA supplycurrent when the load is off. DC-7
  • 38. APPLICATION NOTE 105: Current Sense Circuit CollectionLevel ShiftingQuite often it is required to sense current flow in a supply V+ Powered Separately from Load Supplyrail that is a much higher voltage potential than the sup-ply voltage for the system electronics. Current sense cir-cuits with high voltage capability are useful to translateinformation to lower voltage signals for processing.To see other chapters in this Application Note, return tothe Introduction.Over-The-Top Current Sense 3V TO 44V R1 200Ω 3V The inputs of the LTC6101 can function from 1.4V above RS 0.2Ω + the device positive supply to 48V DC. In this circuit the LT1637 Q1 current flow in the high voltage rail is directly translated 2N3904 – VOUT to a 0V to 3V range. ILOAD (0V TO 2.7V) R2 VOUT 2k Voltage Translator LOAD ILOAD = (RS)(R2/R1) 1637 TA06 + RIN VINThis circuit is a variation on the “classic” high-side cir- – 4 3cuit, but takes advantage of Over-the-Top input capability –to separately supply the IC from a low-voltage rail. This +provides a measure of fault protection to downstream 2 5circuitry by virtue of the limited output swing set by the +low-voltage supply. The disadvantage is VOS in the Over- VTRANSLATE –the-Top mode is generally inferior to other modes, thus 1less accurate. The finite current gain of the bipolar tran- LTC6101 VOUT ROUTsistor is a source of small gain error. This is a convenient usage of the LTC6101 current sense amplifier as a high voltage level translator. Differential voltage signals riding on top of a high common mode voltage (up to 105V with the LTC6101HV) get converted to a current, through RIN, and then scaled down to a ground referenced voltage across ROUT. Level Shifting-1
  • 39. APPLICATION NOTE 105: Current Sense Circuit CollectionLow Power, Bidirectional 60V Precision Hi Side Current SenseUsing a very precise zero-drift amplifier as a pre-amp the 60V limit of the LT1787HV circuit. Overall gain of thisallows for the use of a very small sense resistor in a high circuit is 1000. A 1mA change in current in either direc-voltage supply line. A floating power supply regulates the tion through the 10mΩ sense resistor will produce avoltage across the pre-amplifier on any voltage rail up to 10mV change in the output voltage.Level Shifting-2
  • 40. APPLICATION NOTE 105: Current Sense Circuit CollectionHigh VoltageMonitoring current flow in a high voltage line often re- Measuring bias current into an Avalanche Photoquires floating the supply of the measuring circuits up Diode (APD) using an instrumentation amplifier.near the high voltage potentials. Level shifting and isola- 1ktion components are then often used to develop a lower VIN 1% BIAS OUTPUToutput voltage indication. 10V TO 33V 35V TO APDTo see other chapters in this Application Note, return to – CURRENT LT1789 MONITOR OUTPUTthe Introduction. 0mA TO 1mA = 0V TO 1V + A=1Over-The-Top Current Sense AN92 F02a 3V TO 44V 1N4684 R1 1k 3.3V 200Ω 1% VIN BIAS OUTPUT 10V TO 35V TO APD 3V 10M RS – 0.2Ω + CURRENT Q1 LT1789 MONITOR OUTPUT LT1637 0mA TO 1mA = 0V TO 1V 2N3904 + – VOUT A=1 ILOAD (0V TO 2.7V) AN92 F02b R2 VOUT 2k LOAD ILOAD = (RS)(R2/R1) The upper circuit uses an instrumentation amplifier (IA) 1637 TA06 powered by a separate rail (>1V above VIN) to measure across the 1kΩ current shunt. The lower figure is similarThis circuit is a variation on the “classic” high-side cir- but derives its power supply from the APD bias line. Thecuit, but takes advantage of Over-the-Top input capability limitation of these circuits is the 35V maximum APDto separately supply the IC from a low-voltage rail. This voltage, whereas some APDs may require 90V or more.provides a measure of fault protection to downstream In the single-supply configuration shown, there is also acircuitry by virtue of the limited output swing set by the dynamic range limitation due to VOL to consider. The ad-low-voltage supply. The disadvantage is VOS in the Over- vantage of this approach is the high accuracy that isthe-Top mode is generally inferior to other modes, thus available in an IA.less accurate. The finite current gain of the bipolar tran-sistor is a source of small gain error. High Voltage-1
  • 41. APPLICATION NOTE 105: Current Sense Circuit CollectionSimple 500V Current Monitor 48V Supply Current Monitor with Isolated Output and 105V SurvivabilityAdding two external Mosfets to hold off the voltage al-lows the LTC6101 to connect to very high potentials andmonitor the current flow. The output current from theLTC6101, which is proportional to the sensed input volt-age, flows through M1 to create a ground referenced The HV version of the LTC6101 can operate with a totaloutput voltage. supply voltage of 105V. Current flow in high supply volt- age rails can be monitored directly or in an isolated fash- ion as shown in this circuit. The gain of the circuit and the level of output current from the LTC6101 depends on the particular opto-isolator used.High Voltage-2
  • 42. APPLICATION NOTE 105: Current Sense Circuit CollectionLow Power, Bidirectional 60V Precision Hi Side Current SenseUsing a very precise zero-drift amplifier as a pre-amp the 60V limit of the LT1787HV circuit. Overall gain of thisallows for the use of a very small sense resistor in a high circuit is 1000. A 1mA change in current in either direc-voltage supply line. A floating power supply regulates the tion through the 10mΩ sense resistor will produce avoltage across the pre-amplifier on any voltage rail up to 10mV change in the output voltage. High Voltage-3
  • 43. APPLICATION NOTE 105: Current Sense Circuit CollectionLow VoltageTo see other chapters in this Application Note, return to 1.25V Electronic Circuit Breakerthe Introduction. SI4864DY VIN VOUT 1.25V 1.25VSingle Supply 2.5V Bidirectional Operation with 3.5AExternal Voltage Reference and I/V Converter SENSEP GATE SENSEN VBIAS VCC VBIAS 2.3V TO 6V ISENSE TO RSENSE LTC4213CHARGER/ 10k LOAD C1 2.5V + VSENSE(MAX) OFF ON ON GND ISEL READY 1 8 1µF FIL– FIL+ – LT1787 2 VS VS+ 7 4213 TA01 2.5V VBIAS 6 3 DNC The LTC4213 provides protection and automatic circuit ROUT C3 1000pF breaker action by sensing Drain-to-Source voltage-drop 4 5 VEE VOUT across the NMOSFET. The sense inputs have a Rail-to- – Rail common mode range, so the circuit breaker can pro- A1 VOUT A 2.5V + LT1495 tect bus voltages from 0V up to 6V. Logic signals flag a 1M 5% LT1389-1.25 trip condition (with the READY output signal) and reini- 1787 F07 tialize the breaker (using the ON input). The ON input may also be used as a command in a “smart switch” ap- plication.The LT1787’s output is buffered by an LT1495 rail-to-railop-amp configured as an I/V converter. This configura-tion is ideal for monitoring very low voltage supplies. TheLT1787’s VOUT pin is held equal to the reference voltageappearing at the op amp’s non-inverting input. This al-lows one to monitor supply voltages as low as 2.5V. Theop-amp’s output may swing from ground to its positivesupply voltage. The low impedance output of the op ampmay drive following circuitry more effectively than thehigh output impedance of the LT1787. The I/V converterconfiguration also works well with split supply voltages. Low Voltage-1
  • 44. APPLICATION NOTE 105: Current Sense Circuit CollectionHigh Current (100mA to Amps)Sensing high currents accurately requires excellent con- Shunt Diode Limits Maximum Input Voltagetrol of the sensing resistance, which is typically a very to Allow Better Low Input Resolutionsmall value to minimize losses, and the dynamic range of Without Over-Ranging the LTC6101the measurement circuitry V+To see other chapters in this Application Note, return tothe Introduction. RSENSE DSENSE 6101 F03aKelvin Input Connection Preserves LOADAccuracy Despite Large Load Currents If low sense currents must be resolved accurately in a V+ system that has very wide dynamic range, more gain can be taken in the sense amplifier by using a smaller value RIN for resistor RIN. This can result in an operating current RSENSE 4 3 greater than the max current spec allowed unless the max current is limited in another way, such as with a + – LOAD Schottky diode across RSENSE. This will reduce the high 2 5 current measurement accuracy by limiting the result, while increasing the low current measurement resolution. This approach can be helpful in cases where an occa- 1 sional large burst of current may be ignored. LTC6101 VOUT ROUT Kelvin Sensing 6101 F02 DIRECTION OF CHARGING CURRENTKelvin connection of the IN– and IN+ inputs to the senseresistor should be used in all but the lowest power appli- RSENSEcations. Solder connections and PC board interconnec-tions that carry high current can cause significant error in 4008 F12measurement due to their relatively large resistances. Byisolating the sense traces from the high current paths, CSP BATthis error can be reduced by orders of magnitude. A In any high current, >1Amp, application, Kelvin contactssense resistor with integrated Kelvin sense terminals will to the sense resistor are important to maintain accuracy.give the best results. This simple illustration from a battery charger application shows two voltage-sensing traces added to the pads of the current sense resistor. If the voltage is sensed with high impedance amplifier inputs, no IxR voltage drop errors are developed. High Current (100mA to Amps)-1
  • 45. APPLICATION NOTE 105: Current Sense Circuit Collection0A to 33A High Side Current Monitor with Filtering Single Supply RMS Current Measurement 4.4V TO 48V 3V SUPPLY 2 7 6 LT6100 VCC A4 A2 + 8 VS RSENSE VOUT 5 VOUT = 2.5V 3mΩ ISENSE = 33A – 1 VS VEE FIL LOAD 6100 TA01a 4 3 CONFIGURED FOR GAIN = 25V/V 220pF The LT1966 is a true RMS-to-DC converter that takes aHigh current sensing on a high voltage supply rail is eas- single-ended or differential input signal with rail-to-railily accomplished with the LT6100. The sense amplifier is range. The output of a pcb mounted current sense trans-biased from a low 3V supply and pin-strapped to a gain former can be connected directly to the converter. Up toof 25V/V to output a 2.5V full scale reading of the current 75A of AC current is measurable without breaking theflow. A capacitor at the FIL pin to ground will filter out signal path from a power source to a load. The accuratenoise of the system (220pF produces a 12KHz low pass operating range of the circuit is determined by the selec-corner frequency). tion of the transformer termination resistor. All of the math is built in to the LTC1966 to provide a dc output voltage that is proportional to the true rms value of the current. This is valuable in determining the power/energy consumption of ac powered appliances.Dual LTC6101’s Allow High-Low Current Ranging CMPZ4697 VLOGIC (3.3V TO 5V) 10k 7 M1 3 Si4465 + VIN 4 ILOAD RSENSE HI – 10m 8 Q1 5 CMPT5551 VOUT RSENSE LO 40.2k 6 301 100m 301 301 301 4.7k 1.74M LTC1540 4 3 4 3 2 1 HIGH 2 + – 5 2 + – 5 RANGE VIN 619k INDICATOR (ILOAD > 1.2A) 1 1 HIGH CURRENT RANGE OUT LTC6101 LTC6101 250mV/A 7.5k VLOGIC BAT54C LOW CURRENT RANGE OUT 2.5V/A R5 7.5k (VLOGIC +5V) ≤ VIN ≤ 60V 0 ≤ ILOAD ≤ 10A 6101 F03bHigh Current (100mA to Amps)-2
  • 46. APPLICATION NOTE 105: Current Sense Circuit CollectionUsing two current sense amplifiers with two values of rents, less than 1.2 Amps, than with higher currents. Asense resistors is an easy method of sensing current comparator detects higher current flow, up to 10 Amps,over a wide range. In this circuit the sensitivity and reso- and switches sensing over to the high current circuitry.lution of measurement is 10 times greater with low cur-LDO Load Balancing VIN BALLAST RESISTANCE: IN OUT 1.8V TO 20V + IDENTICAL LENGTH 10µF LT1763 0.01µF 10µF THERMALLY MATED SHDN BYP WIRE OR PCB TRACE FB R2 R1 2k 2k ⎛ R1⎞ IN OUT VOUT = 1.22V ⎜1 + ⎟ ⎝ R2⎠ LT1763 0.01µF 10µF SHDN BYP 100Ω FB ILOAD LOAD 2k 2k 1k 0.1µF – A 10k + IN OUT LT1763 0.01µF 10µF SHDN BYP 100Ω FB 2k 2k 0 ≤ ILOAD ≤ 1.5A 1k 1.22V ≤ VOUT ≤ VDD 0.1µF LDO LOADS MATCH TO WITHIN VDD 1mA WITH 10mΩ OF BALLAST – RESISTANCE (2 INCHES OF AWG 28 GAUGE STRANDED WIRE) B A, B: LTC6078 10k + 60789 TA09As system design enhancements are made there is often and servo’ed to match the master regulator output volt-the need to supply more current to a load than originally age. The precise low offset voltage of the LTC6078 dualexpected. A simple way to modify power amplifiers or op amp (10uV) balances the load current provided byvoltage regulators, as shown here, is to parallel devices. each regulator to within 1mA. This is achieved using aWhen paralleling devices it is desired that each device very small 10mΩ current sense resistor in series withshares the total load current equally. In this circuit two each output. This sense resistor can be implementedadjustable “slave” regulator output voltages are sensed with pcb copper traces or thin gauge wire. High Current (100mA to Amps)-3
  • 47. APPLICATION NOTE 105: Current Sense Circuit CollectionSensing Output Current VCC0V TO 1V 12V VCSRC VCSNK EN +IN VCC V+ ISRC ISNK RS TSD 0.2Ω LT1970 OUT SENSE+ SENSE– RLOAD FILTER – –IN V VEE COMMON LT1787 R4 255k –12V VS– VS+ RG RF BIAS –12V 12V R1 20k 60.4k – VEE VOUT R2 LT1880 2.5V 10k + ±5mV/mA R3 1kHz FULL CURRENT –12V 20k –12V BANDWIDTH 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACKThe LT1970 is a 500mA power amplifier with voltage in a microprocessor controlled system. For closed loopprogrammable output current limit. Separate DC voltage control of the current to a load an LT1787 can monitorinputs and an output current sensing resistor control the the output current. The LT1880 op amp provides scalingmaximum sourcing and sinking current values. These and level shifting of the voltage applied to an A-to-Dcontrol voltages could be provided by a D-to-A Converter Converter for a 5mV/mA feedback signal.High Current (100mA to Amps)-4
  • 48. APPLICATION NOTE 105: Current Sense Circuit CollectionLow Current (Picoamps to Milliamps)For low current applications the easiest way to sense cur- Gain of 50 Current Senserent is to use a large sense resistor. This however causes ISENSE RSENSElarger voltage drops in the line being sensed which may VSUPPLY 6.4V TO 48Vnot be acceptable. Using a smaller sense resistor and + LT6100 VS VS– LOADtaking gain in the sense amplifier stage is often a betterapproach. Low current implies high source impedance – +measurements which are subject approach. Low currentimplies high source impedance measurements which are 5V VCCsubject to noise pickup and often require filtering of FILsome sort. VOUT 50 • RSENSE • ISENSE VEE A2 A4To see other chapters in this Application Note, return to 6100 TA04the Introduction.Filtered Gain of 20 Current Sense The LT6100 is configured for a gain of 50 by grounding both A2 and A4. This is one of the simplest current sens- ISENSE VSUPPLY RSENSE ing amplifier circuits where only a sense resistor is re- 4.4V TO 48V quired. + LT6100 VS V S– LOAD 0nA to 200nA Current Meter + – 100pF 3V VCC R1 FIL 10M 1000pF VOUT 20 • RSENSE • ISENSE R4 – VEE A2 A4 10k 1/2 6100 TA03 – 1.5V LT1495 –3dB AT 2.6kHz INPUT 1/2 CURRENT LT1495 + R2The LT6100 has pin strap connections to establish a vari- + 9k 1.5Vety of accurate gain settings without using external com- R3 IS = 3µA WHEN IIN = 0 2kponents. For this circuit grounding A2 and leaving A4 FULL-SCALE NO ON/OFF SWITCH ADJUST REQUIREDopen set a gain of 20. Adding one external capacitor to 0µA TOthe FIL pin creates a low-pass filter in the signal path. A µA 200µAcapacitor of 1000pF as shown sets a filter corner fre- 1495 TA06quency of 2.6KHz. A floating amplifier circuit converts a full-scale 200nA flowing in the direction indicated at the inputs to 2V at the output of the LT1495. This voltage is converted to a current to drive a 200µA meter movement. By floating the power to the circuit with batteries, any voltage poten- tial at the inputs are handled. The LT1495 is a micro- power op amp so the quiescent current drain from the batteries is very low and thus no on/off switch is re- quired. Low Current (Picoamps to Milliamps)-1
  • 49. APPLICATION NOTE 105: Current Sense Circuit CollectionLock-In Amplifier Technique Permits 1% Accurate APD Current Measurement Over 100nA to 1mA Range. FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO 1k* APD BIAS REGULATOR, SEE APPENDIX A APD 1% VOUT = 20V TO 90V HIGH VOLTAGE TO APD BIAS INPUT 1µF 1µF 100V 100k* 100k* 100V Q1 1N4690 1M* 5.6V 5V 0.2µF 5V – 1µF 6 A1 20k 2 + S2 OUTPUT 10k LT1789 A2 0V TO 1V = 30k 1µF LT1006 + 5 0mA TO 1mA – Q2 0.2µF 20k* MPSA42 1M* –3.5V –3.5V 20k 12 200k* 13 14 S1 –3.5V TO 5V 18 AMPLIFIERS 22µF 5V 3 S3 + * = 0.1% METAL FILM RESISTOR 15 22µF 1µF 100V = TECATE CMC100105MX1825 + # CIRCLED NUMBERS = LTC1043 PIN NUMBER = 1N4148 = TP0610L 16 17 4 0.056µF 5V AN92 F04Avalanche Photodiodes, APDs, require a small amount of which feeds A1 through 0.2µF AC coupling capacitors.current from a high voltage supply. The current into the A1’s single-ended output biases demodulator S2, whichdiode is an indication of optical signal strength and must presents a DC output to buffer amplifier A2. A2’s outputbe monitored very accurately. It is desirable to power all is the circuit output.of the support circuitry from a single 5V supply. Switch S3 clocks a negative output charge pump whichThis circuit utilizes AC carrier modulation techniques to supplies the amplifier’s V– pins, permitting output swingmeet APD current monitor requirements. It features to (and below) zero volts. The 100k resistors at Q10.4% accuracy over the sensed current range, runs from minimize its on-resistance error contribution and preventa 5V supply and has the high noise rejection character destructive potentials from reaching A1 (and the 5V rail)stics of carrier based “lock in” measurements. if either 0.2µF capacitor fails. A2’s gain of 1.1 corrects for the slight attenuation introduced by A1’s input resistors.The LTC1043 switch array is clocked by its internal oscil- In practice, it may be desirable to derive the APD biaslator. Oscillator frequency, set by the capacitor at Pin 16, voltage regulator’s feedback signal from the indicatedis about 150Hz. S1 clocking biases Q1 via level shifter point, eliminating the 1kΩshunt resistor’s voltage drop.Q2. Q1 chops the DC voltage across the 1k current Verifying accuracy involves loading the APD bias lineshunt, modulating it into a differential square wave signal with 100nA to 1mA and noting output agreement.Low Current (Picoamps to Milliamps)-2
  • 50. APPLICATION NOTE 105: Current Sense Circuit CollectionDC Coupled APD Current Monitor FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO 1N4690 1k* APD BIAS REGULATOR, SEE APPENDIX A APD 5.6V CURRENT SHUNT VOUT = 20V TO 90V HIGH VOLTAGE TO APD BIAS INPUT 10M 1k* 51K + + 1N4702 A1 1µF 15V 51k LT1077 – Q1 100k ZVP0545A Q2 5V MPSA42 10k LT1460 1k* 5V Hi-Z OUTPUT 2.5V 0V TO 1V = 0mA TO 1mA 1k* VIN VREF FO BUFFERED OUTPUT LTC2400 SCK 5V 0mA TO 1mA = 0V TO 1V 5V A-TO-D DIGITAL SDO * = 0.1% METAL FILM RESISTOR + INTERFACE 1k 10µF CS = BAT85 A2 LTC1150 OPTIONAL + – CLK OUT 39k Q2 10µF DIGITAL OUTPUT V– 2N3904 + 100k ≈ –3.5V HERE OPTIONAL BUFFERED OUTPUT AN92 F05Avalanche Photodiodes, APDs, require a small amount of across the 20V to 90V APD bias voltage range. The 5.6Vcurrent from a high voltage supply. The current into the zener assures A1’s inputs are always within their com-diode is an indication of optical signal strength and must mon mode operating range and the 10MΩ resistor main-be monitored very accurately. It is desirable to power all tains adequate zener current when APD current is at veryof the support circuitry from a single 5V supply. low levels.This circuit’s DC coupled current monitor eliminates the Two output options are shown. A2, a chopper stabilizedprevious circuit’s trim but pulls more current from the amplifier, provides an analog output. Its output is able toAPD bias supply. A1 floats, powered by the APD bias rail. swing to (and below) zero because its V– pin is suppliedThe 15V zener diode and current source Q2 ensure A1 with a negative voltage. This potential is generated bynever is exposed to destructive voltages. The 1kΩ cur- using A2’s internal clock to activate a charge pumprent shunt’s voltage drop sets A1’s positive input poten- which, in turn, biases A2’s V– pin.3 A second output op-tial. A1 balances its inputs by feedback controlling its tion substitutes an A-to-D converter, providing a serialnegative input via Q1. As such, Q1’s source voltage format digital output. No V– supply is required, as theequals A1’s positive input voltage and its drain current LTC2400 A-to-D will convert inputs to (and slightly be-sets the voltage across its source resistor. Q1’s drain cur- low) zero volts.rent produces a voltage drop across the ground referred1kΩ resistor identical to the drop across the 1kΩ currentshunt and, hence, APD current. This relationship holds Low Current (Picoamps to Milliamps)-3
  • 51. APPLICATION NOTE 105: Current Sense Circuit CollectionSix Decade (10nA to 10mA) Current Log Amplifier – C 100Ω + – B 100Ω + 33µF Q1 Q2 100k 133k VDD – 1000pF A – 1.58k + D PRECISIONIIN + VOUT RESISTOR PT146 1k VCC LT6650 +3500ppm/°C IN OUT 60789 TA07 GND 10nA ≤ IIN ≤ 10mA 1µF 1µF Q1, Q2: DIODES INC. DMMT3906W A TO D: LTC6079 VOUT ≈ 150mV • log (IIN) + 1.23V, IIN IN AMPSUsing precision quad amplifiers like the LTC6079, (10µVoffset and <1pA bias current) allow for very wide rangecurrent sensing. In this circuit a six decade range of cur-rent pulled from the circuit input terminal is converted toan output voltage in logarithmic fashion increasing150mV for every decade of current change.Low Current (Picoamps to Milliamps)-4
  • 52. APPLICATION NOTE 105: Current Sense Circuit CollectionMotors and Inductive LoadsThe largest challenge in measuring current through in- Conventional H-Bridge Current Monitorductive circuits is the transients of voltage that often oc- BATTERY BUScur. Current flow can remain continuous in one direction +while the voltage across the sense terminals reverses inpolarity.To see other chapters in this Application Note, return tothe Introduction. RS + DIFF AMPElectronic Circuit Breaker IM – ON/OFF IN VS CT CD RD *RSEN 0.22µF 0.01µF 100k 0.1Ω CT DS Z5U LTC1153 DN374 F03 TO µP STATUS G IRLR024 Many of the newer electric drive functions, such as steer- 51k GND SHUTDOWN 51k ing assist, are bidirectional in nature. These functions are 5V SENSITIVE generally driven by H-bridge MOSFET arrays using pulse- **70°C PTC 5V LOAD width-modulation (PWM) methods to vary the com- manded torque. In these systems, there are two main ALL COMPONENTS SHOWN ARE SURFACE MOUNT. purposes for current monitoring. One is to monitor the * IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700 current in the load, to track its performance against the ** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591 LTC1153 • TA01 desired command (i.e., closed-loop servo law), and an- other is for fault detection and protection features.The LTC1153 is an Electronic Circuit Breaker. Sensed cur-rent to a load opens the breaker when 100mV is devel- A common monitoring approach in these systems is tooped between the supply input, Vs, and the Drain Sense amplify the voltage on a “flying” sense resistor, aspin, DS. To avoid transient, or nuisance trips of the break shown. Unfortunately, several potentially hazardous faultcomponents RD and CD delay the action for 1msec. A scenarios go undetected, such as a simple short tothermistor can also be used to bias the Shutdown input ground at a motor terminal. Another complication is theto monitor heat generated in the load and remove power noise introduced by the PWM activity. While the PWMshould the temperature exceed 70°C in this example. A noise may be filtered for purposes of the servo law, in-feature of the LTC1153 is timed Automatic Reset which formation useful for protection becomes obscured. Thewill try to re-connect the load after 200msec using the best solution is to simply provide two circuits that indi-0.22µF timer capacitor shown. vidually protect each half-bridge and report the bidirec- tional load current. In some cases, a smart MOSFET bridge driver may already include sense resistors and offer the protection features needed. In these situations, the best solution is the one that derives the load informa- tion with the least additional circuitry. Motors and Inductive Loads-1
  • 53. APPLICATION NOTE 105: Current Sense Circuit CollectionMotor Speed Control Practical H-Bridge Current Monitor Offers Fault OV TO 5V Detection and Bidirectional Load Information TORQUE/STALLCURRENT CONTROL 15V – VCSRC BATTERY BUS DIFF VCSNK OUTPUT EN TO ADC +IN VCC V+ + ISRC LTC6101 RIN RIN LTC6101 ISNK RS 1Ω ROUT ROUT TSD RS RS LT1970 OUT SENSE+ SENSE– 12V DC + FILTER MOTOR –IN V– FOR IM RANGE = ±100A, VEE DIFF OUT = ±2.5V COMMON GND 15V RS = 1mΩ C1 RIN = 200Ω R1 –15V 1µF TACH ROUT = 4.99k1.2k FEEDBACK REVERSE 3V/1000rpm IM R4 R5 49.9k 49.9k R2 1970 F13 10k FORWARD R3 DN374 F041.2k –15V This circuit implements a differential load measurementThis uses an LT1970 power amplifier as a linear driver of for an ADC using twin unidirectional sense measure-a DC motor with speed control. The ability to source and ments. Each LTC6101 performs high side sensing thatsink the same amount of output current provides for bi- rapidly responds to fault conditions, including loaddirectional rotation of the motor. Speed control is man- shorts and MOSFET failures. Hardware local to the switchaged by sensing the output of a tachometer built on to module (not shown in the diagram) can provide the pro-the motor. A typical feedback signal of 3V/1000rpm is tection logic and furnish a status flag to the control sys-compared with the desired speed-set input voltage. Be- tem. The two LTC6101 outputs taken differentially pro-cause the LT1970 is unity-gain stable, it can be config- duce a bidirectional load measurement for the controlured as an integrator to force whatever voltage across servo. The ground-referenced signals are compatiblethe motor as necessary to match the feedback speed with most ∆ΣADCs. The ∆ΣADC circuit also provides asignal with the set input signal. Additionally, the current “free” integration function that removes PWM contentlimit of the amplifier can be adjusted to control the torque from the measurement. This scheme also eliminates theand stall current of the motor. need for analog-to-digital conversions at the rate needed to support switch protection, thus reducing cost and complexity.Motors and Inductive Loads-2
  • 54. APPLICATION NOTE 105: Current Sense Circuit CollectionLamp Driver Relay Driver 12V 12V + + 100µF 470µF 10k 0.02Ω 2Ω 0.02Ω IN VS 100k 10k IN VS5V CT DS 0.01µF 5V 1N4148 0.33µF LTC1153 VN2222LL CT DS STATUS G 1µF LTC1153 MTD3055E 0.1µF 1M STATUS G TO 12V LOAD GND SD IRFZ34 15V 12V GND SD 12V/2A 1N4001 BULB COIL CURRENT LIMITED TO 350mA CONTACT CURRENT LIMITED TO 5A LTC1153 • TA07 LTC1153 • TA08The inrush current created by a lamp during turn-on can This circuit provides reliable control of a relay by usingbe 10 to 20 times greater than the rated operating cur- an Electronic Circuit Breaker circuit with two-level over-rent. This circuit shifts the trip threshold of an LTC1153 current protection. Current flow is sensed through twoElectronic Circuit Breaker up by a factor of 11:1 (to 30A) separate resistors, one for the current into the relay coilfor 100ms while the bulb is turned on. The trip threshold and the other for the current through the relay contacts.then drops down to 2.7A after the inrush current has When 100mV is developed between the Vs supply pinsubsided. and the Drain Sense pin, DS, the N-channel MOSFET is turned off opening the contacts. As shown, the relay coilIntelligent High Side Switch current is limited to 350mA and the contact current to 5 Amps.The LT1910 is a dedicated high side MOSFET driver withbuilt in protection features. It provides the gate drive fora power switch from standard logic voltage levels. It pro-vides shorted load protection by monitoring the currentflow to through the switch. Adding an LTC6101 to thesame circuit, sharing the same current sense resistor,provides a linear voltage signal proportional to the loadcurrent for additional intelligent control. Motors and Inductive Loads-3
  • 55. APPLICATION NOTE 105: Current Sense Circuit CollectionFull-Bridge Load Current Monitor +VSOURCE 5V LT1990 900k 10k 8 7 1M 100k 2 – RS 6 VOUT – + 3 1M + IL VREF = 1.5V 4 10k 5 IN OUT 54.9k 1nF LT6650 40k 900k GND FB 40k 100k 20k –12V ≤ VCM ≤ 73V VOUT = VREF ± (10 • IL • RS) 1 1990 TA01 1µFThe LT1990 is a difference amplifier that features a very the output away from ground. The output will movewide common mode input voltage range that can far ex- above or below 1.5V as a function of which direction theceed its own supply voltage. This is an advantage to re- current in the load is flowing. As shown, the amplifierject transient voltages when used to monitor the current provides a gain of 10 to the voltage developed acrossin a full bridge driven inductive load such as a motor. The resistor RS.LT6650 provides a voltage reference of 1.5V to bias upMotors and Inductive Loads-4
  • 56. APPLICATION NOTE 105: Current Sense Circuit CollectionBatteriesThe science of battery chemistries and the charging and Charge/Discharge Current Monitordischarging characteristics is a book of its own. This on Single Supply with Shifted VBIASchapter is intended to provide a few examples of TO RSENSEmonitoring current flow into and out of batteries of any CHARGER/ 3.3V LOAD C1chemistry. 1 8 1µF TO 60V FIL– FIL+ 3.3V LT1787HV VS– VS+To see other chapters in this Application Note, return to 2 7 20k 5% VBIAS 6the Introduction. 3 DNC ROUT C2 4 5 1µF LT1634-1.25Input Remains Hi-Z when LT6100 is Powered Down VEE VOUT C3* ISENSE 1000pF RSENSE TO LOAD *OPTIONAL OUTPUT 1787 F04 + BATTERY LT6100 VS – V S+ 4.1V TO 48V Here the LT1787 is used in a single supply mode with the POWER VBIAS pin shifted positive using an external LT1634 volt- DOWN OK – + age reference. The VOUT output signal can swing above VCC 3V and below VBIAS to allow monitoring of positive or nega- VCC 0V INPUTS tive currents through the sense resistor. The choice of REMAIN FIL reference voltage is not critical except for the precaution Hi-Z VOUT that adequate headroom must be provided for VOUT to VEE A2 A4 6100 F08 swing without saturating the internal circuitry. The com- ponent values shown allow operation with VS supplies as low as 3.1V.This is the typical configuration for an LT6100, monitor-ing the load current of a battery. The circuit is powered Battery Current Monitorfrom a low-voltage supply rail rather than the battery be- ILing monitored. A unique benefit of this configuration is CHARGE RSENSE 0.1Ωthat when the LT6100 is powered down, its battery senseinputs remain high impedance, drawing less than 1uA of DISCHARGE 5V 12Vcurrent. This is due to an implementation of Linear Tech- – RA RA –nology’s Over-The-Top® input technique at its front end. A2 A1 1/2 LT1495 RA RA 1/2 LT1495 + + 2N3904 2N3904 DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSE RB RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05 One LT1495 dual op-amp package can be used to estab- lish separate charge and discharge current monitoring outputs. The LT1495 features Over-the-Top operation allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage. Batteries-1
  • 57. APPLICATION NOTE 105: Current Sense Circuit CollectionInput Current Sensing Application Coulomb Counter 5V CHARGER + C1 RSENSE22µF 1µF + LOAD RP1 3k C2 1 8 1µF SENSE AVG 1% 4.7µF 2 7 IOUT PROG RL RL RP2 3 LT1620MS8 6 12k SENSE – SENSE + VDD GND VCC 1% CF+ INT 4.7µF 4 5 LTC4150 CLR CHG µP IN – IN+ CF– DISCHG R1 POL 0.033Ω SHDN TO GND + SYSTEM LOAD 4150 TA01a 22µF L1B 10µH MBRS340 The LTC4150 is a micropower high-side sense circuit that VBATT = 12.3V 7 VIN VSW 5 includes a V/F function. Voltage across the sense resistor 4.7µF L1A 57k is cyclically integrated and reset to provide digital transi- LT1513 10µH + 6 2 22µF Li-ION tions that represent charge flow to or from the battery. A RUN S/S VFB ×2 4 GND IFB 3 24Ω 6.4k polarity bit indicates the direction of the current. Supply GND potential for the LTC4150 is 2.7V to 8.5V. In the free- TAB VC 8 1 0.22µF RSENSE running mode (as shown, with CLR & INT connected 0.1Ω 0.1µF together) the pulses are approximately 1µs wide and X7R around 1Hz full-scale. 1620/21 • F04 Li-Ion Gas GaugeThe LT1620 is coupled with an LT1513 SEPIC batterycharger IC to create an input over current protected POWER-DOWN SWITCHcharger circuit. The programming voltage (VCC – VPROG) 2.5V CL LOAD 47µFis set to 1.0V through a resistor divider (RP1 and RP2) RL RLfrom the 5V input supply to ground. In this configuration, 1 10 3k 3k SENSE +if the input current drawn by the battery charger com- RSENSE INT LTC4150 CLR 9 0.1Ω 8bined with the system load requirements exceeds a cur- 2 3 SENSE – VDD C2 2-CELL + 4.7µF C F+ 7rent limit threshold of 3A, the battery charger current will Li-Ion 6V ~ 8.4V CF GND µPbe reduced by the LT1620 such that the total input supply 4.7µF 4 C F– 5 6current is limited to 3A. SHDN POL SHUTDOWN This is the same as the Coulomb Counter circuit, except that the microprocessor clears the integration cycle complete condition with software, so that a relatively slow polling routine may be used.Batteries-2
  • 58. APPLICATION NOTE 105: Current Sense Circuit CollectionNiMH Charger Q3 INPUT SWITCH DCIN 0V TO 20V R8 C1 147k 0.1µF 0.25% VLOGIC BATMON DCIN RCL R11 R12 C4 VFB INFET 0.02Ω 100k 100k 0.1µF 1% SYSTEM ICL ICL LTC4008 CLP R1 5.1k 1% LOAD C2 ACP ACP/SHDN CLN RSENSE 20µF L1 0.025Ω FAULT FAULT TGATE Q1 10µH 1% FLAG FLAG BGATE NiMH R10 32.4k 1% BATTERY NTC PGND Q2 D1 C3 PACK 20µF RT CSP ITH BAT R4 3.01k 1% R9 R7 GND PROG R5 3.01k 1% C7 13.3k 6.04k CHARGING 0.47µF 0.25% 1% CURRENT C5 MONITOR THERMISTOR RT 0.0047µF D1: MBRS130T3 C6 10k 150k R6 Q1: Si4431ADY 0.12µF NTC 26.7k Q2: FDC645N 1% 4008 TA02The LTC4008 is a complete NiMH battery pack controller. connected the battery pack is always kept charged andIt provides automatic switchover to battery power when ready for duty.the external DC power source is removed. When power isSingle Cell Li-Ion Charger Li-Ion Charger VIN 800mA (WALL) 5V TO 22V WALL LTC4076 500mA (USB) ADAPTER DCIN BAT USB USBIN HPWR + 4.2V PORT 1µF SINGLE CELL IUSB 0.1µF 10µF Li-Ion BATTERY VCC 2k BAT 1µF IDC ITERM GATE 1% 1.24k GND 1k 2k 1% 1% LTC4002ES8-4.2CHARGE 6.8µH 4076 TA01STATUS CHRG SENSE Just a few external components are required for this sin- 68mΩ gle Li-Ion cell charger. Power for the charger can come COMP BAT NTC GND from a wall adapter or a computer’s USB port. 0.47µF + Li-Ion 22µF BATTERY 2.2k 10k T 4002 TA01 NTC NTC: DALE NTHS-1206N02Controlling the current flow in Lithium-Ion battery charg-ers is essential for safety and extending useful batterylife. Intelligent battery charger ICs can be used in fairlysimple circuits to monitor and control current, voltageand even battery pack temperature for fast and safecharging. Batteries-3
  • 59. APPLICATION NOTE 105: Current Sense Circuit CollectionBattery Monitor RS RA 0.2Ω 2k Q1 CHARGER VOLTAGE + 2N3904 A IBATT RA 1/4 LT1491 – 2k C – 1/4 LT1491 LOGIC + RB 2k Q2 + 2N3904 LOGIC HIGH (5V) = CHARGING B LOGIC LOW (0V) = DISCHARGING RB 1/4 LT1491 2k LOAD – + + D RG VOUT 1/4 LT1491 10k VBATT = 12V – S1 10k 90.9k VOUT V S1 = OPEN, GAIN = 1 RA = R B IBATT = = OUT AMPS (RS)(RG /RA)(GAIN) GAIN S1 = CLOSED, GAIN = 10 VS = 5V, 0V 1490/91 TA01Op-amp sections A & B form classical high-side sense cating whether the current is a charge or discharge flow.circuits in conjunction with Q1 & Q2 respectively. Each S1 sets the section D buffer op-amp gain to +1 or +10.section handles a different polarity of battery current flow Rail-to-Rail op-amps are required in this circuit, such asand delivers metered current to load resistor RG. Section the LT1491 quad in the example.C operates as a comparator to provide a logic signal indi-Batteries-4
  • 60. APPLICATION NOTE 105: Current Sense Circuit CollectionHigh SpeedCurrent monitoring is not normally a particularly high To see other chapters in this Application Note, return tospeed requirement unless excessive current flow is the Introduction.caused by a fault of some sort. The use of fast amplifiersin conventional current sense circuits is usually sufficientto obtain the response time desired.Fast Compact –48V Current Sense VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A ACCURACY ≈ 3% VOUT Q1 R1 1k FMMT493 4.7k 1% VS = 3V 30.1Ω 1% – 3.3k R1 REDUCES Q1 DISSIPATION 0805 LT1797 ×3 + 0.1µF SETTLES TO 1% IN 2µs, BZX84C6V8 1V OUTPUT STEP VZ = 6.8V 0.003Ω 1% 3W –48V SUPPLY –48V LOAD (–42V TO –56V) – + 1797 TA01 ISENSEThis amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-mentary implementation to the classic high-side configu- age is referenced to a positive potential and movesration. The op amp used must support common-mode downward when representing increasing –48V loading.operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors usedlocal supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.sistor provides metered current to an output load resis- High Speed-1
  • 61. APPLICATION NOTE 105: Current Sense Circuit CollectionConventional H-Bridge Current Monitor Single Supply 2.5V Bidirectional Operation with BATTERY BUS External Voltage Reference and I/V Converter + ISENSE TO RSENSE CHARGER/ LOAD C1 2.5V + VSENSE(MAX) 1 8 1µF FIL– FIL+ – LT1787 RS + 2 VS VS+ 7 DIFF 2.5V 3 VBIAS 6 AMP DNC IM – ROUT C3 4 5 1000pF VEE VOUT – A1 VOUT A 2.5V + LT1495 1M DN374 F03 5% LT1389-1.25 1787 F07Many of the newer electric drive functions, such as steer-ing assist, are bidirectional in nature. These functions aregenerally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-railwidth-modulation (PWM) methods to vary the com- op-amp configured as an I/V converter. This configura-manded torque. In these systems, there are two main tion is ideal for monitoring very low voltage supplies. Thepurposes for current monitoring. One is to monitor the LT1787’s VOUT pin is held equal to the reference voltagecurrent in the load, to track its performance against the appearing at the op amp’s non-inverting input. This al-desired command (i.e., closed-loop servo law), and an- lows one to monitor supply voltages as low as 2.5V. Theother is for fault detection and protection features. op-amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op ampA common monitoring approach in these systems is to may drive following circuitry more effectively than theamplify the voltage on a “flying” sense resistor, as high output impedance of the LT1787. The I/V convertershown. Unfortunately, several potentially hazardous fault configuration also works well with split supply voltages.scenarios go undetected, such as a simple short toground at a motor terminal. Another complication is the Battery Current Monitornoise introduced by the PWM activity. While the PWM IL RSENSE CHARGEnoise may be filtered for purposes of the servo law, in- 0.1Ωformation useful for protection becomes obscured. The DISCHARGE 5V 12Vbest solution is to simply provide two circuits that indi- RA RAvidually protect each half-bridge and report the bidirec- – – A2 A1tional load current. In some cases, a smart MOSFET 1/2 LT1495 RA RA 1/2 LT1495 + +bridge driver may already include sense resistors andoffer the protection features needed. In these situations, 2N3904 2N3904the best solution is the one that derives the load informa-tion with the least additional circuitry. DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSE RB RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05 One LT1495 dual op-amp package can be used to estab- lish separate charge and discharge current monitoring outputs. The LT1495 features Over-the-Top operation allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage.High Speed-2
  • 62. APPLICATION NOTE 105: Current Sense Circuit CollectionFast Current Sense with Alarm Fast Differential Current Source 15V R* 2 10pF R* VIN2 – VIN1 7 VIN1 – IOUT = R 6 LT1022 R* 3 VIN2 + 4 R* –15V IOUT RL *MATCH TO 0.01% FULL-SCALE POWER BANDWIDTH = 1MHz FOR IOUTR = 8VP-P = 400kHz FOR IOUTR = 20VP-P MAXIMUM IOUT = 10mAP-P IOUTP-P • RL COMMON-MODE VOLTAGE AT LT1022 INPUT =The LT1995 is shown as a simple unity gain difference 2amplifier. When biased with split supplies the input cur- LT1022 • TA07rent can flow in either direction providing an output volt- This is a variation on the Howland configuration, whereage of 100mV per Amp from the voltage across the load current actually passes through a feedback resistor100mΩ sense resistor. With 32MHz of bandwidth and as an implicit sense resistance. Since the effective sense1000V/usec slew rate the response of this sense ampli- resistance is relatively large, this topology is appropriatefier is fast. Adding a simple comparator with a built in for producing small controlled currents.reference voltage circuit such as the LT6700-3 can beused to generate an over-current flag. With the 400mVreference the flag occurs at 4A. High Speed-3
  • 63. APPLICATION NOTE 105: Current Sense Circuit CollectionFault SensingThe lack of current flow or the dramatic increase of cur- Schottky Prevents Damage During Supply Reversalrent flow very often indicates a system fault. In these cir- RSENSEcuits it is important to not only detect the condition, but R1also ensure the safe operation of the detection circuitry 100itself. System faults can be destructive in many unpre- 4 3dictable ways. L 2 + – 5 O A D VBATTTo see other chapters in this Application Note, return tothe Introduction. LTC6101 1High Side Current Sense and Fuse Monitor D1 R2 4.99k RSENSE 6101 F07 TO LOAD 2mΩ FUSE BATTERY BUS 1 8 + The LTC6101 is not protected internally from external VS– VS+ ADC 2 7 reversal of supply polarity. To prevent damage that may POWER VCC A4 ≥2.7V C2 occur during this condition, a Schottky diode should be – added in series with V–. This will limit the reverse current + 0.1µF 3 FIL A2 6 through the LTC6101. Note that this diode will limit the low voltage performance of the LTC6101 by effectively 4 OUT 5 OUTPUT reducing the supply voltage to the part by VD. VEE 2.5V = 25A LT6100 DN374 F02 Additional Resistor R3 Protects Output During Supply ReversalThe LT6100 can be used as a combination current sensor RSENSEand fuse monitor. This part includes on-chip output buff-ering and was designed to operate with the low supply R1 VBATT 100voltage (≥2.7V), typical of vehicle data acquisition sys- 4 3tems, while the sense inputs monitor signals at the L 2 + – 5higher battery bus potential. The LT6100 inputs are toler- O Aant of large input differentials, thus allowing the blown- D R3fuse operating condition (this would be detected by an LTC6101 1 1k ADCoutput full-scale indication). The LT6100 can also be D1 R2powered down while maintaining high impedance sense 4.99k 6101 F08inputs, drawing less than 1µA max from the battery bus. If the output of the LTC6101 is wired to an independently powered device that will effectively short the output to another rail or ground (such as through an ESD protec- tion clamp) during a reverse supply condition, the LTC6101’s output should be connected through a resistor or Schottky diode to prevent excessive fault current. Fault Sensing-1
  • 64. APPLICATION NOTE 105: Current Sense Circuit CollectionElectronic Circuit Breaker 1.25V Electronic Circuit Breaker SI4864DY VIN VOUT 0.033Ω Si9434DY 5V AT 1A 1.25V 1.25V5V PROTECTED 3.5A 0.1µF 1k VBIAS SENSEP GATE SENSEN FAULT VCC VBIAS 2.3V TO 6V CDELAY LTC4213 10k 100Ω 1 8 1N4148 OFF ON ON GND ISEL READY 33k SENSE AVG 2 7 2N3904 IOUT PROG 100k 3 LT1620MS8 6 4.7k 33k 4213 TA01 GND VCC 4 –IN +IN 5 The LTC4213 provides protection and automatic circuit breaker action by sensing Drain-to-Source voltage-drop TYPICAL DC TRIP AT 1.6A 2N3904 3A FAULT TRIPS LT1620/21 • TA03 across the NMOSFET. The sense inputs have a Rail-to- IN 2ms WITH CDELAY = 1.0µF Rail common mode range, so the circuit breaker can pro-The LT1620l current sense amplifier is used to detect an tect bus voltages from 0V up to 6V. Logic signals flag aover-current condition and shut off a P-MOSFET load trip condition (with the READY output signal) and reini-switch. A fault flag is produced in the over-current condi- tialize the breaker (using the ON input). The ON inputtion and a self-reset sequence is initiated. may also be used as a command in a “smart switch” ap- plication.Electronic Circuit Breaker Lamp Outage Detector ON/OFF IN VS CT CD RD *RSEN 5V TO 44V 3V 0.22µF 0.01µF 100k 0.1Ω 1M CT DS Z5U LTC1153 LAMP 100k ON/OFF TO µP STATUS G IRLR024 5k – 51k 51k 0.5Ω LT1637 OUT GND SHUTDOWN 5V + **70°C SENSITIVE PTC 5V LOAD OUT = 0V FOR GOOD BULB 3V FOR OPEN BULB ALL COMPONENTS SHOWN ARE SURFACE MOUNT. 1637 TA05 * IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700 ** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591 LTC1153 • TA01 In this circuit, the lamp is monitored in both the on and off condition for continuity. In the off condition, the fila-The LTC1153 is an Electronic Circuit Breaker. Sensed cur- ment pull-down action creates a small test current in therent to a load opens the breaker when 100mV is devel- 5kΩ that is detected to indicate a good lamp. If the lampoped between the supply input, Vs, and the Drain Sense is open, the 100kΩ pull-up, or the relay contact, providespin, DS. To avoid transient, or nuisance trips of the break the op-amp bias current through the 5kΩ, that is oppo-components RD and CD delay the action for 1msec. A site in polarity. When the lamp is powered and filamentthermistor can also be used to bias the Shutdown input current is flowing, the drop in the 0.05Ω sense resistorto monitor heat generated in the load and remove power will exceed that of the 5kΩ and a lamp-good detectionshould the temperature exceed 70°C in this example. A will still occur. This circuit requires particular Over-the-feature of the LTC1153 is timed Automatic Reset which Top input characteristics for the op-amp, so part substi-will try to re-connect the load after 200msec using the tutions are discouraged (however, this same circuit also0.22µF timer capacitor shown. works properly with an LT1716 comparator, also an Over- the-Top part).Fault Sensing-2
  • 65. APPLICATION NOTE 105: Current Sense Circuit CollectionSimple Telecom Power Supply Fuse Monitor 47k –48V 5V RETURN FUSE STATUS R1 R2 100k 100k 3 MOC207 SUPPLY A SUPPLY B RTN 47k VA VB STATUS STATUS 1 4 5V VA OUT F OK OK 0 0 SUPPLY A OK UV OR OV 0 1 8 STATUS UV OR OV OK 1 0 VB UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION 2 MOC207 OV: OVERVOLTAGE FUSE A 47k UV: UNDERVOLTAGE 5V 7 5 VFUSE A VFUSE B FUSE STATUS FUSE B OUT A SUPPLY B STATUS = VA = VB 0 = VA ≠ VB 1 ≠ VA = VB 1 MOC207 ≠ VA ≠ VB 1* 6 OUT B 0: LED/PHOTODIODE ON R3 47k 1: LED/PHOTODIODE OFF F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, SUPPLY A 1/4W –48V OUT ALL STATUS OUTPUTS WILL BE HIGH –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48VThe LTC1921 provides an all-in-one telecom fuse and status flags are generated that indicate the condition ofsupply-voltage monitoring function. Three opto-isolated the supplies and the fuses.Conventional H-Bridge Current Monitor A common monitoring approach in these systems is to BATTERY BUS amplify the voltage on a “flying” sense resistor, as + shown. Unfortunately, several potentially hazardous fault scenarios go undetected, such as a simple short to ground at a motor terminal. Another complication is the noise introduced by the PWM activity. While the PWM noise may be filtered for purposes of the servo law, in- RS + DIFF formation useful for protection becomes obscured. The AMP best solution is to simply provide two circuits that indi- IM – vidually protect each half-bridge and report the bidirec- tional load current. In some cases, a smart MOSFET bridge driver may already include sense resistors and offer the protection features needed. In these situations, DN374 F03 the best solution is the one that derives the load informa- tion with the least additional circuitry.Many of the newer electric drive functions, such as steer-ing assist, are bidirectional in nature. These functions aregenerally driven by H-bridge MOSFET arrays using pulse-width-modulation (PWM) methods to vary the com-manded torque. In these systems, there are two mainpurposes for current monitoring. One is to monitor thecurrent in the load, to track its performance against thedesired command (i.e., closed-loop servo law), and an-other is for fault detection and protection features. Fault Sensing-3
  • 66. APPLICATION NOTE 105: Current Sense Circuit CollectionSingle Supply 2.5V Bidirectional Operation with Fast Current Sense with AlarmExternal Voltage Reference and I/V Converter ISENSE TO RSENSECHARGER/ LOAD C1 2.5V + VSENSE(MAX) 1 8 1µF FIL– FIL+ – LT1787 2 VS VS+ 7 2.5V 3 VBIAS 6 DNC C3 ROUT 4 5 1000pF VEE VOUT – A1 VOUT A 2.5V + 1M LT1495 The LT1995 is shown as a simple unity gain difference 5% LT1389-1.25 amplifier. When biased with split supplies the input cur- 1787 F07 rent can flow in either direction providing an output volt- age of 100mV per Amp from the voltage across theThe LT1787’s output is buffered by an LT1495 rail-to-rail 100mΩ sense resistor. With 32MHz of bandwidth andop-amp configured as an I/V converter. This configura- 1000V/usec slew rate the response of this sense ampli-tion is ideal for monitoring very low voltage supplies. The fier is fast. Adding a simple comparator with a built inLT1787’s VOUT pin is held equal to the reference voltage reference voltage circuit such as the LT6700-3 can beappearing at the op amp’s non-inverting input. This al- used to generate an over-current flag. With the 400mVlows one to monitor supply voltages as low as 2.5V. The reference the flag occurs at 4A.op-amp’s output may swing from ground to its positivesupply voltage. The low impedance output of the op ampmay drive following circuitry more effectively than thehigh output impedance of the LT1787. The I/V converterconfiguration also works well with split supply voltages.Battery Current Monitor IL RSENSE CHARGE 0.1Ω DISCHARGE 5V 12V – RA RA – A2 A1 1/2 LT1495 RA RA 1/2 LT1495 + + 2N3904 2N3904 DISCHARGE OUT CHARGE OUT VO = IL () RB RA RSENSE RB RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05One LT1495 dual op-amp package can be used to estab-lish separate charge and discharge current monitoringoutputs. The LT1495 features Over-the-Top operationallowing the battery potential to be as high as 36V withonly a 5V amplifier supply voltage.Fault Sensing-4
  • 67. APPLICATION NOTE 105: Current Sense Circuit CollectionDigitizingIn many systems the analog voltage quantity indicating To see other chapters in this Application Note, return tocurrent flow must be input to a system controller. In this the Introduction.chapter several examples of the direct interface of a cur-rent sense amplifier to an A to D converter are shown.Sensing Output Current VCC0V TO 1V 12V VCSRC VCSNK EN +IN VCC V+ ISRC ISNK RS TSD 0.2Ω LT1970 OUT SENSE+ SENSE– RLOAD FILTER –IN V– VEE COMMON LT1787 R4 255k –12V VS– VS+ RG RF BIAS –12V 12V R1 20k 60.4k – VEE VOUT R2 LT1880 2.5V 10k + ±5mV/mA R3 1kHz FULL CURRENT –12V 20k –12V BANDWIDTH 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACKThe LT1970 is a 500mA power amplifier with voltage in a microprocessor controlled system. For closed loopprogrammable output current limit. Separate DC voltage control of the current to a load an LT1787 can monitorinputs and an output current sensing resistor control the the output current. The LT1880 op amp provides scalingmaximum sourcing and sinking current values. These and level shifting of the voltage applied to an A-to-Dcontrol voltages could be provided by a D-to-A Converter Converter for a 5mV/mA feedback signal. Digitizing-1
  • 68. APPLICATION NOTE 105: Current Sense Circuit CollectionSplit or Single Supply Operation, Bidirectional Output into A/D 1Ω 1% IS = ±125mA VCC 5V VSRCE 1 8 FIL– FIL+ ≈4.75V LT1787 – VS+ 7 10µF 2 VS 16V 3 VBIAS 6 1 DNC 7 20k CONV VEE 4 5 VOUT (±1V) 2 6 CLOCKING VEE AIN LTC1404 CLK –5V VOUT 3 CIRCUITRY OPTIONAL SINGLE VREF 5 DOUT SUPPLY OPERATION: 10µF GND DISCONNECT VBIAS 16V 4 8 FROM GROUND AND CONNECT IT TO VREF. 10µF DOUT REPLACE –5V SUPPLY 16V WITH GROUND. VEE 1787 TA02 OUTPUT CODE FOR ZERO –5V CURRENT WILL BE ~2430In this circuit, split supply operation is used on both the LT1787 pin 6 is driven by VREF, the bidirectional meas-LT1787 and LT1404 to provide a symmetric bidirectional urement range is slightly asymmetric due to VREF beingmeasurement. In the single-supply case, where the somewhat greater than mid-span of the ADC input range.16-Bit Resolution Unidirectional 12-Bit Resolution UnidirectionalOutput into LTC2433 ADC Output into LTC1286 ADC RSENSE TO I = 100A 0.0016Ω LOAD 1 8 2.5V TO 60V FIL– FIL+ – LT1787HV + 2 VS VS 7 R1 C1 5V VBIAS 6 15k 1µF 3 DNC ROUT 20k VREF VCC 4 5 CS VEE +IN VOUT LTC1286 CLK TO µP –IN D GND OUT C2 1787 TA01 0.1µF LT1634-1.25 VOUT = VBIAS + (8 • ILOAD • RSENSE) While the LT1787 is able to provide a bidirectional out-The LTC2433-1 can accurately digitize signal with source put, in this application the economical LTC1286 is usedimpedances up to 5kΩ. This LTC6101 current sense cir- to digitize a unidirectional measurement. The LT1787 hascuit uses a 4.99kΩ output resistance to meet this re- a nominal gain of eight, providing a 1.25V full-scale out-quirement, thus no additional buffering is necessary. put at approximately 100A of load current.Digitizing-2
  • 69. APPLICATION NOTE 105: Current Sense Circuit CollectionCurrent ControlThis chapter collects a variety of techniques useful in To see other chapters in this Application Note, return togenerating controlled levels of current in circuits. the Introduction.800 mA/1A White LED Current Regulator D2 LED WARNING! VERY BRIGHT DO NOT OBSERVE DIRECTLY L1 LED 3µH CURRENT D1 0.030Ω LT6100 B130 VS+ VS– VIN 3.3V TO 4.2V VIN VSW VCC SINGLE Li-Ion LT3436 22µF – + SHDN FB 16V LED 124k CER ON GND VC 1210 VOUT VEE A4 A2 4.7µF MMBT2222 6.3V 8.2k 0.1µF OPEN: 1A CER 4.99k CLOSED: 800mA 6100 TA02 D1: DIODES INC. D2: LUMILEDS LXML-PW09 WHITE EMITTER L1: SUMIDA CDRH6D28-3R0The LT6100 is configured for a gain of either 40V/V or ered. The LT3436 is a boost switching regulator which50V/V depending on whether the switch between A2 and governs the voltage/current supplied to the LED. TheVEE is closed or not. When the switch is open (LT6100 switch “LED ON” connected to the SHDN pin allows forgain of 40V/V), 1A is delivered to the LED. When the external control of the ON/OFF state of the LED.switch is closed (LT6100 gain of 50V/V), 800mA is deliv-Bidirectional Current Source Two Terminal Current Regulator +V 3 7 VCTL + 6 LT1990 2 – 4 RSENSE 1 REF –V ILOAD ILOAD = VCTL/RSENSE ≤ 5mA EXAMPLE: FOR RSENSE =100Ω, OUTPUT IS 1mA PER 100mV INPUT 1990 AI03The LT1990 is a differential amplifier with integrated pre-cision resistors. The circuit shown is the classic Howland The LT1635 combines an op amp with a 200mV refer-current source, implemented by simply adding a sense ence. Scaling this reference voltage to a potential acrossresistor. resistor R3 forces a controlled amount of current to flow from the +terminal to the –terminal. Power is taken from the loop. Current Control-1
  • 70. APPLICATION NOTE 105: Current Sense Circuit CollectionVariable Current Source Precision Voltage Controlled Current SourceA basic high-side current source is implemented at theoutput, while an input translation amplifier section pro-vides for flexible input scaling. A Rail-to-Rail input capa-bility is required to have both amplifiers in one package, The ultra-precise LTC2053 instrumentation amplifier issince the input stage has common-mode near ground configured to servo the voltage drop on sense resistor Rand the second section operates near VCC. to match the command VC. The LTC2053 output capabil- ity limits this basic configuration to low current applica-Precision Voltage Controlled Current tions.Source with Ground Referred Input and Output 5V Switchable Precision Current Source INPUT 3 5 0V TO 3.7V + 1 4V TO 44V LTC2050 + 4 4.7µF LT1004-1.2 – 2 2k R 0.68µF R* 5V + 1k 3 LT1637 TP0610 1/2 LTC6943 7 6 – IOUT = 1.2 R 9 IOUT e.g., 10mA = 120Ω 1µF 1µF 1k 10 SHDN *OPTIONAL FOR LOW OUTPUT CURRENTS, 1637 TA01 R* = R 12 11 VIN IOUT = 15 14 1000Ω This is a simple current-source configuration where the 0.001µF OPERATES FROM A op amp servos to establish a match between the drop on SINGLE 5V SUPPLY 6943 • TA01a the sense resistor and that of the 1.2V reference. ThisThe LTC6943 is used to accurately sample the voltage particular op amp includes a shutdown feature so theacross the 1kΩ sense resistor and translate it to a current source function can be switched off with a logicground reference by charge balancing in the 1µF capaci- command. The 2kΩ pull-up resistor assures the outputtors. The LTC2050 integrates the difference between the MOSFET is off when the op amp is in shutdown mode.sense voltage and the input command voltage to drivethe proper current into load.Current Control-2
  • 71. APPLICATION NOTE 105: Current Sense Circuit CollectionBoosted Bidirectional Controlled Current Source Fast Differential Current Source +V 15V R* 2 10pF R* VIN2 – VIN1 7 1k VIN1 – IOUT = R CZT751 6 LT1022 R* 3 3 7 VIN2 + VCTL + 4 R* 6 LT1990 –15V IOUT RL 2 + – 4 10µF RSENSE 1 *MATCH TO 0.01% REF ILOAD FULL-SCALE POWER BANDWIDTH = 1MHz FOR IOUTR = 8VP-P 1k = 400kHz FOR IOUTR = 20VP-P CZT651 MAXIMUM IOUT = 10mAP-P IOUTP-P • RL COMMON-MODE VOLTAGE AT LT1022 INPUT = 2 –V LT1022 • TA07 ILOAD = VCTL/RSENSE ≤ 100mA EXAMPLE: FOR RSENSE =10Ω, OUTPUT IS 1mA PER 10mV INPUT 1990 AI04 This is a variation on the Howland configuration, where load current actually passes through a feedback resistorThis is a classical Howland bidirectional current source as an implicit sense resistance. Since the effective senseimplemented with an LT1990 integrated difference ampli- resistance is relatively large, this topology is appropriatefier. The op amp circuit servos to match the RSENSE for producing small controlled currents.voltage drop to the input command VCTL. When the loadcurrent exceeds about 0.7mA in either direction, one of 1A Voltage-Controlled Current Sinkthe boost transistors will start conducting to provide theadditional commanded current.0A to 2A Current Source This is a simple controlled current sink, where the op amp drives the NMOSFET gate to develop a match be- tween the 1Ω sense resistor drop and the VIN current command. Since the common-mode voltage seen by theThe LT1995 amplifies the sense resistor drop by 5V/V op amp is near ground potential, a “single-supply” orand subtracts that from VIN, providing an error signal to Rail-to-Rail type is required in this application.an LT1880 integrator. The integrated error drives thePMOSFET as required to deliver the commanded current. Current Control-3
  • 72. APPLICATION NOTE 105: Current Sense Circuit CollectionVoltage Controlled Current Source Adjustable High-Side Current Source V+ VCC RSENSE 5V 0.2Ω 1k 2.5k 0.0033µF LT1004-1.2 – 100Ω Q1 1/2 LT1366 MTP23P06 – RP RS 10k + 1Ω ILOAD + 40k +IN 5V < VCC < 30V LTC6101 0A < ILOAD < 1A AT VCC = 5V 0mA < ILOAD < 160mA AT VCC = 30V FOR VIN = 0V TO 5V, IOUT = 500mA TO 0mA Q2 2N4340 IOUT = 100mA/V LT1366 F07 10µF + RLOAD The wide-compliance current source shown takes advan- tage of the LT1366’s ability to measure small signals near – 0.2V REF 1k 24k the positive supply rail. The LT1366 adjusts Q1’s gate LT3021 VIN voltage to force the voltage across the sense resistor (RSENSE) to equal the voltage between VDC and the poten- tiometer’s wiper. A rail-to-rail op amp is needed becauseAdding a current sense amplifier in the feedback loop of the voltage across the sense resistor is nearly the samean adjustable low dropout voltage regulator creates a as VDC. Q2 acts as a constant current sink to minimizesimple voltage controlled current source. The range of error in the reference voltage when the supply voltageoutput current sourced by the circuit is set only by the varies. At low input voltage, circuit operation is limited bycurrent capability of the voltage regulator. The current the Q1 gate drive requirement. At high input voltage, cir-sense amplifier senses the output current and feeds back cuit operation is limited by the LT1366’s absolute maxi-a current to the summing junction of the regulator’s error mum ratings.amplifier. The regulator will then source whatever currentis necessary to maintain the internal reference voltage atthe summing junction. For the circuit shown a 0V to 5Vcontrol input produces 500mA to 0mA of output current.Current Control-4
  • 73. APPLICATION NOTE 105: Current Sense Circuit CollectionProgrammable Constant Current Source 6V D45VH10 0.1Ω IOUT TO 28V 0A TO 1A 0.1µF 470Ω LT1121CS8-5 8 1 IN OUT + SHDN GND 1µF 0.1µF 0.1µF 10k 5 3 18k 0.1µF 1 8 1% SENSE AVG SHUTDOWN 2 7 VN2222LM IOUT PROG 3 LT1620MS8 6 2N3904 GND VCC IPROG RPROG 22Ω 4 5 –IN +IN IOUT = (IPROG)(10,000) RPROG = 40k FOR 1A OUTPUT LT1620/21 • TA01The current output can be controlled by a variable resis- shutdown command to the LT1121 powers down thetor (RPROG) connected from the PROG pin to ground on LT1620 and eliminates the base-drive to the current regu-the LT1620. The LT1121 is a low-dropout regulator that lation pass transistor, thereby turning off IOUT.keeps the voltage constant for the LT1620. Applying aSnap Back Current Limiting 12V R2 R1 R3 39.2k 54.9k 2.55k VCSRC VCSNK EN 500mA IMAX VIN +IN VCC V+ ISRC 50mA ILOW ISNK RS 1Ω IOUT 0 TSD LT1970 OUT SENSE+ – –500mA SENSE RL FILTER – VCC • R2 –IN V VEE IMAX ≈ (R1 + R2) • 10 • RS COMMON VCC • (R2||R3) ILOW ≈ [R1 + (R2||R3)] • 10 • RS RG –12V RF 10k 10k 1970 F04The LT1970 provides current detection and limiting fea- command to a lower level. When the load condition per-tures built-in. In this circuit, the logic flags that are pro- mits the current to drop below the limiting level, then theduced in a current-limiting event are connected in a feed- flags clear and full current drive capability is restoredback arrangement that in turn reduces the current limit automatically. Current Control-5
  • 74. APPLICATION NOTE 105: Current Sense Circuit CollectionPrecisionOffset voltage and bias current are the primary sources High Side Power Supply Current Senseof error in current sensing applications. To maintain pre-cision operation the use of zero-drift amplifier virtuallyeliminates the offset error terms.To see other chapters in this Application Note, return tothe Introduction.Precision High Side Power Supply Current Sense 1.5mΩ VREGULATOR 2 – 8 OUT 7 100mV/A The low offset error of the LTC6800 allows for unusually LTC6800 3 + 6 10k OF LOAD CURRENT low sense resistance while retaining accuracy. 5 4 0.1µF ILOAD LOAD Second Input R Minimizes Error Due to Input Bias Current 150Ω V+ 6800 TA01 RIN–This is a low-voltage, ultra-high-precision monitor featur-ing a Zero-Drift Instrumentation Amplifier (IA) that pro- RSENSEvides Rail-to-Rail inputs and outputs. Voltage gain is set RIN+ 4 3by the feedback resistors. Accuracy of this circuit is set + –by the quality of resistors selected by the user, small- LOAD 2 5signal range is limited by VOL in single-supply operation.The voltage rating of this part restricts this solution toapplications of <5.5V. This IA is sampled, so the output isdiscontinuous with input changes, thus only suited to 1 LTC6101 VOUTvery low frequency measurements. ROUT 6101 F04 RIN+ = RIN– – RSENSE The second input resistor decreases input error due caused by the input bias current. For smaller values of RIN this may not be a significant consideration. Precision-1
  • 75. APPLICATION NOTE 105: Current Sense Circuit CollectionWide RangeTo measure current over a wide range of values requires To see other chapters in this Application Note, return togain changing in the current sense amplifier. This allows the Introduction.the use of a single value of sense resistor. The alternativeapproach is to switch values of sense resistor. Both ap-proaches are viable for wide range current sensing.Dual LTC6101’s Allow High-Low Current Ranging CMPZ4697 VLOGIC (3.3V TO 5V) 10k 7 M1 3 Si4465 + VIN 4 ILOAD RSENSE HI – 10m 8 Q1 5 CMPT5551 VOUT RSENSE LO 40.2k 6 301 100m 301 301 301 4.7k 1.74M LTC1540 4 3 4 3 2 1 HIGH 2 + – 5 2 + – 5 RANGE VIN 619k INDICATOR (ILOAD > 1.2A) 1 1 HIGH CURRENT RANGE OUT LTC6101 LTC6101 250mV/A 7.5k VLOGIC BAT54C LOW CURRENT RANGE OUT 2.5V/A R5 7.5k (VLOGIC +5V) ≤ VIN ≤ 60V 0 ≤ ILOAD ≤ 10A 6101 F03bUsing two current sense amplifiers with two values of rents, less than 1.2 Amps, than with higher currents. Asense resistors is an easy method of sensing current comparator detects higher current flow, up to 10 Amps,over a wide range. In this circuit the sensitivity and reso- and switches sensing over to the high current circuitry.lution of measurement is 10 times greater with low cur- Wide Range-1
  • 76. APPLICATION NOTE 105: Current Sense Circuit CollectionAdjust Gain Dynamically for Enhanced Range RSENSE ISENSE TO LOAD FROM SOURCE – VS+ LT6100 VS – + 5V VCC FIL VOUT VEE A2 A4 6100 TA05 2N7002 5V 0V (GAIN = 50) (GAIN = 10)Instead of having fixed gains of 10, 12.5, 20, 25, 40, and50, this circuit allows selecting between two gain set-tings. An NMOSFET switch is placed between the twogain-setting terminals (A2, A4) and ground to provideselection of gain = 10 or gain = 50, depending on thestate of the gate drive. This provides a wider currentmeasurement range than otherwise possible with just asingle sense resistor.Wide Range-2

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