APPLICATION NOTE—105                                                                                Application Note 105  ...
APPLICATION NOTE 105: Current Sense Circuit CollectionCurrent Sense BasicsThis chapter introduces the basic techniques use...
APPLICATION NOTE 105: Current Sense Circuit CollectionFULL-RANGE (HIGH AND LOW SIDE)                              SUMMARY ...
APPLICATION NOTE 105: Current Sense Circuit Collection                                                                    ...
APPLICATION NOTE 105: Current Sense Circuit Collection                                                                    ...
APPLICATION NOTE 105: Current Sense Circuit Collection                                                                    ...
APPLICATION NOTE 105: Current Sense Circuit Collection                                                                    ...
APPLICATION NOTE 105: Current Sense Circuit CollectionHigh SideThis chapter discusses solutions for high side current     ...
APPLICATION NOTE 105: Current Sense Circuit CollectionSelf-Powered High Side Current Sense                                ...
APPLICATION NOTE 105: Current Sense Circuit CollectionPrecision Current Sensing in Supply Rails                           ...
APPLICATION NOTE 105: Current Sense Circuit CollectionSimple 500V Current Monitor                                Bidirecti...
APPLICATION NOTE 105: Current Sense Circuit CollectionLTC6101 Supply Current                                           Sim...
APPLICATION NOTE 105: Current Sense Circuit CollectionHigh-Side Transimpedance Amplifier                           Intelli...
APPLICATION NOTE 105: Current Sense Circuit Collection48V Supply Current Monitor withIsolated Output and 105V Survivabilit...
APPLICATION NOTE 105: Current Sense Circuit CollectionLow SideThis chapter discusses solutions for low side current       ...
APPLICATION NOTE 105: Current Sense Circuit Collection–48V Hot Swap Controller                                GND         ...
APPLICATION NOTE 105: Current Sense Circuit CollectionFast Compact –48V Current Sense                                     ...
APPLICATION NOTE 105: Current Sense Circuit CollectionIn this circuit an economical ADC is used to acquire the            ...
APPLICATION NOTE 105: Current Sense Circuit CollectionSimple Telecom Power Supply Fuse Monitor                            ...
APPLICATION NOTE 105: Current Sense Circuit CollectionNegative VoltageThis chapter discusses solutions for negative voltag...
APPLICATION NOTE 105: Current Sense Circuit Collection–48V Low Side Precision Current SenseThe first stage amplifier is ba...
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  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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