AN-1072                                                                                                                   ...
AN-1072                                                                                                                   ...
Application Note                                                                                                          ...
AN-1072                                                                                                                App...
Application Note                                                                                                          ...
AN-1072                                                                                                                 Ap...
Application Note                                                                                                          ...
AN-1072                                                                                     Application NoteNOTES©2010 Ana...
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  1. 1. AN-1072 APPLICATION NOTEOne Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com How to Successfully Apply Low Dropout Regulators by Ken MarascoINTRODUCTIONA low dropout (LDO) regulator is capable of maintaining its As portable systems rapidly evolve, however, the integratedspecified output voltage over a wide range of load currents and PMIC cannot keep up with peripheral power requirements.input voltages, down to a very small difference between input Dedicated LDOs must be added in the later stages of systemand output voltages. This difference, known as the dropout development to power such optional items as camera modules,voltage or headroom requirement, can be as low as 80 mV at Bluetooth®, WiFi®, and other bolt-on functions. LDOs have also2 A. The adjustable output, low dropout regulator (see “Break been used to reduce noise, to solve voltage-regulation problemsLoose from Fixed IC Regulators” by Dobkin in the References caused by electromagnetic interference (EMI) and printedsection) first came to public attention in 1977. Current portable circuit board (PCB) routing, and to improve system powerdevices often require up to 20 low dropout linear regulators. efficiency by switching off unneeded functions.Many of the LDOs in today’s portable devices are integrated This application note reviews the basic LDO topology, explainsinto multifunction power management ICs (PMICs), highly key specifications, and shows the application of low dropoutintegrated systems with many power domains for audio, battery voltage regulators in systems. Examples are given using designcharging, housekeeping, lighting, communications, and other characteristics of Analog Devices, Inc., LDO families. For morefunctions. For more information on power management ICs, information on LDOs, visit www.analog.com/linear-regulators.visit www.analog.com/power-management. VBIAS VIN VOUT PASS TRANSISTOR EN 08965-001 REFERENCE Figure 1. An LDO Regulates the Output Voltage with a Low Dropout Voltage (The Difference Between VOUT and the Lowest Specified Value of VIN at the Rated Load Current) Rev. 0 | Page 1 of 8
  2. 2. AN-1072 Application NoteTABLE OF CONTENTSIntroduction ...................................................................................... 1  Dropout Voltage ............................................................................5 Basic LDO Architecture ................................................................... 3  Start-Up Time ................................................................................5  LDO Efficiency ............................................................................. 3  Current-Limit Threshold .............................................................5  Circuit Features Enhance LDO Performance ........................... 3  Operating Temperature Range ....................................................5 Understanding Linear Regulator Requirements .......................... 4  Thermal Shutdown (TSD) ...........................................................6  LDOs for Digital Loads ............................................................... 4  Enable Input ...................................................................................6  LDOs for Analog and RF Loads ................................................. 4  Undervoltage Lockout ..................................................................6 Key LDO Specifications and Definitions ...................................... 5  Output Noise ..................................................................................6  Input Voltage Range ..................................................................... 5  Power-Supply Ripple Rejection ...................................................6  Ground (Quiescent) Current ...................................................... 5  Minimum Input and Output Capacitance .................................6  Shutdown Current ........................................................................ 5  Reverse-Current Protection Feature ...........................................6  Output Voltage Accuracy ............................................................ 5  Soft Start .........................................................................................6  Line Regulation ............................................................................. 5  Conclusion..........................................................................................7  Dynamic Load Regulation .......................................................... 5  References .......................................................................................7  Rev. 0 | Page 2 of 8
  3. 3. Application Note AN-1072BASIC LDO ARCHITECTUREAn LDO consists of a voltage reference, an error amplifier, a between supply voltage and load voltage. For example, as afeedback voltage divider, and a pass transistor, as shown in lithium-ion battery drops from 4.2 V (fully charged) to 3.0 VFigure 1 (see “Ask The Applications Engineer—37, Low-Dropout (discharged), a 2.8 V LDO connected to the battery maintains aRegulators” by Patoux in the References section). Output cur- constant 2.8 V at the load (dropout voltage less than 200 mV),rent is delivered via the pass transistor. Its gate voltage is but its efficiency increases from 67% with the fully chargedcontrolled by the error amplifier, which compares the reference battery to 93% with the discharged battery.voltage with the feedback voltage, amplifying the difference to To improve efficiency, LDOs can be connected to an intermediatereduce the error voltage. If the feedback voltage is lower than voltage rail generated by a high efficiency switching regulatorthe reference voltage, the gate of the pass transistor is pulled lower, such as the ADP2108. With a 3.3 V switching regulator, forallowing more current to pass and increasing the output voltage. example, the LDO efficiency is constant at 85%, and the overallIf the feedback voltage is higher than the reference voltage, the system efficiency is 81%, assuming 95% efficiency for thegate of the pass transistor is pulled higher, restricting the switching regulator.current flow and decreasing the output voltage. CIRCUIT FEATURES ENHANCE LDOThe dynamics of this closed-loop system are based on two main PERFORMANCEpoles: the internal pole formed by the error amplifier/pass transis-tor and the external pole formed by the output impedance of An enable input permits external control of LDO turn-on andthe amplifier and the equivalent series resistance (ESR) of the turn-off, allowing supplies to be sequenced in proper order inoutput capacitor. The output capacitance and its ESR affect the multirail systems. Soft start limits inrush current and controlsloop stability and the response to transient changes in load output voltage rise time during power-up. A sleep statecurrent. An ESR of ≤1 Ω is recommended to ensure stability. minimizes power drain, especially useful in battery-basedAlso, LDOs require input and output capacitors to filter noise systems, while allowing fast turn-on. Thermal shutdown turnsand control load transients. Larger values improve the transient the LDO off if its temperature exceeds the specified value.response of the LDO, but increase the start-up time. Analog Overcurrent protection limits the LDO’s output current andDevices LDOs are designed to be stable over the specified power dissipation. Undervoltage lockout disables the outputoperating conditions when the specified capacitors are used. when the supply voltage is below the specified minimum value. Figure 2 shows a simplified typical power system for portableLDO EFFICIENCY designs.Increased efficiency is a constant demand from the design VBAT 3.0V TO 4.2V VINengineer. This translates into a reduction of the quiescent VUSB 3.3V USB ENcurrent (IQ) and forward-voltage drop. VIN VANA 2.75V ANALOG  V I  EN LDO efficiency   OUT OUT   100%  VIN I   OUT  I Q    VIN EN VCORE 1.5V COREWith IQ in the denominator, it is evident that the higher IQ is, VIN VMEM 1.8V MEMORY ENthe lower the efficiency becomes. Current LDOs have reasonablylow IQ, and for simplicity, IQ can be neglected in efficiency VIN VDIG 2.8V MODEM ENcalculations if IQ is very small compared to ILOAD. Then LDO VINefficiency is simply (VOUT/VIN) × 100%. Because the LDO has VBT 2.8V BLUETOOTH ENno way to store significant amounts of unused energy, powernot delivered to the load is dissipated as heat within the LDO. SEQUENCING Power dissipated (PD) = (VIN − VOUT) × IIN RTC 08965-002 I2 CProviding a stable power supply voltage independent of loadand line variations, changes in ambient temperature, and the Figure 2. Typical Power Domains in a Portable Systempassage of time, LDOs are most efficient with small differences Rev. 0 | Page 3 of 8
  4. 4. AN-1072 Application NoteUNDERSTANDING LINEAR REGULATOR REQUIREMENTSLDOS FOR DIGITAL LOADS voltage must not overshoot excessively. Excessive overshoot can result in system latch-up, sometimes requiring the removal ofDigital linear regulators such as the ADP170 and ADP1706 are the battery or activation of the master reset button to correctdesigned to support the main digital power supply requirements the problem and restart the system.of a system, usually microprocessor cores and system input/output(I/O) circuitry. LDOs for DSPs and microcontrollers have to LDOS FOR ANALOG AND RF LOADSwork with good efficiency and handle high and rapidly varying Low noise and high power supply ripple rejection (PSRR) ascurrents. Newer application requirements put tremendous found in the ADP121 and ADP130 are important for LDOsstrain on the digital LDO because processor cores often change used in the analog environment because analog devices areclock speed to save energy. The clock speed variation, in more sensitive to noise than digital devices. Analog LDO requi-response to software-induced loading, translates into a sites are mainly driven by the wireless interface requirements:demanding need for LDO load regulation capability. do no harm to the receiver or transmitter, and create no pop orThe important characteristics for digital loads are line and load hum in the audio system. The wireless connection is highlyregulation and transient undershoot and overshoot. When susceptible to noise, and the receiver’s effectiveness can be reducedpowering low voltage microprocessor cores, accurate output if the noise interferes with the signal. When considering an analogcontrol is very important at all times; inadequate regulation can linear regulator, it is important for the device to suppress noiseallow the core to latch up. These parameters are not always from upstream sources and downstream loads, while not addingfeatured in data sheets, and graphs of transient response may further noise itself. Analog regulator noise is measured in voltsshow optimistic rise and fall speed in response to transient rms and should be below 35 mV when used in sensitive circuits.signals. Line and load regulation can be expressed as percent PSRR, the LDOs ability to suppress upstream noise on the powerdeviation of output voltage with load change, actual V/I values, supply line should be greater than 60 dB. The ADP150 ultralowor both, at a specific load current. noise LDO with an output noise of 9 mV and a PSRR of 70 dB is an excellent choice to power sensitive analog circuits. Adding anTo save energy, digital LDOs are designed with low IQ to increase external filter or a bypass capacitor can also reduce noise, butbattery life, and portable systems have long periods of low adds cost and increases PCB solution size. Noise reduction andpower operation when the software is idling. During periods supply noise rejection can also be achieved by care andof inactivity, the system goes to sleep, requiring the LDO to shut ingenuity in the internal design of the LDO. When selectingdown and consume less than 1 μA. While the LDO is in sleep LDOs, it is important to review the product details in relation tomode, all of the circuits, including the band gap reference, are the overall performance needed for each system.switched off. When the system returns to active mode, fastturn-on times are required, during which the digital supply Rev. 0 | Page 4 of 8
  5. 5. Application Note AN-1072KEY LDO SPECIFICATIONS AND DEFINITIONSSpecifications on the front page of manufacturer data sheets are DYNAMIC LOAD REGULATIONbrief summaries, often presented in a way that emphasizes the Most LDOs can easily hold the output voltage nearly constant asattractive features of the device. The key parameters often emphas- long as the load current changes slowly. When the load currentize typical performance characteristics that can be more fully changes quickly, however, the output voltage changes. How muchunderstood only when consulting the complete specifications the output voltage changes when subjected to a change in loadand other data within the body of the data sheet. Because there current defines load transient performance.is little standardization among manufacturers in the way specifica-tions are presented, power designers need to understand the DROPOUT VOLTAGEdefinition and the methodology used to obtain key parameters Dropout refers to the smallest difference between input andlisted in the electrical specifications table. The system designer output voltages required to maintain regulation; that is, an LDOshould pay close attention to key parameters, such as ambient can hold the output load voltage constant as the input is decreasedand junction temperature ranges, X and Y scales of graphic until the input reaches the output voltage plus the dropout voltage,information, loads, rise and fall times of transient signals, and at which point the output drops out of regulation. The dropoutbandwidth. The following sections describe some important voltage should be as low as possible to minimize power dissipa-parameters relating to the characterization and application of tion and maximize efficiency. Typically, dropout is consideredAnalog Devices LDOs. to be reached when the output voltage has dropped to 100 mVINPUT VOLTAGE RANGE below its nominal value. The load current and junction tempera- ture can affect the dropout voltage. The maximum dropoutAn LDO’s input voltage range determines the lowest usable voltage should be specified over the full operating temperatureinput supply voltage. The specifications may show a wide input range and load current.voltage range, but the lowest input voltage must be greater thanthe dropout voltage plus the desired output voltage. For example, START-UP TIMEa 150 mV dropout means that the input voltage must be above Start-up time is defined as the time between the rising edge of2.95 V for a regulated 2.8 V output. If the input voltage drops the enable signal to VOUT reaching 90% of its nominal value.below 2.95 V, the output voltage drops below 2.8 V. This test is usually performed with VIN applied and the enableGROUND (QUIESCENT) CURRENT pin toggled from off to on. Note that, in some cases where the enable is connected to VIN, the start-up time can substantiallyThe quiescent current, IQ, is the difference between the input increase because the band gap reference takes time to stabilize.current, IIN, and the load current, IOUT, measured at the specified Start-up time of a regulator is an important consideration forload current. For fixed-voltage regulators, IQ is the same as the applications where the regulator is frequently turned off and onground current, IG. For adjustable-voltage regulators, such as to save power in portable systems.the ADP171, the quiescent current is the ground current minusthe current from the external resistance divider network. CURRENT-LIMIT THRESHOLDSHUTDOWN CURRENT Current-limit threshold is defined as the load current at which the output voltage drops to 90% of the specified typical value.Shutdown current is the input current consumed when the device For example, the current limit for a 3.0 V output voltage ishas been disabled. Usually below 1.0 μA for portable LDOs, this defined as the current that causes the output voltage to drop tospecification is important for battery charge life during long 90% of 3.0 V, or 2.7 V.standby times when the portable device is turned off. OPERATING TEMPERATURE RANGEOUTPUT VOLTAGE ACCURACY Operating temperature range can be specified by ambient andAnalog Devices LDOs are designed for high output voltage junction temperature. Because an LDO dissipates heat, the ICaccuracy; they are factory-trimmed to within ±1% at 25°C. always operates above the ambient. How much it operates aboveOutput voltage accuracy is specified over the operating tempera- the ambient temperature depends on the operating conditionsture, input voltage, and load current ranges; error is specified and the PCB thermal design. A maximum junction temperatureas ±x% worst case. (TJ) is specified because operation above the maximum junctionLINE REGULATION temperature for extended periods may affect device reliability,Line regulation is the change in output voltage for a change in statistically specified as mean time to failure (MTTF).the input voltage. To avoid inaccuracy due to changes in chiptemperature, the measurement is made under conditions of lowpower dissipation or by using pulse techniques. Rev. 0 | Page 5 of 8
  6. 6. AN-1072 Application NoteTHERMAL SHUTDOWN (TSD) range (1 kHz to 100 kHz). In an LDO, PSRR can be characte-Most LDOs incorporate silicon thermostats to protect the IC rized in two frequency bands. Band 1 is from dc to the controlfrom thermal runaway. They are used to power down the LDO loop’s unity-gain frequency; PSRR is set by the open-loop gainif the junction temperature exceeds the specified thermal shut- of the regulator. Band 2 is above the unity-gain frequency;down threshold. Hysteresis is required to allow the LDO to cool PSRR is unaffected by the feedback loop. Here PSRR is set bydown before restarting. TSD is important because it protects more the output capacitor and any leakage paths from the input to thethan the LDO alone; excessive heat affects more than just the output pins. Choosing a suitably high value output capacitanceregulator. Heat conducted from the LDO to the PCB (or to the typically improves PSRR in this latter band. In Band 1, the AnalogLDO from hotter elements on the board) can damage the PCB Devices proprietary circuit design reduces changes in PSRR duematerial and solder connections over time and damage nearby to input voltage and load variations. For optimum supply rejec-components, reducing the life of the device. Thermal shutdown tion, the PCB layout must be considered to reduce the leakagealso affects system reliability. Thermal design to control the board from input to output, and grounding should be robust.temperature (for example, heat sinking, cooling) is thus an MINIMUM INPUT AND OUTPUT CAPACITANCEimportant system consideration. The minimum input and output capacitance should be greaterENABLE INPUT than specified over the full range of operating conditions,LDO enables, offered in positive and negative logic, turn the especially operating voltage and temperature. The full range ofdevice on and off. Active-high logic enables the device when the operating conditions in the application must be consideredenable voltage exceeds the logic high threshold. Active-low logic during device selection to ensure that the minimum capacitanceenables the device when the enable voltage is below the logic specification is met. X7R-type and X5R-type capacitors arelow threshold. The enable input permits external control of LDO recommended; Y5V and Z5U capacitors are not recommendedturn-on and turn-off, an important feature in the sequencing of for use with any LDO.supplies in multirail systems. Some LDOs have substantially REVERSE-CURRENT PROTECTION FEATURElower start-up times because their band-gap reference is on A typical LDO with a PMOS pass transistor has an intrinsicwhile the LDO is disabled, allowing the LDO to turn on faster. body diode between VIN and VOUT. When VIN is greater thanUNDERVOLTAGE LOCKOUT VOUT, this diode is reverse-biased. If VOUT is greater than VIN, theUndervoltage lockout (UVLO) ensures that voltage is supplied intrinsic diode becomes forward-biased and conducts currentto the load only when the system input voltage is above the from VOUT to VIN, potentially causing destructive power dissipa-specified threshold. UVLO is important because it only allows tion. Some LDOs, such as the ADP1740/ADP1741 for example,the device to power on when the input voltage is at or above have additional circuitry to protect against reverse current flowwhat the device requires for stable operation. from VOUT to VIN. The reverse current protection circuitry detects when VOUT is greater than VIN and reverses the directionOUTPUT NOISE of the intrinsic diode connection, reverse-biasing the diode.The LDO internal band gap voltage reference is the source of SOFT STARTnoise, usually specified in microvolts rms over a specific band-width. For example, the ADP121 has an output noise of 40 μV rms Programmable soft start is useful for reducing inrush currentfrom 10 kHz to 100 kHz at a VOUT of 1.2 V. When comparing upon startup and for providing voltage sequencing. For applica-data sheet specifications, the specified bandwidth and operating tions that require a controlled inrush current at startup, LDOsconditions are important considerations. such as the ADP1740/ADP1741 provide a programmable soft start (SS) function. To implement soft start, a small ceramicPOWER-SUPPLY RIPPLE REJECTION capacitor is connected from SS to GND.PSRR, expressed in decibels, is a measure of how well the LDOrejects ripple from the input power supply over a wide frequency Rev. 0 | Page 6 of 8
  7. 7. Application Note AN-1072CONCLUSIONLDOs perform a vital function. Though simple in concept, there REFERENCESare many factors to consider in applying them. This application Dobkin, Robert. April 12, 1977. “Break Loose from Fixed ICnote has reviewed basic LDO topology, and explained key Regulators.” Electronic Design.specifications and application of low dropout voltage regulatorsin systems. Data sheets contain much helpful information; and Patoux, Jerome. 2007. “Ask The Applications Engineer—37,more information (selection guide, data sheets, application Low-Dropout Regulators.” Analog Dialogue 41-2, pp. 8-10.notes), as well as ways to obtain assistance, are available on theAnalog Devices power management web site. For more informa-tion, visit www.analog.com/power-management. Also availableis ADIsimPower™, the fastest, most accurate dc-to-dc powermanagement design tool (visit www.analog.com/ADIsimPower). Rev. 0 | Page 7 of 8
  8. 8. AN-1072 Application NoteNOTES©2010 Analog Devices, Inc. All rights reserved. Trademarks andregistered trademarks are the property of their respective owners. AN08965-0-4/10(0) Rev. 0 | Page 8 of 8

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