LDO Performance Near Dropout - TI.com

1 ApplicationReportSBVA029 PMOSlow- Dropout (LDO) ,theeffectonseveralkeyelectricalcharacte risticsisexamed:lineandloadregulation,po wer-supplyrejectionratio(PSRR),controllo opstabilityandtransientresponse, (OutputImpedanceIgnored).. March2013 LDOP erformanceNearDropoutSubmitDocumentation FeedbackCopyright 2013,TexasInstrumentsIncorporatedGACGgmC INVINCOZOZO_RegRAROR1R2 RESRRLoadVREFERENCEE rrorAmplifierPMOSPass FETII (dc)DOO VV(dc)DOUTOUT LDO showsthesimplifiedblockdiagramfora controllooptoprovidea dcregulatedvoltage,VOUT, attheload(RLoad) theproductofthreefactors:thevoltagedivid ergain[ = R2/ (R1+ R2)];thegainoftheErrorAmplifier(GA); andthevoltagegainofthecommon-sourceFETam plifier(GF= gm [rO|| (R1+ R2) ||ZO]),wheregmis a functionofthedesignedloopbandwidth,loopg ain,theaccuracyofthereferencevoltage,and mostsignificantly,theregionofoperationof thepassFET:thatis,thesaturationregionort helinearregionapproachingdropout(VDO).

2 OperationoftheLDOwiththeFETbiasedin theactivesaturationregionaffordsoptimala cperformanceoftheloopwhileoperationin thelinear(ohmic)region,neardropout,yield spoorerperformance, fora depictionofa setofcharacteristiccurvesfora MOSFET; ,thepassFETtransitionsfromitssaturationr egionthroughitslinearregion,beginningatP 2,tofinallyreachthedropoutvoltage(VDO) atP3wheretheFETactsonlyasa linear, (ontheright-handsideofthereddashedline), thevoltagegainofaFETamplifieris atitsmaximumasa resultofboththehightransconductancegain( gm= IDS/ VGS, shownhereasanincreasingfunctionofdrain-s ourcecurrent)andthehighoutputimpedance,r O, illustratedbytheflat, (totheleftofthereddashedline),thephysics ofthetransistorchange:theFETchangesfroma transconductancegainstagetosomethingsimi lartoa illustratedbythenarrowerspacingbetweencu rves,indicativeofverylowtransconductance ,andthewidervariationsin ,then,itsinput-outputresistance,rO, is atminimum;thegate-sourcedrivevoltageis atmaximumlimit;andtheLDOceasestoregulate theoutput(VOUT= VIN IO RDSon, whereRDSon= rOatminimum).

3 2 LDOP erformanceNearDropoutSBVA029 March2013 SubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporatedDDVVOUT IN1[g G(r + R)OLoadmAb]=DDVIOUTOUT1g Gm Ab= Current(Arbitrary Units)Drain to Source Voltage (Arbitrary Units)1 V2 V3 V4 V5 V6 VV= 7 VGSL inear Region(Ohmic)Saturation Region(Active)P1P2P3 ILIMITIRATEDV= VV = :Thefigureillustratestherelativedependen ceofthedrain-sourcecurrent,IDS, onthegate-sourcevoltage,VGS, anddrain-sourcevoltage, ,orlow-frequency,modelforanLDOcanbeunder stoodfromFigure1, IOUT, thenominaloutputvoltagethenchangesbysome VOUT value,dependingonthedcloopgainaloneassho wnbyEquation1 ( ).(1)Thelargertheproductofthetranconduct ancegain,theerroramplifiergain,andthevol tagedividergain,thesmallertheoutputerror ( VOUT) willbefora givenchangein IOUT. Astheinput-outputvoltagedecreasestowardd ropout, ,thisdecreasetendstoincreasetheoutputerr or, VOUT.

4 Forthewell-designedLDO,thegainoftheerror amplifier,GA, is sufficientlylargetoensureregulation(usua llybya widemargin)untilthedrain-sourcevoltageis ,thegatedrivereachesitsoperationallimita ndtheLDOstopsregulating; influencedbya changetotheinputvoltage,VIN. WhenthedcinputlinevoltagechangestoVIN VIN, thenVOUT againchangesbysome VOUT valueasEquation2 shows( ).(2)AstheLDOapproachesdropout,boththetr ansconductancegain(gm) andtheoutputimpedanceofthepassFET(rO) decrease,causingthe , March2013 LDOP erformanceNearDropoutSubmitDocumentation FeedbackCopyright 2013,TexasInstrumentsIncorporated6050403 020100 PSRR Magnitude [dB (Power/Hz)]1101001 k10 k100 k1 MFrequency (Hz) V(1) V1 VVat Full LoadDOTPS79501 PSRR AT VARIOUS Dropout VOLTAGESrOVINVOUT(1 + )]sPaG gAmb[G gAbmZO_Reg=rOZO_Reg, where:PSR =(Z)(r + Z)O_RegOO_Reg(1 + )sPa(1 + )]sPaG gAmb[G gAbm!

5 (PSRR)is theabilityoftheLDOcontrollooptorejectinp utripple(forexample,1 Hzto10 MHz)ontheVINrailbymaintainingtheoutput(V OUT) in a isolationfromitsoutputimpedanceasa voltagedividercircuit( ). Thischaracteristicis seenin Figure3 astheimpedancefrominputtooutput(rO) andthentheimpedanceoftheregulatedoutputt oground,ZO_Reg. ThePSRR hasbeenderivedin Equation3 forthemodelshownin Figure3 ( ). Theloopgainisbandwidth-limitedbytheinten alpole,Pa, causedbyRAandCG(refertoFigure1) oftheLDO,wherePa=1/2 RACA).(3) (OutputImpedanceIgnored)Accordingtothism odel,PSRRis entirelya functionoftheloopgainandtheoneinternalpo le, ,thePSRR decreasesdramatically(logarithmically)as a resultofthereductionin transconductancegainofthepassFET,theredu cedeffectofthegate-sourcevoltageonthedra incurrent, showsa setoftypicalPSRR plots,in thiscasefora TPS79501 LDO, toillustratethereductionin gainastheinput-outputvoltageapproachesth epointoffullloaddropout(whereVDO= 110mV).

6 ThesegraphsshowthesuggestedmodelofFigure 3 tobevalidtoabout100kHz.(1) March2013 SubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporated(a)Inpu t-Output Voltage = 1 V(c)Input-Output Voltage = V(b)Input-Output Voltage = VG (s) =GgROLAmpb 1 + sC ROESR(1 + sC R ) [1 + sC (R + R)]GOpESRA , ,however, givesthesimplifiedopen-loopgainequationf orGOL, basedonthePMOSLDOofFigure1.(4)WhereRp= rO|| (R1+ R2) || RLoadand = R2/ (R1+ R2).Decreasingtheinput-outputvoltagetoap proachdropoutreducestheloopgainandbandwi dth,andshiftsthefrequencyoftheoutputpole [FP_OUT= 1/2 CO(Rp+ RESR)].Thoughit is beyondthescopeofthisreport,it canbeshownbyFigure1 thattypically,a reductionin gainimprovesstability(thatis,phasemargin willincrease) illustratesthischangein stepresponsebyemployingtheTPS71728 LDOtodrivea steploadfrom45mAto150mAforthreecaseswher etheinput-outputvoltageis.

7 Oscilloscopechannel4 showstheloadcurrentstep,andchannel1 March2013 LDOP erformanceNearDropoutSubmitDocumentation FeedbackCopyright 2013, ,thedrivevoltagetothegateofthepassFETand variousothercircuitsis ,whenalltheseinternalcircuitsarethensatu rated,thereis oftena largerisein ,however,usebalanced,differentialdriveci rcuitrythatalwaysdrawsthesamecurrentwhet herthedeviceis optimalwhentheinput-to-outputvoltagediff erenceis greatenoughtoassurethattheFETis operatingin (VDO), thetransconductancegainoftheFETrollsoff, theoutputimpedanceoftheFETdecreases, ,thesedocumentsareavailablefordownloadfr omtheTIwebsite( ). , (1999). ,V.,Rinc n-Mora, , (2004).Analysisanddesignofmonolithic,hig hPSR, ,E.(1999).Stabilityanalysisoflow- Dropout linearregulatorswitha , March2013 SubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,enhancements,improvementsandotherchang estoitssemiconductorproductsandservicesp erJESD46,latestissue,andtodiscontinueany productorserviceperJESD48, (alsoreferredtohereinas components )

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