Understanding the load-transient response of LDOs

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1、TexasInstrumentsIncorporatedPowerManagementUnderstandingtheload-transientresponseofLDOsByBrianM.KingAdvancedAnalogProductsIntroductionFigure1.TypicalLDOcomponentsLow-dropoutlinearregulators(LDOs)arecommonlyusedtoprovidepowertolow-voltagedigitalcircuits,wherepoint-of-Inputloadregulationisimportant.I

2、nPasstheseapplications,itiscommonElementforthedigitalcircuittohaveReference+severaldifferentmodesofopera-Errortion.AsthedigitalcircuitswitchesAmplifierInputOutputfromonemodeofoperationtoCapacitor–CapacitorDynamicFeedbackanother,theloaddemandontheLoadNetworkLDOcanchangequickly.Thisquickchangeofloadr

3、esultsinatemporaryglitchoftheLDOout-putvoltage.Mostdigitalcircuitsdonotreactfavorablytolargevoltagetransients.Forthedigitalcircuitdesigner,minimizinganLDO’stransientresponseisanimportanttask.LDOsareavailableinawidevarietyofoutputvoltagesandcurrentcapacities.SomeLDOcompensationLDOsaretailoredtoappli

4、cationswhereagoodresponsetoTheprimaryfeedbackloopoftheLDO,consistingoftheafasttransientisimportant.TheTPS751xx,TPS752xx,outputcapacitor,feedbacknetwork,erroramplifier,andTPS753xx,andTPS754xxfamiliesofLDOsfromTexaspasselement,determinestheLDO’sfrequencyresponse.Instrumentsareexamplesoffast-transient

5、-responseLDOs.TheunitygaincrossoverfrequencyandstabilityoftheTheTPS751xxandTPS753xxfamiliesareratedat1.5AofLDOcircuitaffecttheoveralltransientresponseoftheLDO.outputcurrent,whiletheTPS752xxandTPS754xxfami-Thecrossoverfrequencyaffectsthesettlingtimeoftheliescanprovideupto2A.AllfourfamiliesusePMOSpas

6、slinearregulatorcircuit,wherethesettlingtimeisthetimeelementstoprovidealowdropoutvoltageandlowgroundelapsedfromtheinitialonsetoftheloadtransienttothecurrent.ThesedevicescomeinaPowerPADTMpackagetimewheretheoutputvoltagereturnstowithinafewper-thatprovidesaneffectivewayofmanagingthepowercentofasteady-

7、statevalue.AhighercrossoverfrequencydissipationinaTSSOPfootprint.willdecreasethedurationofatransientcondition.InmostFigure1showsthecircuitelementsofatypicalLDOLDOs,theoutputcapacitoranditsassociatedequivale