Low-Drop-out-Regulator

36IEEEJOURNALOFSOLID-STATECIRCUITS,VOL.33,NO.1,JANUARY1998ALow-Voltage,LowQuiescentCurrent,LowDrop-OutRegulatorGabrielA.Rincon-Mora,Member,IEEE,andPhillipE.Allen,Fellow,IEEEAbstract—Thedemandforlow-voltage,lowdrop-out(LDO)regulatorsisincreasingbecauseofthegrowingdemandforportableelectronics,i.e.,cellularphones,pagers,laptops,etc.LDO’sareusedcoherentlywithdc-dcconvertersaswellasstandaloneparts.Inpowersupplysystems,theyaretypicallycascadedontoswitchingregulatorstosuppressnoiseandprovidealownoiseoutput.Theneedforlowvoltageisinnatetoportablelowpowerdevicesandcorroboratedbylowerbreakdownvoltagesresultingfromreductionsinfeaturesize.Lowquiescentcurrentinabattery-operatedsystemisanintrinsicperformanceparameterbecauseitpartiallydeterminesbatterylife.Thispaperdiscussessometechniquesthatenablethepracticalrealizationsoflowqui-escentcurrentLDO’satlowvoltagesandinexistingtechnologies.Theproposedcircuitexploitsthefrequencyresponsedependenceonload-currenttominimizequiescentcurrentflow.Moreover,theoutputcurrentcapabilitiesofMOSpowertransistorsareenhancedanddrop-outvoltagesaredecreasedforagivendevicesize.Otherapplications,likedc-dcconverters,canalsoreapthebenefitsoftheseenhancedMOSdevices.AnLDOprototypeincorporatingtheaforementionedtechniqueswasfabricated.Thecircuitwasoperabledowntoinputvoltagesof1Vwithazero-loadquiescentcurrentflowof23A.Moreover,theregulatorprovided18and50mAofoutputcurrentatinputvoltagesof1and1.2V,respectively.IndexTerms—Lowdrop-out,low-voltageregulators,powersupplycircuits,regulators.I.INTRODUCTIONTHElowdrop-outnatureoftheregulatormakesitappro-priateforuseinmanyapplications,namely,automotive,portable,industrial,andmedicalapplications[1].Theautomo-tiveindustryrequireslowdrop-out(LDO)regulatorstopowerupdigitalcircuits,especiallyduringcold-crankconditionswherethebatteryvoltagecanbebelow6V.Theincreasingdemand,however,isespeciallyapparentinmobilebattery-operatedproducts,suchascellularphones,pagers,camerarecorders,andlaptops[2].Inacellularphone,forinstance,switchingregulatorsareusedtoboostupthevoltagebutLDO’sarecascadedinseriestosuppresstheinherentnoiseassociatedwithswitchers.LDO’sbenefitfromworkingwithlowinputvoltagesbecausepowerconsumptionisminimizedaccordingly,.Lowvoltageandlowquiescentcurrentareintrinsiccircuitcharacteristicsforincreasedbatteryefficiencyandlongevity[3].Lowvoltageoperationisalsoaconsequenceofprocesstechnology.ThisisbecauseisolationManuscriptreceivedOctober4,1996;revisedMarch30,1997.G.A.Rincon-MoraiswiththeStandardLinearDesignBranch,TexasInstrumentsIncorporated,Dallas,TX75243USA.P.E.AlleniswiththeSchoolofElectricalandComputerEngineering,GeorgiaInstituteofTechnology,Atlanta,GA30332-0250USA.PublisherItemIdentifierS0018-9200(98)00368-0.Fig.1.Typicallowdrop-outregulatortopology.barriersdecreaseasthecomponentdensitiesperunitareaincrease,therebyexhibitinglowerbreakdownvoltages[4],[5].Therefore,lowpowerandfinerlithographyrequireregulatorstooperateatlowvoltages,producepreciseoutputvoltages,andhavecharacteristicallylowerquiescentcurrentflow[5].Bytheyear2004,thepowersupplyvoltageisexpectedtobeaslowas0.9Vin0.14-mtechnologies[5],[6].Drop-outvoltagesalsoneedtobeminimizedtomaximizedynamicrangewithinagivenpowersupplyvoltage.Thisisbecausethesignal-to-noiseratiotypicallydecreasesasthepowersupplyvoltagesdecreasewhilenoiseremainsconstant[7].Lastly,financialconsiderationsalsorequirethatthesecircuitsberealizedinrelativelysimpleprocesses,suchasstandardCMOS,bipolar,andinexpensiveBiCMOStechnologies[8].AnexampleoftherelativelyinexpensiveBiCMOSprocessisthe2-mMOSIStechnology(informationisavailablethroughtheInternetat).ThisisavanillaCMOSprocesswithanaddedp-baselayertorealizeverticalNPNtransistors.Fig.1illustratesthegeneralcomponentsofatypicallowdrop-outregulator,namely,anerroramplifier,apassdevice,areferencecircuit,afeedbacknetwork,andsomeloadingelements.Theassociatedgatecapacitanceofthepassdeviceisdepictedas.II.CURRENTEFFICIENTBUFFERA.CurrentEfficiencyCurrentefficiencyisanimportantcharacteristicofbattery-poweredproducts.Itisdefinedastheratiooftheload-currenttothetotalbatterydraincurrent,whichiscomprisedofload-0018–9200/98$10.001998IEEERINCON-MORAANDALLEN:LOW-VOLTAGE,LOWQUIESCENTCURRENT,LOWDROP-OUTREGULATOR37currentandthequiescentcurrentoftheregulatorEfficiency(1)Currentefficiencydetermineshowmuchthelifetimeofthebatteryisdegradedbythemereexistenceoftheregulator.Batterylifeisrestrictedbythetotalbatterycurrentdrain.Duringconditionswheretheload-currentismuchgreaterthanthequiescentcurrent,operationlifetimeisessentiallydeterminedbytheload-current,whichisaninevitablechar-acteristicoflinearregulators.Ontheotherhand,theeffectsofquiescentcurrentonbatterylifearemostprevalentduringlowload-currentconditionswhencurrentefficiencyislow.Formanyapplications,highload-currentisusuallyatemporarycondition,whereastheoppositeistrueforlowload-currents.Asaresult,currentefficiencyplaysapivotalroleindesigningbattery-poweredsupplies.Thetwoperformancespecificationsthatpredominantlylimitthecurrentefficiencyoflowdrop-outregulatorsaremaximumload