An-Output-Capacitorless-Low-Dropout-Regulator-With

458IEEEJOURNALOFSOLID-STATECIRCUITS,VOL.45,NO.2,FEBRUARY2010AnOutput-CapacitorlessLow-DropoutRegulatorWithDirectVoltage-SpikeDetectionPuiYingOrandKaNangLeung,SeniorMember,IEEEAbstract—Anoutput-capacitorlesslow-dropoutregulator(LDO)withadirectvoltage-spikedetectioncircuitispresentedinthispaper.Theproposedvoltage-spikedetectionisbasedoncapacitivecoupling.ThedetectioncircuitmakesuseoftherapidtransientvoltageattheLDOoutputtoincreasethebiascurrentmomentarily.Hence,thetransientresponseoftheLDOissignif-icantlyenhancedduetotheimprovementoftheslewrateatthegateofthepowertransistor.Theproposedvoltage-spikedetectioncircuitisappliedtoanoutput-capacitorlessLDOimplementedinastandard0.35-mCMOStechnology(whereand).ExperimentalresultsshowthattheLDOconsumes19Aonly.Itregulatestheoutputat0.8Vfroma1-Vsupply,withdropoutvoltageof200mVatthemaximumoutputcurrentof66.7mA.ThevoltagespikeandtherecoverytimeoftheLDOwiththeproposedvoltage-spikedetectioncircuitarereducedtoabout70mVand3s,respectively,whereastheyaremorethan420mVand30sfortheLDOwithouttheproposeddetectioncircuit.IndexTerms—Capacitivecoupling,low-dropoutregulator,voltagespike.I.INTRODUCTIONTRANSIENTresponseisacriticaldynamicspecificationinlow-dropoutregulator(LDO)design.Boththeampli-tudeofthevoltagespikeandtherecoverytimeoftheregulatedoutputvoltageaffectitsoverallaccuracy,whichindirectlyimpactstheperformanceofthecircuitssuppliedbytheLDO.Infact,thetransientresponseofaLDOisrelatedtodifferentde-signparameterssuchastheclosed-loopstability,theloopband-widthandtheslewrateatthegateofthepowertran-sistor[1]–[5].Theclosed-loopstabilityandaresmall-signalparametersrelatedtothepositionsofthepolesandthezerosinthefeedbacksystem,whileisalarge-signalparameterthatdependsheavilyonthemagnitudeofthebiascurrent[6].Undoubtedly,typicalmeasurestooptimizethetran-sientresponseofaLDOaretoincreasetheoutputcapacitance,usealowequivalent-series-resistance(ESR)capacitor,andin-creasethebiascurrentoftheerroramplifier/voltagebuffer[1].However,intheSoCdesign,itisexpectedtoplaceanon-chipoutput-capacitorlessLDOadjacenttoindividualcircuitblocks,ManuscriptreceivedMay02,2009;revisedAugust17,2009.CurrentversionpublishedFebruary05,2010.ThispaperwasapprovedbyAssociateEditorAn-dreasKaiser.TheworkdescribedinthispaperwasfullysupportedbyagrantfromtheResearchGrantCouncilofHongKongSARGovernmentunderprojectno.CUHK414209.TheauthorsarecurrentlywiththeDepartmentofElectronicEngineering,TheChineseUniversityofHongKong,Shatin,HongKong(e-mail:[e-mail];[e-mail]).Colorversionsofoneormoreofthefiguresinthispaperareavailableonlineat[8]–[10].sothatthepowersupplyofeachcircuitblockcanbeoptimizedindependently(i.e.accuracy,magnitude,power-supplyrejec-tionandnoise)toimprovetheoverallperformanceofthesystem[7].TheregulatedpowersuppliesaregeneratedinsidetheSoCchip.Underthiscircumstance,intheSoCdesign,thegenericapproachusingexternalcapacitorsisnolongerusefultoeffec-tivelyreducethevoltagespikeduetothenon-zerobondwireinductanceandthelongpower-lineroutings(i.e.delay).Therefore,morepowerisneededtoincreasebothandoftheLDOtosuppressthevoltagespikeandreducetherecoverytime.Fig.1showsonereportedLDOstructureusedin[8]–[10]toenableoutputcapacitorlessLDO.TheLDOstructureisba-sicallybasedontheflippedvoltagefollower[11],whichisamodifiedstructureofthesupersourcefollower[6].istheunregulatedinputvoltageoftheLDO.isthepowertran-sistor,whileandformafoldederroramplifierinthecommon-gateconfiguration[10].ThesourceofdetectstheLDOoutputforcomparingwithacontrolvoltagede-finedatthegateof.Anerrorsignalisthengeneratedatthegateofthepowertransistortoachieveclosed-loopvoltagereg-ulationbythenegativefeedback.TheoutputimpedanceoftheLDOisreduceddrasticallybytheloopgainoftheshuntfeed-back[6].Sincethereisnolargeoutputcapacitorinanoutput-ca-pacitorlessLDO,theshuntfeedbackcanpushthepolecreatedattheLDOoutputawayfrom(itisnotedthattheparasiticcapacitancefromthepowerlinedoesnotaffecttheclosed-loopstability,asitsvalueis,ingeneral,intheorderofpFornFatmost)[10].Generally,iswide,asthedominantpoleisafunctionofthegatecapacitanceofthepowertransistor,whichismuchsmallerthanthevalueoftheoutputcapacitancetypicallyintheorderof[8]–[10].Thus,thetransientre-sponseoftheLDOshowninFig.1isdominatedbythelimit.Thereareseveralreportedapproachestosolvetheproblem.Hazuchaetal.proposedtouseheavybiascurrentof6mA,0018-9200/$26.00©2010IEEEORANDLEUNG:ANOUTPUT-CAPACITORLESSLOW-DROPOUTREGULATORWITHDIRECTVOLTAGE-SPIKEDETECTION459Fig.2.RelationshipbetweentheoutputcurrentandthequiescentcurrentoftheLDO(a)constantbiasing[8](b)dynamicbiasing[9](c)current-efficientcurrentbuffer[1],[12].wherethebiascurrentisindependentoftheoutputcurrentasshowninFig.2(a),andconnecta600-pFon-chipoutputcapac-itortotheirLDO,implementedinCMOS90-nmtechnology,todeliveramaximumoutputcurrentof100mA[8].However,thisapproachisnotalwaysapplicabletothepower-saving,chip-area-limitedSoCdesignsimplementedininexpensivetechnolo-gi