optimizing-the-load-transient-response-of-the-buck

OptimizingtheLoadTransientResponseoftheBuckConverterRichardRed1BrianP.ErismanandZoltanZanskyELF1S.A.Derrey-la-CabucheCH-1756OnnensFRSwitzerlandAnalogDevices,Inc.1500SpaceParkDriveSantaClara,CA95052-8020U.S.A.Abstracf-Optimizedgainofthevoltage-erroramplifierandcurrent-modecontrolprovideinstantaneoustransientresponseandsmalldcshiftoftheoutputvoltageforstepchangesintheloadcurrent.Thispaperanalyzesthesystem,determinesthebestcompensation,providesdesignguidelinesforbuckconverterspoweringPentiumBIIprocessors,andpresentsresultsofsimulationsandexperiments.TableI[I]presentsthevoltageandcurrentspecificationsfortheprocessorduringthethreestates.TABLEIPENTIUMBI1PROCESSORVOLTAGEANDCURRENTSPECIFICATIONS-Max.SymbolParameter'rocessorMin.:orefreq.m)I.INTRODUCTION'Dc/dcconvertersforpoweringhigh-performancemicroprocessorsmusthavesmalltransientdeviationtoastepchangeintheloadcurrent.Thestepload-currentchangeistheresultofactivepowermanagement.Therateofchangeofloadcurrentisintheorderof30Alps.Iftheoutputvoltagedeviatesmorethanafewpercentfromthenominalvalueduetothestepload-currentchangeproperoperationisnotguaranteed.Thispaperdealswiththeissueofoptimizingtheloadtransientresponseofthesynchronousbuckconverter.(Thatconverteristhepreferredchoiceinmicroprocessorpower-supplyapplications.)First,thepower-supplyspecificationsoftheIntelPentiumBI1processorarereviewed.Thenthetheoreticallimitsoftheloadtransientresponseofthebuckconverteraredetermined.Thisisfollowedbyabriefoverviewoftheknownloadtransientreductiontechniques(increasingtheoutputcapacitance,load-currentorcapacitor-currentfeedfonvard,VZarchitecture,zero-impedanceconverter).Aftertheoverview,theoptimal,loadtransientresponseoftheprocessorpowersupplyisdetermined.Wethensynthesizetherequiredoutputimpedanceofthebuckconverterwithaconstant-off-timecurrent-modecontroller,determinethecompensationoftheerroramplifier,andprovideadesignprocedure.Computersimulationsandexperimentalresultscomplementthetheoreticalderivations.2.8__processorcoreVcc,,,statictolerance0.100__0.1400.1400.130-0.060266300233266300transienttoleranceGrantvcc,,,6.97.88.711.812.714.20.80.9TBD1.11.2TBDLO700.0800.020ICCforDeepSleepVcc,,,30-dIcc,,,/dtICCslewrateAccordingto[l],theoutputvoltagemeasuredattheconverteroutputpinsonthesystemboardmustbewithinthetransientrangeshowninTableI,includingthetransitionfromICC,GWCORE(Stop-Grantstate)toIccC~RE(Maximum)orfromIcccoRE(Maximum)toIcc~G~CORE(Stop-Grantstate),exceptforinputvoltageturn-onandturn-off.Thistolerancemustincludethevariationduetodcvoltageregulationplustheeffectsofanoutputloadtransient(slewrate)of30Alpsecattheconverteroutputpins.Theloadtransientresponsemaynotexceedthestaticvoltagespecificationforlongerthan2microseconds.Thetoggleratefortheoutputloadtransitionmayrangefrom100Hzto100kHz.Undertheaboveconditionsandforalltogglerates,thetransientresponsemustbemeasuredovera20MHzfrequencyband,andatambienttemperaturesbetween25OCand5OOC.11.OBJECTIVESPECIFICATIONSDuetothepresenceofactivepowermanagement,thesupplycurrentofthenewPentiumBI1familyofIntelmicroprocessorschangesrapidlyandoverawiderangeduringnormaloperationofthecomputer.Thechangesinthesupplycurrentaretheresultsofstoppingtheclocktovariousinternalsectionsoftheprocessor.Threestatesaretobeconsideredwithreducedpowerconsumption,Stop-Grant,Sleep,andDeepSleep.0-7803-4340-9/98/$10.0001998IEEE.170LoadcurrentI12-11AInductorcurrentICapacitor*-t7ycurrentFig.1.Outputvoltage(top)andloadcurrent(bottom).111.THEORETICALLOADTRANSIENTRESPONSEPERFORMANCELIMITSFig.1showsthetypicaloutputvoltagetransient(top)andtheloadcurrentthatgeneratesit(bottom).Thetransientcomprisesfourdistinctsections.Insection1theoutputvoltagestepsdownwardfromitssteady-statevalueV,,byavoltageVI.Thisstepiscausedbytheequivalentseriesinductance,ESL(orLe),oftheoutputcapacitoroftheswitchingregulator.ThemagnitudeVIofthestepisequaltotheproductofLeandtherateofchange,ordi/dt,oftheloadcurrent.Thedi/dtisequalto(I,-Il)/Trjse.Insection2theoutputvoltagecontinuestomovedownward,butasarampfunctionratherthanastepfunction.ThemagnitudeVzoftherampvoltageisequaltotheproductoftheloadcurrentandtheequivalentseriesresistance,ESR(orRe)oftheoutputcapacitor.Insection3theoutputvoltagestepsintheoppositedirection(upward).Again,thisstepiscausedbythecombinationofthechangeinthedi/dtoftheloadcurrentandtheESLoftheoutputcapacitor.ThemagnitudeofthestepisVI.Insection4,theoutputvoltagecontinuestomovebutatamuchslowerratethaninthefirstthreesections.ThepeakdeviationoftheoutputvoltagefromtheinitialvalueisV3;eventuallytheoutputvoltagesettlestoanewvalueVo2.Insection4thetimefunctionoftheoutputvoltageisdeterminedbythedynamicbehavioroftheconverterandbytheappliedcontrolmethod.Consideringthepresentlypracticalswitchingfrequencies(100to500kHz)andthe30A/psecrateofchangeoftheloadcurrent,evenwithanoptimalcontrolleritwouldnotbepossibletoreducethepeakdeviationoftheoutputvoltagebelowthevalue:v,+v2=-12--11Le+(I2-Z1)Re(1)TrimTheminimumachievablepeakdeviationofthecapacitorvoltageofthebuckconverter(neglectingforthemomenttheESLandESRofthecapacitorandassumi