LM26365-BitProgrammableSynchronousBuckRegulatorControllerMarch2005
LM2636
5-BitProgrammableSynchronousBuckRegulatorController
GeneralDescription
TheLM2636isahighspeedcontrollerdesignedspecificallyforuseinsynchronousDC/DCbuckconvertersforadvancedmicroprocessors.A5-bitDACacceptstheVIDcodedi-rectlyfromtheCPUandadjuststheoutputvoltagefrom1.3Vto3.5V.Itprovidesthepowergood,over-voltageprotection,andoutputenablefeaturesasrequiredbyIntelVRMspeci-fications.Currentlimitingisachievedbymonitoringthevolt-agedropacrosstherDS_ONofthehighsideMOSFET,whicheliminatesanexpensivecurrentsenseresistor.
TheLM2636employsafixed-frequencyvoltagemodePWMarchitecture.Toprovideafasterresponsetoalargeandfastloadtransient,twoultra-fastcomparatorsarebuiltintomoni-tortheoutputvoltageandoverridetheprimarycontrolloopwhennecessary.ThePWMfrequencyisadjustablefrom50kHzto1MHzthroughanexternalresistor.ThewiderangeofPWMfrequencygivesthepowersupplydesignertheflexibil-itytomaketrade-offsbetweenloadtransientresponseper-formance,MOSFETcostandtheoverallefficiency.Theadaptivenon-overlappingMOSFETgatedrivershelpavoidanypotentialshoot-throughproblemwhilemaintaininghighefficiency.BiCMOSgatedriverswithrail-to-railswingensurethatnospuriousturn-onoccur.Whenonly5Visavailable,abootstrapstructurecanbeemployedtoaccommodateanNMOShighsideswitch.Theprecisionreferencetrimmedto
2.5%overtemperatureisavailableexternallyforusebyotherregulators.Dynamicpositioningofloadvoltage,whichhelpscutthenumberofoutputcapacitors,canalsobeimplementedeasily.
Features
nnnnnnnnnnn
1.3Vto3.5V5-bitprogrammableoutputvoltageSynchronousrectification
PowerGoodflagandoutputenableOver-voltageprotection
InitialOutputAccuracy:1.5%overtemperatureCurrentlimitwithoutexternalsenseresistorAdaptivenon-overlappingMOSFETgatedrivesAdjustableswitchingfrequency:50kHzto1MHzDynamicoutputvoltagepositioning
1.256VreferencevoltageavailableexternallyPlasticSO-20packageandTSSOP-20package
Applications
nMotherboardpowersupply/VRMforCyrixGxm,CyrixGxi,CyrixMII,Pentium™II,PentiumPro,6x86andK6processors
n5Vto1.3V–3.5Vhighcurrentpowersupplies
ConnectionDiagrams
TOPVIEW
TOPVIEW
1008340310083403
PlasticSO-20
OrderNumberLM2636MSeeNSPackageNumberM20BPlasticTSSOP-20
OrderNumberLM2636MTCSeeNSPackageNumberMTC20
Pentium™isatrademarkofIntelCorporation.
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LM2636TypicalApplication
10083401
FIGURE1.5Vto1.3V–3.5V,14APowerSupply
PinDescriptions
LSGATE(Pin1):Gatedriveforthelow-sideN-channelMOSFET.ThissignalisinterlockedwithHSGATE(Pin20)toavoidashoot-throughproblem.
BOOTV(Pin2):Powersupplyforhigh-sideN-channelMOSFETgatedrive.Thevoltageshouldbeatleastonegatethresholdabovetheconverterinputvoltagetoproperlyop-eratethehigh-sideN-FET.
PGND(Pin3):Groundforhighcurrentcircuitry.Itshouldbeconnectedtosystemground.
SGND(Pin4):Groundforsignallevelcircuitry.Itshouldbeconnectedtosystemground.
VCC(Pin5):Powersupplyforthecontroller.
SENSE(Pin6):Converteroutputvoltagesensing.Itpro-videsinputforpowergood,fastdualcomparatorcontrolloop,andover-voltageprotectioncircuitry.Itisrecom-mendedthata0.1µFcapacitorbeconnectedbetweenthispinandgroundtoavoidpotentialnoiseproblems.
IMAX(Pin7):Currentlimitthresholdsetting.Itsinksafixed180µAcurrent.ByconnectingaresistorbetweenthehighsideMOSFETdrainandthispin,afixedvoltagedropcanbebuiltacrosstheresistor.ThisvoltagedropiscomparedwiththeVDSofthehigh-sideN-MOSFETtodetermineifanover-currentconditionhasoccurred.
IFB(Pin8):High-sideN-MOSFETsourcevoltagesensing.ThispinisoneVDSbelowdrainvoltage.WhenthisvoltageislowerthanthatofIMAXpinduringthetimethehigh-sideFETison,itmeansVDSishigherthanthepresetvoltageacrosstheIMAXresistor,whichcanbeinterpretedasanover-currentcondition.
VREF(Pin9):Bandgapreferencevoltage.Thisvoltageismainlyforusebyotherpowersuppliesonthemotherboardwhichneedareference.
EA_OUT(Pin10):Outputoftheerroramplifier.Thevoltagelevelonthispiniscomparedwithaninternallygeneratedrampsignaltodeterminethedutycycle.Thispinisneces-saryforcompensatingtheprimarycontrolloop.
FB(Pin11):Invertinginputoftheerroramplifier.Apinnecessaryforcompensatingthecontrolloop.
FREQ_ADJ(Pin12):Switchingfrequencyadjustment.Switchingfrequencycanbeadjustedbychangingthegroundingresistanceonthispin.
PWRGD(Pin13):PowerGood.TherearetwowindowsaroundtheDACoutputvoltagethatareassociatedwithPWRGDpin,the±10%windowandthe±8%window.IfPWRGDisinitiallyhigh(opendrainstate)andoutputvoltagetravelsoutof±10%window,PWRGDgoestolow(lowimpedancetoground).IfPWRGDisinitiallylowandoutputvoltagetravelsintothe±8%windowandhasstayedwithinthewindowforatleast10ms,PWRGDgoestohigh.APWRGDhighmeanstheoutputvoltageisatleastwithinthe±10%windowwhereasaPWRGDlowindicatestheoutputvoltageisdefinitelyoutsidethe±8%window.
VID4:0(Pins14,15,16,17,18):VoltageIdentificationCode.Thefivepinsacceptanopen-groundpattern5-bitbinarycodefromoutsidethechip(typicallyfromtheCPU)forgeneratingthedesiredoutputvoltage.EachVIDpinisinter-nallypulleduptoVCCviaa90µAcurrentsource.Table1showsthecodetable.
OUTEN(Pin19):OutputEnable.Theoutputvoltageisdisabledwhenthispinispulledlow.ItisinternallypulleduptoVCCviaa90µAcurrentsource.
HSGATE(Pin20):Gatedriveforthehigh-sideN-channelMOSFET.ThissignalisinterlockedwithLSGATE(Pin1)toavoidashoot-throughproblem.
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LM2636PinDescriptions
(Continued)
TABLE1.VIDCodeandDACOutput
VID4VID3VID2VID1VID000000000000000001111111111111111
11111111000000001111111100000000
11110000111100001111000011110000
11001100110011001100110011001100
10101010101010101010101010101010
RatedOutputVoltage(V)
1.301.351.401.451.501.551.601.651.701.751.801.851.901.952.002.05(shutdown)
2.12.22.32.42.52.62.72.82.93.03.13.23.33.43.5
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LM2636AbsoluteMaximumRatings(Note1)
IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.(AllvoltagesarereferencedtothePGNDandSGNDpins.)VCCBOOTV
JunctionTemperature
DCPowerDissipation(Note2)StorageTemperature
7V18V150˚C1.42W
−65˚Cto+150˚C
SolderingTime,TemperatureWave(4seconds)Infrared(10seconds)
VaporPhase(75seconds)ESDSusceptibility(Note3)
260˚C240˚C219˚C2kV
RecommendedOperatingConditions(Note1)
SupplyVoltageRange(VCC)JunctionTemperatureRange
4.5Vto5.5V0˚Cto+125˚C
ElectricalCharacteristics
VCC=5VunlessotherwiseindicatedundertheConditionscolumn.TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=+25˚C.Limitsappearinginboldfacetypeapplyover0˚Cto+70˚C.SymbolVBOOTVVDACOUTParameter
FETDriverSupplyVoltage
5-BitDACOutputVoltage
VID4:0=01111VID4:0=01101VID4:0=01011VID4:0=01001VID4:0=00111VID4:0=00101VID4:0=00001VID4:0=11101VID4:0=11010VID4:0=10111
∆VOUTGEASREABWEAIQ_VCCIQ_BOOTVDMAXDMINRSENSERDS_SRCDCLoadRegulationDCLineRegulationErrorAmplifierDCGainErrorAmplifierSlewRate
ErrorAmplifierUnityGainBandwidthOperatingVCCCurrentShutdownVCCCurrentBOOTVPinQuiescentCurrent
MaximumDutyCycleMinimumDutyCycleSENSEPinResistancetoGround
FETDriverDrain-SourceONResistancewhenSourcingCurrentFETDriverDrain-SourceONResistancewhenSinkingCurrentOscillatorFrequency
BOOTV=5V
7
(IndependentofBOOTVVoltage)
1.7
RFA=84kΩRFA=22kΩRFA=10.5kΩ
IMAXIMAXPinSinkCurrent
VIMAX=5V,VIFB=6V,VCC=5V
130250
30010002000180
230
µA
350
kHzΩΩ
7
OUTEN=VCC=5V,VID=10111OUTENFloating,VID0:4FloatingBOOTV=12V,OUTEN=0,VID0:4Floating
1.51
IOUT=0to14AFigure2VIN=4.75Vto5.25VFigure2
1.2841.3851.4831.5851.6831.7841.9832.1732.4712.768
1.3041.4061.5061.6091.7091.8112.0132.2062.5092.81−5185652.51.5490011.5
1643
Conditions
Min
Typ
Max181.3241.4271.5291.6331.7351.8382.0432.2392.5472.852
mVdBV/µsMHzmAµA%%kΩVUnitsV
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LM2636ElectricalCharacteristics
SymbolVOUTEN_IHParameter
OUTENPinInputLogicLowtoLogicHighTripPoint
OUTENPinInputLogicHightoLogicLowTripPoint
BandGapReferenceReferenceVoltageatFullLoad
ReferenceVoltageatHighLine
ReferenceVoltageatLowLine
ReferenceVoltageLoadRegulation
ReferenceVoltageLineRegulation
RampSignalValleyVoltage
RampSignalPeakVoltage
PWRGDPin↓TripPoints(seePin
DescriptionforPin13)PWRGDPin↑TripPoints(seePin
DescriptionforPin13)Over-voltageProtectionTripPoint
PowerGoodResponseTime
PowerNotGoodResponseTimeOUTENPinInternalPull-UpCurrent
VIDPinsLogicHighTripPoint
VIDPinsLogicLowTripPoint
VID0:4InternalPull-UpCurrent
SoftStartDuration
(Continued)
VCC=5VunlessotherwiseindicatedundertheConditionscolumn.TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=+25˚C.Limitsappearinginboldfacetypeapplyover0˚Cto+70˚C.
Conditions
OUTENVoltage↑
3.5
OUTENVoltage↓
1.8
IVREF=0mA
IVREF=0.5mA,SourcingIVREF=0mA,VCC=5.25VIVREF=0mA,VCC=4.75VIVREF=0.5mA,SourcingIVREF=0mA,VCCChangesfrom5.25Vto4.75V
1.2251.2231.2261.224
1.2561.2541.2571.255−2−0.51.253.25
%aboveDACOutputVoltage,whenOutputVoltage↑
%belowDACOutputVoltage,whenOutputVoltage↓
%aboveDACOutputVoltage,whenOutputVoltage↓
%belowDACOutputVoltage,whenOutputVoltage↑
%aboveDACOutputVoltageVSENSERisesfrom0VtoRatedVOUTVSENSEFallsfromRatedVOUTto0V
10
%
−108
%
−815
22603.5
66903.01.8
60
902048
1.31301515130
%µsµsµAVVµAclockcycles
1.51.2871.2851.2881.286
VVVVVmVmVVV
3.0
V
Min
Typ
Max
Units
VOUTEN_ILVREFVREF_LOADVREF_525VREF_475∆VREF_LOAD∆VREF_LINEVSAWLVSAWHVPWRBAD_GDVPWRGD_BADVOVPtPWRGDtPWRBADIOUTENVVID_IHVVID_ILIVIDtSSNote1:AbsoluteMaximumRatingsarelimitsbeyondwhichdamagetothedevicemayoccur.RecommendedOperatingConditionsareconditionsunderwhichthedeviceoperatescorrectly.RecommendedOperatingConditionsdonotimplyguaranteedperformancelimits.
Note2:MaximumallowableDCpowerdissipationisafunctionofthemaximumjunctiontemperature,TJMAX,thejunction-to-ambientthermalresistance,θJA,andtheambienttemperature,TA.Themaximumallowablepowerdissipationatanyambienttemperatureiscalculatedusing:
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LM2636ElectricalCharacteristics
(Continued)
Thejunction-to-ambientthermalresistance,θJA,forLM2636intheM20Bpackageis88˚C/W,and120˚C/WfortheMTC20package.Note3:Allpinsareratedfor2kV,exceptfortheIMAXpin(Pin7)whichisratedfor1.5kV.
BlockDiagram
10083402
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LM2636TestCircuit
10083404
FIGURE2.
ApplicationsInformation
OVERVIEW
TheLM2636isahighspeedsynchronousPWMbuckregu-latorcontrollerdesignedforVRMvendorsormotherboardmanufacturerswhoneedtobuildon-boardpowersuppliesforCyrixMII,PentiumIIorDeschutesmicroprocessors.Ithasabuilt-in5-bitDACtodecodethe5-bitwordprovidedbytheCPUandsupplythecorrespondingvoltage.Italsohasthepowergood(PWRGD)andoutputenable(OUTEN)func-tionsrequiredbytheVRMspecification.Itemploysavoltagemodecontrolschemeplustwofastrespondingcomparatorstoquicklyrespondtolargeloadtransients.IthastwofastFETdriverstodrivethehigh-sideandlow-sideNMOSswitchesofasynchronousbuckregulator.ThePWMfre-quencyisadjustablefrom50kHzto1MHzthroughanexternalresistor.Over-voltageprotectionisachievedbyshuttingoffthehigh-sidedriverandturningonthelow-sidedriver100%ofthetime.CurrentlimitingisimplementedbysensingVDSofthehigh-sideNMOSswitchandshuttingitoffforthepresentswitchingcyclewhenanovercurrentcondi-tionisdetected.SoftstartfunctionalityisrealizedthroughaninternaldigitalcounterandaninternalDAC.THEORYOFOPERATION
StartUp
WhenVCCvoltageexceeds4.2V,OUTENpinisalogichighandtheVIDcodeisvalid,thesoftstartcircuitrystartstowork.Thedurationofthesoftstartisdeterminedbyaninternaldigitalcounterandtheswitchingfrequency.Duringsoftstart,theoutputoftheerroramplifierisallowedtoincreasegradually.Whenthecounterhascounted2,048clockcycles,thesoftstartsessionendsandtheoutputvoltageleveloftheerroramplifierisreleasedandallowedtogotoavaluethatisdeterminedbythefeedbackloop.PWRGDpinisforcedlowduringsoftstartandisturnedover
tooutputvoltagemonitoringcircuitryafterthat.BeforeVCCreaches4.2V,allinternallogicisinapoweronresetstateandthetwoFETdriversaredisabled.
Duringnormaloperation,ifVCCvoltagedropsbelow3.8V,theinternalcircuitrywillgointopoweronresetagain.ThehysteresishelpsdecreasethenoisesensitivityontheVCCpin.Aftersoftstartsendsandduringnormaloperation,iftheconverteroutputvoltageexceeds115%oftheDACoutputvoltage,theLM2636willlockintoovervoltageprotectionmode.Thehighsidedrivewillbedisabled,andthelowsidedrivewillbehigh.Therearetwowaystoclearthemode.OneistocycleVCCvoltageonce.TheotheristotoggletheOUTENlevel.Aftertheovervoltageprotectionmodeiscleared,theLM2636willenterthesoftstartsessionandstartover.
NormalOperation
Duringthenormaloperationmode,theLM2636regulatestheconverteroutputvoltagebyadjustingthedutyratio.Theoutputvoltageisdeterminedbythe5-bitVIDcodesetbytheuser/load.
ThePWMfrequencyissetbytheexternalresistorbetweenFREQ_ADJpinandground.Theresistanceneededforadesiredswitchingfrequencyis:
Forexample,ifthedesiredswitchingfrequencyis300kHz,theresistanceshouldbearound84kΩ.
TheminimumallowablePWMfrequencyis5kHz.
MOSFETGateDrive
TheLM2636hastwogatedrivesthataresuitablefordrivingexternalN-MOSFETsinasynchronousbucktopology.ThepowerforthetwoFETdriversissuppliedbytheBOOTVpin.ThisBOOTVvoltageneedstobeatleastoneVGS(th)higherthantheconverterinputvoltageforthehighsideFETtobe
7
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LM2636ApplicationsInformation
(Continued)
fullyturnedon.Thevoltagecanbeeithersuppliedfromaseparatesourceotherthantheinputvoltageorcanbegeneratedlocallybyutilizingachargepumpstructure.Inatypicaldesktopmicroprocessorapplication,if5Vischosentobetheinputvoltage,then12VcanbeusedfortheBOOTV.If12Visnotavailable,asimplechargepumpcircuitryconsistingofadiodeandasmallcapacitorcanbeused,asshowninFigure3.
transient,dependingonthecompensationdesign,thechangeindutyratiocanbeasfastaslessthanoneswitch-ingcycle.RefertoDesignConsiderationssectionformoredetails.
Besidestheusualvoltagemodefeedbackcontrolloop,theLM2636alsohasapairoffastcomparators(theMINandMAXcomparators)tohelpmaintaintheoutputvoltagedur-ingalargeandfastloadtransient.Thetrippointsofthecomparatorsaresetto±5%oftheDACoutputvoltage.Whentheloadtransientissolargethattheoutputvoltagegoesoutsidethe±5%window,theMINorMAXcomparatorwillbypasstheprimaryvoltagecontrolloopandimmediatelysetthedutyratiotoeithermaximumvalueortozero.Thisprovidesthefastestpossiblewaytoreacttosuchalargeloadtransientinaclassicalbuckconverter.PowerGoodSignal
Thepowergoodsignalisusedtoindicatethattheoutputvoltageiswithinspecifiedrange.IntheLM2636,therangeissettoa±10%windowoftheDACoutputvoltage.Duringsoftstart,thepowergoodsignalisalwayslow.Attheendofthesoftstartsession,theoutputvoltageischeckedandthePWRGDpinwillbeassertedifthevoltageiswithinspecifiedrange.
OverVoltageProtection
Whentheoutputvoltageexceeds115%oftheDACoutputvoltageaftertheendofsoftstart,theLM2636willenterovervoltageprotectionmodeinwhichitshutsitselfdown.Theuppergatedriverisheldlowwhilethelowergatedriverisheldhigh.PWRGDwillbelow.ForLM2636torecoverfromOVPmode,eitherOUTENorVCCvoltagehastobetoggled.AnothermoresubtlewaytorecoveristofloatalltheVIDpinsandreapplythecorrectcode.
CurrentLimit
CurrentlimitisrealizedbysensingtheVDSvoltageofthehighsideMOSFETwhenitison.SincetherDS_ONofaMOSFETisaknownvalue,currentthroughtheMOSFETcanbeknownbymonitoringVDS.Therelationshipbetweenthethreeparametersis:
10083406
FIGURE3.BOOTVVoltageSuppliedbyaChargePumpWhenthelowsideFETison,thechargepumpcapacitorischargedtoneartheinputvoltagethroughthediode.WhenlowsideFETisturnedoff,thehighsideFETdriverisenabled,andthechargepumpcapacitorstartstochargethehighsideFETgateuntilitisfullyon.BythistimethehighsideFETsourcenodewillflytoclosetoinputvoltagelevelandtheuppernodeofthecapacitorwillalsoflytooneinputvoltagehigherthantheinputvoltage,enablingthehighsideFETdrivertocontinueworking.
ForaBOOTVof12V,theinitialgatechargingcurrentistypically2A,andtheinitialgatedischargingcurrentistypi-cally6A,goodforhighspeedswitching.
TheLM2636gatedrivesareofBiCMOSdesign.UnlikesomeotherbipolarVRMcontrolICs,thegatedrivehasrail-to-railswingthatensuresnospuriousturn-onduetocapacitivecoupling.
AnotherfeatureoftheFETgatedrivesistheadaptivenon-overlappingmechanism.Agatedriverisnotturnedonuntiltheotherisfullyoff.Thedeadtimeinbetweenistypically20ns.Thisavoidsthepotentialshoot-throughproblemandhelpsimproveefficiency.LoadTransientResponse
InatypicalmodernMPUapplicationsuchasthePentiumIIcorevoltagepowersupply,loadtransientresponseisacriticalissue.TheLM2636utilizestheconventionalvoltagefeedbacktechnologyastheprimaryfeedbackcontrolmethod.Whentheloadtransienthappens,theerrorintheoutputvoltagelevelisfedtotheerroramplifier.TheoutputoftheerroramplifieristhencomparedwithaninternallygeneratedPWMrampsignalandtheresultofthecomparisonisaseriesofpulseswithcertaindutyratios.Thesepulsesareusedtocontroltheturn-onandturn-offoftheMOSFETgatedrivers.Inthisway,theerrorintheoutputvoltagegets“compensated”orcancelledbythechangeinthedutyratiooftheFETswitches.Duringalargeload
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Toimplementthecurrentlimitfunction,anexternalresistorRIMAXisneed.TheresistorshouldbeconnectedbetweenthedrainofthehighsideMOSFETandtheIMAXpin.Aconstantcurrentofaround180µAisforcedintotheIMAXpinandcausesafixedvoltagedropacrosstheRIMAXresistor.ThisvoltagedropisthencomparedwiththeVDSofthehighsideMOSFETandifthelatterishigher,overcurrentisreached.SotheappropriatevalueofRIMAXforapre-determinedcurrentlimitlevelILIMcanbecalculatedbythefollowingequation:
Forexample,ifweknowthattherDS_ONoftheMOSFETis20mΩ,andthecurrentlimitwewanttosetis20A,thenweshouldchoosethevalueofRIMAXtobe2.2kΩ.
ToprovidethegreatestprotectionoverthehighsideMOS-FET,cyclebycycleprotectionisimplemented.ThesamplingoftheVDSstartsasearlyasabout300nsaftertheswitchis
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LM2636ApplicationsInformation
(Continued)
turnedon.Wheneveranovercurrentconditionisdetected,thehighsideswitchisimmediatelyturnedoffandthelowsideswitchturnedon,untilthenextswitchingcyclecomes.Thedelayof300nsistocircumventswitchingnoisewhentheMOSFETisfirstturnedon.DESIGNCONSIDERATIONSControlLoopCompensation
Aswitchingregulatorshouldbeproperlycompensatedtoachieveastablecondition.Forasynchronousbuckregula-torthatneedstomeetstringentloadtransientrequirementsuchasaPentiumIIMPUcorevoltagesupply,asimple2-pole-1-zerocompensationnetworkshouldsuffice,suchastheoneshowninFigure4(C1,C2,R1andR2).ThisisbecausetheESRzeroofthetypicaloutputcapacitorsislowenoughtomakethecontrol-to-outputtransferfunctionasingle-pole-roll-off.
Asanexample,letusfigureoutthevaluesofthecompen-sationnetworkcomponentsinFigure4.Assumethefollow-ingparameters:R=20Ω,RL=20mΩ,RC=9mΩ,L=2µH,C=7.5mF,VIN=5V,Vm=2Vandswitchingfrequency=300kHz.TheseparametersarebasedonthetypicalapplicationinFigure1.NoticeRListhesumoftheinductorDCresistanceandtheonresistanceoftheMOSFETs.
10083409
FIGURE4.BuckConverterfromaControlPointof
View
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LM2636ApplicationsInformation
Thecontrol-to-outputtransferfunctionis
(Continued)
where
TheESRzerofrequencyis:
OneofthepolesislocatedatorigintohelpachievethehighestDCgain.Sotherearethreeparameterstodeter-mine,thepositionofthezero,thepositionofthesecondpole,andtheconstantA.Todeterminethecutofffrequencyandphasemargin,theloopbodeplotsneedtobegener-ated.Thelooptransferfunctionis:
TF=−TF1xTF2
Bychoosingthezeroclosetothedoublepolepositionandthesecondpoletohalfoftheswitchingfrequency,theclosedlooptransferfunctionturnsouttobeverygood.
Thatis,iffZ=1.32kHz,fP=153kHz,andA=4.8x10−6ΩF,thenthecutofffrequencywillbe50kHz,thephasemarginwillbe72˚,andtheDCgainwillbethatoftheerroramplifier.SeeFigure6below.
Thepowerstagedoublepolefrequencyis:
ThecorrespondingBodeplotsareshowninFigure5.
10083413
FIGURE5.Control-to-OutputBodePlots
SincetheESRzerofrequencyissolow,iteffectivelycancelsthephaseshiftfromoneofthepowerstagepoles.Thislimitsthetotalphaseshiftto90%.
Althoughthisregulatordesignisstable(phaseshiftis<90˚whengain=0dB),itneedscompensationtoimprovetheDCgainandcutofffrequency(0dBfrequency).Otherwise,thelowDCgainmaycauseapoorlineregulation,andthelowcutofffrequencywillhurttransientresponseperformance.Thetransferfunctionforthe2-pole-1-zerocompensationnetworkshowninFigure4is:
10083417
FIGURE6.LoopBodePlots
Thecompensationnetworkcomponentvaluescanbedeter-minedbythefollowingequations:
Noticetherearethreeequationsbutfourvariables.Sooneofthevariablescanbechosenarbitrarily.Sincethecurrent
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LM2636ApplicationsInformation
(Continued)
drivingcapabilityoftheerroramplifierislimitedtoaround3mA,itisagoodideatohaveahighimpedancepathfromtheoutputoftheerroramplifiertotheoutputoftheconverter.FromtheaboveequationsitcanbetoldthatalargerR2willresultinasmallerC1,C2andalargerR1.However,toolargeanR1canalsobringerrorduetothebiascurrentrequiredbytheinvertinginputpinoftheerroramplifier.Calculationsshowthatthefollowingcombinationisagoodone:R2=51Ω,C1=0.022µF,R1=5.6kΩ,C2=820pF.
Foradifferentapplicationordifferenttypeofoutputcapaci-tors,adifferentcompensationschememaybenecessary.Theusercaneitherfollowthestepsabovetofiguretheappropriatecomponentvaluesorcontactthefactoryforhelp.
MOSFETSELECTION
TheselectionofMOSFETswitchesaffectsboththeeffi-ciencyofthewholeconverterandthecurrentlimitsetting.Fromanefficiencypointofviewitissuggestedthatforthehigh-sideswitch,onlylogiclevelMOSFETsbeused.Stan-dardMOSFETscanbeusedforthelowsideswitchwhen12VisusedtopowertheBOOTVpin.Thelowerlossasso-ciatedwiththeMOSFETsistwo-fold—Ohmiclossandswitchingloss.TheOhmiclossiseasytocalculatewhereastheswitchinglossismuchmoredifficulttoestimate.Ingeneraltheswitchinglossisdirectlyproportionaltotheswitchingfrequency.AsthepowerMOSFETtechnologyad-vances,lowerandlowergatechargedeviceswillbeavail-able.Thatshouldallowtheusertogotohigherswitchingfrequencieswithoutthepenaltyoflosingtoomuchefficiency.Asanexample,letusselecttheMOSFETsforaconverterwithatargetefficiencyof80%ataloadof2.8V,14A.Assumetheinductorslose1W,thecapacitorslose0.75Wandthetotalswitchinglossat300kHzis3.2W.Thetotalallowedpowerlossis9.8W,sotheMOSFETOhmiclossshouldnotexceed4.9W.Assumethetwoswitcheshavethesameconductionloss,i.e.,2.5Weach,thentheONresis-tanceforthetwoswitchesis:
ingconverterhastorelyontheoutputcapacitorstotakecareofthefirstfewmicroseconds.Undersuchacurrentslewrate,ESRoftheoutputcapacitorsismoreofaconcernthantheESL.Dependingonthekindofcapacitorsbeingused,capacitanceoftheoutputcapacitorsmayormaynotbeanimportantfactor.Whentheoutputcapacitanceistoolow,theconvertermayhavetohaveasmalloutputinductortoquicklysupplycurrenttotheoutputcapacitorswhentheloadsuddenlykicksinandtoquicklystopsupplyingcurrentwhentheloadissuddenlyremoved.
Multilayerceramic(MLC)capacitorscanhaveverylowESRbutalsoalowcapacitancevaluecomparedtootherkindsofcapacitors.LowESRaluminumelectrolyticcapacitorstendtohavelargesizesandcapacitances.TantalumelectrolyticcapacitorscanhaveafairlylowESRwithamuchsmallersizeandcapacitancethanthealuminumcapacitors.CertainOSCONcapacitorspresentultralowESRandlonglifespan.BythetimethetotalESRoftheoutputcapacitorbankreachesaround9mΩ,thecapacitanceofthealuminum/tantalum/OSCONcapacitorsisusuallyalreadyinthemilli-faradrange.Forthosecapacitors,ESRistheonlyfactortoconsider.MLCscanhavethesameamountoftotalESRwithmuchlesscapacitance,mostprobablyunder100µF.Averysmallinductor,ultrafastcontrolloopandahighswitchingfrequencybecomenecessaryinsuchacasetodealwiththefastcharging/dischargingrateoftheoutputcapacitorbank.Fromacostsavingspointofview,aluminumelectrolyticcapacitorsarethemostpopularchoiceforoutputcapacitors.Theyhavereasonablylonglifespanandtheytendtohavehugecapacitancetowithstandthechargingordischargingprocessduringaloadtransientforafairlylongperiod.SanyoMV-GXseriesgivesgoodperformancewhenenoughofthecapacitorsareparalleled.The6MV1500GXcapacitorhasatypicalESRof44mΩ.Fiveofthesecapacitorsshouldbesufficientinthecaseofon-boardpowersupplyforaPentiumIImotherboard.
Thechallengeforinputcapacitorsistheripplecurrent.ThelargeripplecurrentdrawnbythehighsideswitchtendstogeneratequitesomeheatduetothecapacitorESR.Theripplecurrentratingsinthecapacitorcatalogsareusuallyspecifiedunderthehighestallowabletemperature.Inthecaseofdesktopapplications,thoseratingsseemtoocon-servative.Agoodwaytoensureenoughnumberofcapaci-torsisthroughlabevaluation.TheinputcurrentRMSripplevaluecanbedeterminedbythefollowingequation:
ThelowsideswitchONresistanceismuchhigherthanthehighsidebecauseat2.8Vthedutycycleishigherthan50%andbecomesevenlargeratfullload.Forthehighsideswitch,anIRL3202(TO-220package)orIRL3202S(D2PAK)shouldbesufficient.Forthelowsideswitch,anIRL3303(TO-220package)orIRL3303S(D2PAK)shouldbesuffi-cient.SinceeachFETisdissipating3.2W/2+2.5W=4.1W,itissuggestedthatappropriateheatsinksbeusedinthecaseofTO-220packageorlargeenoughcopperareabeconnectedtothedraininthecaseofsurfacemountpack-age.
CAPACITORSELECTION
TheselectionofcapacitorsisanextremelyimportantstepwhendesigningaconverterforaloadsuchasthePentiumII.Sincethetypicalslewrateoftheloadcurrentduringalargeloadtransientisaround20A/µsto30A/µs,theswitch-11
andthepowerlossineachinputcapacitoris:
InthecaseofPentiumIIpowersupply,themaximumoutputcurrentisaround14A.Undertheworstcasewhendutycycleis50%,themaximuminputcapacitorRMSripplecurrentishalfofoutputcurrent,i.e.,7A.ItisfoundthatthreeSanyo16MV820GXcapacitorsareenoughunderroomtempera-ture.ThetypicalESRofthosecapacitorsis44mΩ.Sothepowerlossineachofthemisaround(7A)2x44mΩ/32=0.24W.Notethatthepowerlossineachcapacitorisin-verselyproportionaltothesquareofthetotalnumberofcapacitors,whichmeansthepowerlossineachcapacitorquicklydropswhenthenumberofcapacitorsincreases.
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LM2636ApplicationsInformation
INDUCTORSELECTION
(Continued)
Thesizeoftheoutputisdeterminedbyanumberofparam-eters.Basicallythelargertheinductor,thesmallertheoutputripplevoltage,buttheslowertheconverter’sresponsespeedduringaloadtransient.Ontheotherhand,asmallerinductorrequireshigherswitchingfrequencytomaintainthesamelevelofoutputripple,andprobablyresultsinamorelossyconverter,buthaslessinertiarespondingtoloadtran-sient.InthecaseofPentiumIIpowersupply,fastrecoveryoftheloadvoltagefromtransientwindowbacktothesteadystatewindowisconsideredimportant.Thislimitsthehighestinductancevaluethatcanbeused.Thelowestinductancevalueislimitedbythehighestswitchingfrequencythatcanbepracticallyemployed.Astheswitchingfrequencyin-creases,theswitchinglossintheMOSFETstendstoin-crease,resultinginlessconverterefficiencyandlargerheatsinks.AgoodswitchingfrequencyisprobablyafrequencyunderwhichtheMOSFETconductionlossishigherthantheswitchinglossbecausethecostoftheMOSFETisdirectlyrelatedtoitsRDSON.Theinductorsizecanbedeterminedbythefollowingequation:
thetotalESRis15mΩ.Wheneverthereisasuddenloadcurrentchange,ithastoinitiallybesupportedbytheinputcapacitorbankinsteadoftheinputinductor.Soforafullloadswingbetween0Aand14A,thevoltageseenbytheinputinductoris∆V=14Ax15mΩ=210mV.Usethefollowingequationtodeterminetheminimuminductancevalue:
where(di/dt)MAXisthemaximumallowableinputcurrentslewrate,whichis0.1A/µsinthecaseofthePentiumIIpowersupply.Sotheinputinductorsize,accordingtotheaboveequation,shouldbe2.1µH.
DYNAMICPOSITIONINGOFLOADVOLTAGE
SincetheIntelVRMspecificationshavedefinedtwooperat-ingwindowsfortheMPUcorevoltage,onebeingthesteadystatewindowandtheotherthetransientwindow,itisagoodideatodynamicallypositionthesteadystateoutputvoltageinthesteadystatewindowwithrespecttoloadcurrentlevelsothattheoutputvoltagehasmoreheadroomforloadtransientresponse.Thisrequiresinformationabouttheloadcurrent.ThereareatleasttwosimplewaystoimplementthisideawithLM2636.OneistoutilizetheoutputinductorDCresistance,seeFigure7.TheaveragevoltageacrosstheoutputinductorisactuallythatacrossitsDCresistance.Thataveragevoltageisproportionaltoloadcurrent.
SincetheswitchingnodevoltageVAbouncesbetweentheinputvoltageandgroundattheswitchingfrequency,itisimpossibletochoosepointAasthefeedbackpoint,other-wisethedynamicperformancewillsufferandthesystemmayhavesomenoiseproblems.Usingalowpassfilternetworkaroundtheinductor,suchastheoneshowninthefigure,seemstobeagoodidea.ThefeedbackpointisC.
whereVO_RIPisthepeak-to-peakoutputripplevoltage,fistheswitchingfrequency.ForcommonlyusedlowRDSONMOSFETs,areasonableswitchingfrequencyis300kHz.Assumeanoutputpeak-peakripplevoltageof18mVistobeguaranteed,thetotaloutputcapacitorESRis9mΩ,theinputvoltageis5V,andoutputvoltageis2.8V.Theinductancevalueaccordingtotheaboveequationwillthenbe2µH.Thehighestslewrateoftheinductorcurrentwhentheloadchangesfromnoloadtofullloadcanbedeterminedasfollows:
whereDMAXisthemaximumalloweddutycycle,whichisaround0.9forLM2636.Foraloadtransientfrom0Ato14A,thehighestcurrentslewrateoftheinductor,accordingtotheaboveequation,is0.85A/µs,andthereforetheshortestpos-sibletotalrecoverytimeis14A/(0.85A/µs)=16.5µs.Noticethattheoutputvoltagestartstorecoverwhenevertheinduc-torstartstosupplycurrent.
Thehighestslewrateoftheinductorcurrentwhentheloadchangesfromfullloadtonoloadcanbedeterminedfromthesameequation,butuseDMINinsteadofDMAX.
SincetheDMINofLM2636at300kHzis0%,theslewrateistherefore−1.4A/µs.Sotheapproximatetotalrecoverytimewillbe14A/(1.4A/µs)=10µs.
Theinputinductorisforlimitingtheinputcurrentslewrateduringaloadtransient.InthecasethatlowESRaluminumelectrolyticcapacitorsareusedfortheinputcapacitorbank,voltagechangeduetocapacitorcharging/dischargingisusu-allynegligibleforthefirst20µs.ESRisbyfarthedominantfactorindeterminingtheamountofcapacitorvoltageundershoot/overshootduetoloadtransient.Sotheworstcaseiswhentheloadchangesbetweennoloadandfullload,underwhichconditiontheinputinductorseesthehigh-estvoltagechangeacrosstheinputcapacitors.Assumetheinputcapacitorbankismadeupofthree16MV820GX,i.e.,
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10083426
FIGURE7.DynamicVoltagePositioningbyUtilizing
OutputInductorDCResistanceSinceattheswitchingfrequencytheimpedanceofthe0.1µFismuchlessthan5kΩ,thebouncingvoltageatpointAwillbemainlyappliedacrosstheresistor5kΩ,andpointCwillbemuchquieterthanA.However,VCBaverageisstillthemajorityofVABaverage,becauseoftheresistordivider.SoinsteadystateVC=IOxrL+VCORE,whererListheinductorDCresistance.Soatnoload,outputvoltageisequaltoVC,andatfullload,outputvoltageisIOxrLlowerthanVC.Tofurtherutilizethesteadystatewindow,aresistorcanbe
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LM2636ApplicationsInformation
(Continued)
connectedbetweentheFBpinandgroundtoincreasethenoloadoutputvoltagetoclosetotheupperlimitofthewindow.
10083428
FIGURE9.VREFUsedinanN-FETLDOSupplyPCBLAYOUTCONSIDERATIONSThereareseveralpointstoconsider.1.
10083427
FIGURE8.DynamicVoltagePositioningbyUsingA
Stand-AloneResistorApossibledrawbackoftheschemeinFigure7isslowtransientrecoveryspeed.Sincethe5kΩresistorandthe0.1µFcapacitorhavealargetimeconstant,thesettlingofpointCtoitssteadystatevalueduringaloadtransientmaytakeafewmilliseconds.Dependingontheinteractionbetweenthecompensationnetworkandthe0.1µFcapacitor,Vcoremaytakedifferentroutestoreachitssteadystatevalue.Thisisundesiredwhentheloadtransientshappensmorethan1000timespersecond.ReducingthetimeconstantwillresultinamorefluctuatingVCduetoalesseffectivelowpassfilter.Finetuningtheparametersmaybalancethetradeoffs.Anotherwaytoimplementthedynamicvoltagepositioningisthroughtheuseofastand-aloneresistor,suchasthe4mΩresistorinFigure8above.Theadvantageofthisimplemen-tationoverthepreviousoneisamuchfasterspeedofVCOREfromtransientleveltosteadystatelevel.Thedisadvantageislessefficiency.Thetotalpowerlosscanbe0.78Wat14Aofloadcurrent.ThecostoftheresistorcanbeminimizedbyimplementingitthroughaPCBtrace.
REFERENCEVOLTAGE
TheVREFpincanhavemanyuses,suchasinthewatchdogcircuitryandinanLDOcontroller.Figure9showsanappli-cationwhereVREFisusedtobuildaN-FETLDOcontroller.Anappropriatecompensationnetworkisnecessarytotailorthedynamicperformanceofthewholepowersupply.
2.
3.
4.5.6.
7.
Trytouse2oz.copperforthegroundplaneiftightloadregulationisdesired.Inthecaseofdynamicvoltagepositioning,thismaynotbeaconcernbecausethelooseloadregulationisdesiredanyway.However,donotforgettotakeintoconsiderationthevoltagedropcausedbythegroundplanewhencalculatingdynamicvoltagepositioningparameters.
Trytokeepgatetracesshort.However,donotmakethemtooshortorelsetheLM2636maystaytooclosetotheMOSFETsandgetheatedupbythem.Forthesamereason,donotusewidetraces,10miltracesshouldbeenough.
Whennotemployingdynamicvoltagepositioning,placethefeedbackpointattheVRMconnectorpinstohaveatightloadregulation.Ifitisanon-boardpowersupply,placethefeedbackpointatSlotIconnectororwhereverisclosesttotheMPU.
StartcomponentplacementwiththepowerdevicessuchasMOSFETsandinductors.
DonotplacetheLM2636directlyunderneaththeMOS-FETswhenwhensurfacemountMOSFETsareused.Ifpossible,keepthecapacitorssomedistanceawayfromtheinductorssothatthecapacitorswillhavealowertemperatureenvironment.
WhenimplementingdynamicvoltagepositioningthroughaPCBtrace,beawarethatthePCBtraceisaheatsourceandtrytoavoidplacingthetracedirectlyunderneaththeLM2636.
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LM2636PhysicalDimensions
unlessotherwisenoted
inches(millimeters)
20-LeadPlasticSOPackageOrderNumberLM2636MNSPackageNumberM20B
20-LeadPlasticTSSOP(MTC)OrderNumberLM2636MTCNSPackageNumberMTC20
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LM26365-BitProgrammableSynchronousBuckRegulatorControllerNotes
Nationaldoesnotassumeanyresponsibilityforuseofanycircuitrydescribed,nocircuitpatentlicensesareimpliedandNationalreservestherightatanytimewithoutnoticetochangesaidcircuitryandspecifications.Forthemostcurrentproductinformationvisitusatwww.national.com.LIFESUPPORTPOLICY
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2.Acriticalcomponentisanycomponentofalifesupportdeviceorsystemwhosefailuretoperformcanbereasonablyexpectedtocausethefailureofthelifesupportdeviceorsystem,ortoaffectitssafetyoreffectiveness.
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