LM2745/8SynchronizationMay2005
LM2745/8
SynchronousBuckControllerwithPre-biasStartup,andOptionalClockSynchronization
GeneralDescription
Features
TheLM2745/8arehigh-speedsynchronousbuckregulatornSwitchingfrequencyfrom50kHzto1MHz
controllerswithanaccuratefeedbackvoltageaccuracyofnSwitchingfrequencysynchronizerange250kHzto1±1.5%.ItcanprovidesimpledownconversiontooutputMHz(LM2745Only)
voltagesaslowas0.6V.ThoughthecontrolsectionsofthenStartupwithapre-biasedoutputload
ICareratedfor3to6V,thedriversectionsaredesignedtoacceptinputsupplyrailsashighas14V.TheuseofadaptivenPowerstageinputvoltagefrom1Vto14Vnon-overlappingMOSFETgatedrivershelpsavoidpotentialnControlstageinputvoltagefrom3Vto6Vshoot-throughproblemswhilemaintaininghighefficiency.nOutputvoltageadjustabledownto0.6VTheICisdesignedforthemorecost-effectiveoptionofnPowerGoodflagandshutdown
drivingonlyN-channelMOSFETsinboththehigh-sideandnOutputovervoltageandundervoltagedetection
low-sidepositions.Itsensesthelow-sideswitchvoltagedropn±1.5%feedbackvoltageaccuracyovertemperatureforprovidingasimple,adjustablecurrentlimit.
nLow-sideadjustablecurrentsensingTheLM2745/8featuresafixed-frequencyvoltage-modenAdjustablesoft-start
PWMcontrolarchitecturewhichisadjustablefrom50kHztonTrackingandsequencingwithshutdownandsoftstart1MHzwithoneexternalresistor.Inaddition,theLM2745pins
alsoallowstheswitchingfrequencytobesynchronizedtoannTSSOP-14package
externalclocksignalovertherangeof250kHzto1MHz.Thiswiderangeofswitchingfrequencygivesthepowersupplydesignertheflexibilitytomakebettertradeoffsbe-Applications
tweencomponentsize,costandefficiency.
nDownConversionfrom3.3VFeaturesincludetheabilitytostartupwithapre-biasedloadnCableModem,DSLandADSLontheoutput,soft-start,inputundervoltagelockout(UVLO)nLaserJetandInkJetPrintersandPowerGood(basedonbothundervoltageandovervolt-nLowVoltagePowerModulesagedetection).Inaddition,theshutdownpinoftheICcanben
DSP,ASIC,CoreandI/O
usedforprovidingstartupdelay,andthesoft-startpincanbeusedforimplementingprecisetracking,forthepurposeofsequencingwithrespecttoanexternalrail.
TypicalApplication
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SynchronousBuckControllerwithPre-biasStartup,andOptionalClock元器件交易网www.cecb2b.com
LM2745/8ConnectionDiagrams
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14-LeadPlasticTSSOP
θJA=155˚C/W
NSPackageNumberMTC1414-LeadPlasticTSSOP
θJA=155˚C/W
NSPackageNumberMTC14
OrderingInformation
OrderNumberLM2745MTCLM2745MTCXLM2748MTCLM2748MTCX
TSSOP-14
MTC14
PackageType
NSCPackageDrawing
94UnitsonRail
2500UnitsonTapeandReel94UnitsonRail
2500UnitsonTapeandReel
EAO(Pin8)-Outputoftheerroramplifier.Thevoltagelevelonthispiniscomparedwithaninternallygeneratedrampsignaltodeterminethedutycycle.Thispinisnecessaryforcompensatingthecontrolloop.
SS/TRACK(Pin9)-Soft-startandtrackingpin.Thispinisinternallyconnectedtothenon-invertinginputoftheerroramplifierduringsoft-start,andinfactanytimetheSS/TRACKpinvoltagehappenstobebelowtheinternalrefer-encevoltage.Forthebasicsoft-startfunction,acapacitorofminimumvalue1nFisconnectedfromthispintoground.Totracktherisingrampofanotherpowersupply’soutput,con-nectaresistordividerfromtheoutputofthatsupplytothispinasdescribedinApplicationInformation.
FB(Pin10)-Feedbackpin.Thisistheinvertinginputoftheerroramplifier,whichisusedforsensingtheoutputvoltageandcompensatingthecontrolloop.
FREQ/SYNC(Pin11)-Frequencyadjustpin.Theswitchingfrequencyissetbyconnectingaresistorofsuitablevaluebetweenthispinandground.Sometypicalvalues(roundeduptotheneareststandardvalues)are150kΩfor200kHz,100kΩfor300kHz,51.1kΩfor500kHz,18.7kΩfor1MHz.ThispinisalsousedintheLM2745tosynchronizetoanexternalclock.
SD(Pin12)-ICshutdownpin.PullthispintoVCCtoensuretheICisenabled.ConnecttogroundtodisabletheIC.Undershutdown,bothhigh-sideandlow-sidedrivesareoff.Thispinalsofeaturesaprecisionthresholdforpowersupplysequencingpurposes,aswellasalowthresholdtoensureminimalquiescentcurrent.
HG(Pin14)-High-gatedrivepin.Thisisthegatedriveforthehigh-sideN-channelMOSFET.ThissignalisinterlockedwithLG(Pin2)toavoidshoot-through.
SuppliedAs
PinDescription
BOOT(Pin1)-Bootstrappin.Thisisthesupplyrailforthehigh-sidegatedriver.Whenthehigh-sideMOSFETturnson,thevoltageonthispinshouldbeatleastonegatethresholdabovetheregulatorinputvoltageVINtoproperlyturnontheMOSFET.SeeMOSFETGateDriversintheApplicationInformationsectionformoredetailsonhowtoselectMOS-FETs.
LG(Pin2)-Low-gatedrivepin.Thisisthegatedriveforthelow-sideN-channelMOSFET.Thissignalisinterlockedwiththehigh-sidegatedriveHG(Pin14),soastoavoidshoot-through.
PGND(Pins3,13)-Powerground.Thisisalsothegroundforthelow-sideMOSFETdriver.BoththepinsmustbeconnectedtogetheronthePCBandformagroundplane,whichisusuallyalsothesystemground.
SGND(Pin4)-Signalground.Itshouldbeconnectedappropriatelytothegroundplanewithdueregardtogoodlayoutpracticesinswitchingpowerregulatorcircuits.VCC(Pin5)SupplyrailforthecontrolsectionsoftheIC.PWGD(Pin6)-PowerGoodpin.Thisisanopendrainoutput,whichistypicallymeanttobeconnectedtoVCCoranyotherlowvoltagesourcethroughapull-upresistor.Choosethepull-upresistorsothatthecurrentgoingintothispiniskeptbelow1mA.Arecommendedvalueforthepull-upresistoris100kΩformostapplications.Thevoltageonthispinisthuspulledlowunderoutputundervoltageorovervolt-agefaultconditionsandalsounderinputUVLO.
ISEN(Pin7)-Currentlimitthresholdsettingpin.Thissourcesafixed40µAcurrent.Aresistorofappropriatevalueshouldbeconnectedbetweenthispinandthedrainofthelow-sideMOSFET(switchnode).Theminimumvalueforthisresistoris1kΩ.
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LM2745/8AbsoluteMaximumRatings(Note1)
IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.VCCBOOTVoltageISENFREQ/SYNCVoltageAllotherpinsJunctionTemperatureStorageTemperature
-0.3to7V-0.3to18V-0.3to14V
-0.5toVCC+0.3V-0.3toVCC+0.3V
150˚C
−65˚Cto150˚C
SolderingInformation
LeadTemperature(soldering,10sec)InfraredorConvection(20sec)ESDRating(Note3)
260˚C235˚C2kV
OperatingRatings
SupplyVoltageRange,VCC(Note2)BOOTVoltageRange
JunctionTemperatureRange(TJ)ThermalResistance(θJA)
3Vto6V1Vto17V−40˚Cto+125˚C155˚C/W
ElectricalCharacteristics
VCC=3.3Vunlessotherwiseindicated.TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=25˚C.LimitsappearinginboldfacetypeapplyoverfullOperatingTemperatureRange.Datasheetmin/maxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysis.SymbolVFBVONParameter
FBPinVoltageUVLOThresholds
VCCRisingVCCFallingVCC=3.3V,VSD=3.3V
fSW=600kHzVCC=5V,VSD=3.3V
fSW=600kHzVCC=3.3V,VSD=0VVFBRisingVFBFallingVSS=0VVSS=2.0V
7
LM2745LM2748LM2745LM2748
1.11.01.31.0
Conditions
VCC=3Vto6V
Min0.591
Typ0.62.792.421.71.521.8110101090
25
Sourcing
4020
55142.32.1
mA
2.62.63
µAµsµsµAµAµAnA
Max0.609
UnitsVV
IQ_VCCOperatingVCCCurrent
ShutdownVCCCurrent
tPWGD1tPWGD2ISS-ONISS-OCPWGDPinResponseTimePWGDPinResponseTimeSSPinSourceCurrent
SSPinSinkCurrentDuringOverCurrent
ISENPinSourceCurrentTripPoint
FBPinCurrent
ErrorAmplifierUnityGainBandwidth
ErrorAmplifierDCGainErrorAmplifierSlewRateEAOPinCurrentSourcingandSinkingCapability
ErrorAmplifierOutputVoltage
ISEN-THIFBGBWGSRIEAOVEAOERRORAMPLIFIER
911821416
MinimumMaximum
12.2
MHzdBV/µsmAVV
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LM2745/8ElectricalCharacteristics
(Continued)
VCC=3.3Vunlessotherwiseindicated.TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=25˚C.LimitsappearinginboldfacetypeapplyoverfullOperatingTemperatureRange.Datasheetmin/maxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysis.Symbol
Parameter
BOOTPinQuiescentCurrentHigh-SideMOSFETDriverPull-UpONresistanceHigh-SideMOSFETDriverPull-DownONresistance
Conditions
VBOOT=12V,VSD=0
VBOOT=5V@350mASourcing350mASinking
Min
Typ182.70.82.70.8
Max90
UnitsµAΩΩΩΩ
GATEDRIVE
IQ-BOOTRHG_UPRHG_DNRLG_UPRLG_DNOSCILLATOR
RFADJ=750kΩ
PWMFrequency
fSWLM2745ExternalSynchronizingSignalFrequency
SYNCLSYNCHDMAXLM2745SynchronizationSignalLowThreshold
LM2745SynchronizationSignalHighThreshold
MaxHigh-SideDutyCycle
RFADJ=100kΩRFADJ=42.2kΩRFADJ=18.7kΩ
VoltageSwing=0VtoVCCfSW=250kHzto1MHzfSW=250kHzto1MHzfSW=300kHzfSW=600kHzfSW=1MHz
VFB=0.575V,VBOOT=3.3VVSDRising
VFB=0.575V,VBOOT=3.3VVSDFallingVSDRisingVSDFallingVFBFallingVFBRisingVFBFallingVFBRising
0.80.4080.677
0.4340.7106090
0.4570.742
0.232
1.3
2
867867
1.1
250475
503006001000
10001
VV
725
kHz
Low-SideMOSFETDriverPull-Up
VBOOT=5V@350mASourcing
ONresistance
Low-SideMOSFETDriverPull-DownONresistance
350mASinking
%
LOGICINPUTSANDOUTPUTSVSTBY-IHVSTBY-ILVSD-IHVSD-ILVPWGD-TH-LOVPWGD-TH-HIVPWGD-HYSStandbyHighTripPointStandbyLowTripPointSDPinLogicHighTripPointSDPinLogicLowTripPointPWGDPinTripPointsPWGDPinTripPointsPWGDHysteresis
VVVVVVmV
Note1:Absolutemaximumratingsindicatelimitsbeyondwhichdamagetothedevicemayoccur.Operatingratingsindicateconditionsforwhichthedeviceoperatescorrectly.OperatingRatingsdonotimplyguaranteedperformancelimits.
Note2:ThepowerMOSFETscanrunonaseparate1Vto14Vrail(Inputvoltage,VIN).PracticallowerlimitofVINdependsonselectionoftheexternalMOSFET.SeetheMOSFETGATEDRIVERSsectionunderApplicationInformationforfurtherdetails.
Note3:ESDusingthehumanbodymodelwhichisa100pFcapacitordischargedthrougha1.5kΩresistorintoeachpin.
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LM2745/8TypicalPerformanceCharacteristics
Efficiency(VOUT=1.2V)VCC=3.3V,fSW=1MHz
InternalReferenceVoltagevsTemperature
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FrequencyvsTemperatureOutputVoltagevsOutputCurrent
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SwitchWaveforms
VCC=3.3V,VIN=5V,VOUT=1.2VIOUT=3A,CSS=12nF,fSW=1MHzStart-Up(Full-Load)
VCC=3.3V,VIN=5V,VOUT=1.2VIOUT=3A,CSS=12nF,fSW=1MHz
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LM2745/8TypicalPerformanceCharacteristics
Start-Up(No-Load)
VCC=3.3V,VIN=5V,VOUT=1.2V
CSS=12nF,fSW=1MHz
(Continued)
Shutdown(Full-Load)
VCC=3.3V,VIN=5V,VOUT=1.2VIOUT=3A,CSS=12nF,fSW=1MHz
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LoadTransientResponse
VCC=3.3V,VIN=14V,VOUT=1.2V
fSW=1MHzLineTransientResponse(VIN=3Vto9V)
VCC=3.3V,VOUT=1.2VIOUT=2A,fSW=1MHz
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Frequencyvs.FrequencyAdjustResistor
MaximumDutyCyclevsFrequency
VCC=3.3V
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LM2745/8TypicalPerformanceCharacteristics
MaximumDutyCyclevsVCCfSW=600kHz
(Continued)
MaximumDutyCyclevsVCCfSW=1MHz
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LM2745/8BlockDiagram
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ApplicationInformation
TheLM2745/8isavoltage-mode,high-speedsynchronousbuckregulatorwithaPWMcontrolscheme.Itisdesignedforuseinset-topboxes,thinclients,DSL/Cablemodems,andotherapplicationsthatrequirehighefficiencybuckconvert-ers.Ithasoutputshutdown(SD),inputundervoltagelock-out(UVLO)modeandpowergood(PWGD)flag(basedonovervoltageandundervoltagedetection).TheovervoltageandundervoltagesignalsareOR-gatedtodrivethepowergoodsignalandprovidealogicsignaltothesystemiftheoutputvoltagegoesoutofregulation.CurrentlimitisachievedbysensingthevoltageVDSacrossthelowsideMOSFET.TheLM2745/8isalsoabletostart-upwiththeoutputpre-biasedwithaload.TheLM2745alsoallowsfortheswitchingfrequencytobesynchronizedwithanexternalclocksource.
STARTUP/SOFT-START
WhenVCCexceeds2.79Vandtheshutdownpin(SD)seesalogichigh,thesoft-startperiodbegins.Thenaninternal,fixed10µAsourcebeginschargingthesoft-startcapacitor.Duringsoft-startthevoltageonthesoft-startcapacitorCSSisconnectedinternallytothenon-invertinginputoftheerroramplifier.Thesoft-startperiodlastsuntilthevoltageonthesoft-startcapacitorexceedstheLM2745/8referencevoltageof0.6V.Atthispointthereferencevoltagetakesoveratthenon-invertingerroramplifierinput.ThecapacitanceofCSSdeterminesthelengthofthesoft-startperiod,andcanbeapproximatedby:
WhereCSSisinµFandtSSisinms.
DuringsoftstartthePowerGoodflagisforcedlowanditisreleasedwhentheFBpinvoltagereaches70%of0.6V.Atthispointthechipentersnormaloperationmode,andtheoutputovervoltageandundervoltagemonitoringstarts.SETTINGTHEOUTPUTVOLTAGE
TheLM2745/8regulatestheoutputvoltagebycontrollingthedutycycleofthehighsideandlowsideMOSFETs(seeTypicalApplicationCircuit).Theequationgoverningoutputvoltageis:
SETTINGTHESWITCHINGFREQUENCY
Duringfixed-frequencymodeofoperationthePWMfre-quencyisadjustablebetween50kHzand1MHzandissetbyanexternalresistor,RFADJ,betweentheFREQ/SYNCpinandground.Theresistanceneededforadesiredfrequency
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LM2745/8ApplicationInformation
(Continued)
isapproximatedbythecurveFREQUENCYvs.FRE-QUENCYADJUSTRESISTORintheTypicalPerformanceCharacteristicssection.
Whenitisdesiredtosynchronizetheswitchingfrequencywithanexternalclocksource,theLM2745hastheuniqueabilitytosynchronizefromthisexternalsourcewithintherangeof250kHzto1MHz.TheexternalclocksignalshouldbeACcoupledtotheFREQ/SYNCpinasshownbelowinFigure1,wheretheRFADJischosensothatthefixedfre-quencyisapproximatelywithin±30%oftheexternalsyn-chronizingclockfrequency.Aninternalprotectiondiodeclampsthelowlevelofthesynchronizingsignaltoapproxi-mately-0.5V.Theinternalclocksychronizestotherisingedgeoftheexternalclock.
switchingcycle.Figure2showstheSWnode,HG,andLGsignalsduringpre-biasstartup.Thepre-biasedoutputvolt-ageshouldnotexceedVCC+VGSoftheexternalHigh-SideMOSFETtoensurethattheHigh-SideMOSFETwillbeabletoswitchduringstartup.
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FIGURE1.ACCoupledClock
ItisrecommendedtochooseanACcouplingcapacitanceintherangeof50pFto100pF.Exceedingtherecommendedcapacitancemayinjectexcessiveenergythroughtheinter-nalclampingdiodestructurepresentontheFREQ/SYNCpin.
Thetypicaltriplevelofthesynchronizationpinis1.5V.ToensurepropersynchronizationandtoavoiddamagingtheIC,thepeak-to-peakvalue(amplitude)shouldbebetween2.5VandVCC.Theminimumwidthofthispulsemustbegreaterthan100ns,andit’smaximumwidthmustbe100nslessthantheperiodoftheswitchingcycle.
TheexternalclocksynchronizationprocessbeginsoncetheLM2745isenabledandanexternalclocksignalisdetected.Duringtheexternalclocksynchronizationprocesstheinter-nalclockinitiallyswitchesatapproximately1.5MHzanddecreasesuntilithasmatchedtheexternalclock’sfre-quency.Thelock-inperiodisapproximately30µsiftheexternalclockisswitchingat1MHz,andabout100µsiftheexternalclockisat200kHz.Whenthereisnoclocksignalpresent,theLM2745entersintofixed-frequencymodeandbeginsswitchingatthefrequencysetbytheRFADJresistor.Iftheexternalclocksignalisremovedafterfrequencysyn-chronization,theLM2745willenterfixed-frequencymodewithintwoclockcycles.Iftheexternalclockisremovedwithinthe30µslock-inperiod,theLM2745willre-enterfixed-frequencymodewithintwointernalclockcyclesafterthelock-inperiod.
OUTPUTPRE-BIASSTARTUP
Ifthereisapre-biasedloadontheoutputoftheLM2745/8duringstartup,theICwilldisableswitchingofthelow-sideMOSFETandmonitortheSWnodevoltageduringtheoff-timeofthehigh-sideMOSFET.Thereisnoloadcurrentsensingwhileinpre-biasmodebecausethelow-sideMOS-FETneverturnson.TheICwillremaininthispre-biasmodeuntilitseestheSWnodestaysbelow0Vduringtheentirehigh-sideMOSFET’soff-time.OnceitisdeterminedthattheSWnoderemainedbelow0Vduringthehigh-sideoff-time,thelow-sideMOSFETbeginsswitchingduringthenext
9
FIGURE2.OutputPre-BiasModeWaveformsTRACKINGAVOLTAGELEVEL
TheLM2745/8cantracktheoutputofamasterpowersupplyduringsoft-startbyconnectingaresistordividertotheSS/TRACKpin.Inthisway,theoutputvoltageslewrateoftheLM2745/8willbecontrolledbythemastersupplyforloadsthatrequireprecisesequencing.Whenthetrackingfunctionisusednosoft-startcapacitorshouldbeconnectedtotheSS/TRACKpin.Howeverinallothercases,aCSSvalueofatleast1nFbetweenthesoft-startpinandgroundshouldbeused.
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FIGURE3.TrackingCircuit
Onewaytousethetrackingfeatureistodesignthetrackingresistordividersothatthemastersupply’soutputvoltage(VOUT1)andtheLM2745/8’soutputvoltage(representedsymbolicallyinFigure3asVOUT2,i.e.withoutexplicitlyshowingthepowercomponents)bothrisetogetherand
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LM2745/8ApplicationInformation
(Continued)
reachtheirtargetvaluesatthesametime.Forthiscase,theequationgoverningthevaluesofthetrackingdividerresis-torsRT1andRT2is:
ThecurrentthroughRT1shouldbeabout4mAforprecisetracking.ThefinalvoltageoftheSS/TRACKpinshouldbesethigherthanthefeedbackvoltageof0.6V(sayabout0.65Vasintheaboveequation).Ifthemastersupplyvoltagewas5VandtheLM2745/8outputvoltagewas1.8V,forexample,thenthevalueofRT1neededtogivethetwosuppliesidenticalsoft-starttimeswouldbe150Ω.Atimingdiagramfortheequalsoft-starttimecaseisshowninFigure4.
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FIGURE5.TrackingwithEqualSlewRates
SEQUENCING
Thestartup/soft-startoftheLM2745/8canbedelayedforthepurposeofsequencingbyconnectingaresistordividerfromtheoutputofamasterpowersupplytotheSDpin,asshowninFigure6.
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FIGURE4.TrackingwithEqualSoft-StartTimeTRACKINGAVOLTAGESLEWRATE
Thetrackingfeaturecanalternativelybeusednottomakebothrailsreachregulationatthesametimebutrathertohavesimilarriserates(intermsofoutputdV/dt).ThismethodensuresthattheoutputvoltageoftheLM2745/8alwaysreachesregulationbeforetheoutputvoltageofthemastersupply.Inthiscase,thetrackingresistorscanbedeterminedbasedonthefollowingequation:
FIGURE6.SequencingCircuit
AdesireddelaytimetDELAYbetweenthestartupofthemastersupplyoutputvoltageandtheLM2745/8outputvolt-agecanbesetbasedontheSDpinlow-to-highthresholdVSD-IHandtheslewrateofthevoltageattheSDpin,SRSD:
tDELAY=VSD-IH/SRSDNoteagain,thatinFigure6,theLM2745/8’soutputvoltagehasbeenrepresentedsymbolicallyasVOUT2,i.e.withoutexplicitlyshowingthepowercomponents.
VSD-IHistypically1.08VandSRSDistheslewrateoftheSDpinvoltage.ThevaluesofthesequencingdividerresistorsRS1andRS2settheSRSDbasedonthemastersupplyoutputvoltageslewrate,SROUT1,usingthefollowingequa-tion:
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FortheexamplecaseofVOUT1=5VandVOUT2=1.8V,withRT1setto150Ωasbefore,RT2iscalculatedfromtheaboveequationtobe265Ω.AtimingdiagramforthecaseofequalslewratesisshowninFigure5.
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LM2745/8ApplicationInformation
(Continued)
Forexample,ifthemastersupplyoutputvoltageslewratewas1V/msandthedesireddelaytimebetweenthestartupofthemastersupplyandLM2745/8outputvoltagewas5ms,thenthedesiredSDpinslewratewouldbe(1.08V/5ms)=0.216V/ms.DuetotheinternalimpedanceoftheSDpin,themaximumrecommendedvalueforRS2is1kΩ.Toachievethedesiredslewrate,RS1wouldthenbe274Ω.AtimingdiagramforthisexampleisshowninFigure7.
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FIGURE8.SDPinLogic
MOSFETGATEDRIVERS
TheLM2745/8hastwogatedriversdesignedfordrivingN-channelMOSFETsinasynchronousmode.Notethatunlikemostothersynchronouscontrollers,thebootstrapcapacitoroftheLM2745/8providespowernotonlytothedriveroftheupperMOSFET,butthelowerMOSFETdrivertoo(bothdriversaregroundreferenced,i.e.nofloatingdriver).
Twothingsmustbekeptinmindhere.First,theBOOTpinhasanabsolutemaximumratingof18V.Thismustneverbeexceeded,evenmomentarily.SincethebootstrapcapacitorisconnectedtotheSWnode,thepeakvoltageimpressedontheBOOTpinisthesumoftheinputvoltage(VIN)plusthevoltageacrossthebootstrapcapacitor(ignoringanyforwarddropacrossthebootstrapdiode).Thebootstrapcapacitorischargedupbyagivenrail(calledVBOOT_DChere)whenevertheupperMOSFETturnsoff.ThisrailcanbethesameasVCCoritcanbeanyexternalground-referencedDCrail.ButcarehastobeexercisedwhenchoosingthisbootstrapDCrailthattheBOOTpinisnotdamaged.Forexample,ifthedesiredmaximumVINis14V,andVBOOT_DCischosentobethesameasVCC,thenclearlyiftheVCCrailis6V,thepeakvoltageontheBOOTpinis14V+6V=20V.Thisisunac-ceptable,asitisinexcessoftheratingoftheBOOTpin.AVCCof3Vwouldbeacceptableinthiscase.OrtheVINrangemustbereducedaccordingly.ThereisalsotheoptionofderivingthebootstrapDCrailfromanother3Vexternalrail,independentofVCC.
Thesecondthingtobekeptinmindhereisthattheoutputofthelow-sidedriverswingsbetweenthebootstrapDCraillevelofVBOOT_DCandGround,whereastheoutputofthehigh-sidedriverswingsbetweenVIN+VBOOT_DCandGround.Tokeepthehigh-sideMOSFETfullyonwhende-sired,theGatepinvoltageoftheMOSFETmustbehigherthanitsinstantaneousSourcepinvoltagebyanamountequaltothe’Millerplateau’.ItcanbeshownthatthisplateauisequaltothethresholdvoltageofthechosenMOSFETplusasmallamountequaltoIo/g.HereIoisthemaximumloadcurrentoftheapplication,andgisthetransconductanceofthisMOSFET(typicallyabout100forlogic-leveldevices).ThatmeanswemustchooseVBOOT_DCtoatleastexceed
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FIGURE7.DelayforSequencing
SDPINIMPEDANCE
WhenconnectingaresistordividertotheSDpinoftheLM2745/8somecarehastobetaken.OncetheSDvoltagegoesaboveVSD-IH,a17µApull-upcurrentisactivatedasshowninFigure8.Thiscurrentisusedtocreatetheinternalhysteresis()170mV);however,highexternalimpedanceswillaffecttheSDpinlogicthresholdsaswell.TheexternalimpedanceusedforthesequencingdividernetworkshouldpreferablybeasmallfractionoftheimpedanceoftheSDpinforgoodperformance(around1kΩ).
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LM2745/8ApplicationInformation
(Continued)
theMillerplateaulevel.ThismaythereforeaffectthechoiceofthethresholdvoltageoftheexternalMOSFETs,andthatinturnmaydependonthechosenVBOOT_DCrail.
Sofar,inthediscussionabove,theforwarddropacrossthebootstrapdiodehasbeenignored.Butsincethatdoesaffecttheoutputofthedriversomewhat,itisagoodideatoincludethisdropinthefollowingexamples.LookingattheTypicalApplicationschematic,thismeansthatthedifferencevoltageVCC-VD1,whichisthevoltagethebootstrapcapacitorchargesupto,mustalwaysbegreaterthanthemaximumtolerancelimitofthethresholdvoltageoftheupperMOS-FET.HereVD1istheforwardvoltagedropacrosstheboot-strapdiodeD1.Thismayplacerestrictionsontheminimuminputvoltageand/ortypeofMOSFETused.
AbasicbootstrapcircuitcanbebuiltusingoneSchottkydiodeandasmallcapacitor,asshowninFigure9.ThecapacitorCBOOTservestomaintainenoughvoltagebetweenthetopMOSFETgateandsourcetocontrolthedeviceevenwhenthetopMOSFETisonanditssourcehasrisenuptotheinputvoltagelevel.ThechargepumpcircuitryisfedfromVCC,whichcanoperateoverarangefrom3.0Vto6.0V.Usingthisbasicmethodthevoltageappliedtothegatesofbothhigh-sideandlow-sideMOSFETsisVCC-VD.ThismethodworkswellwhenVCCis5V±10%,becausethegatedriveswillgetatleast4.0VofdrivevoltageduringtheworstcaseofVCC-MIN=4.5VandVD-MAX=0.5V.LogiclevelMOSFETsgenerallyspecifytheiron-resistanceatVGS=4.5V.WhenVCC=3.3V±10%,thegatedriveatworstcasecouldgoaslowas2.5V.LogiclevelMOSFETsarenotguaranteedtoturnon,ormayhavemuchhigheron-resistanceat2.5V.Sub-logiclevelMOSFETs,usuallyspeci-fiedatVGS=2.5V,willwork,butaremoreexpensive,andtendtohavehigheron-resistance.ThecircuitinFigure9workswellforinputvoltagesrangingfrom1Vupto14VandVCC=5V±10%,becausethedrivevoltagedependsonlyonVCC.
powersboththeVCCandthebootstrapcircuit,providingefficientdriveforlogiclevelMOSFETs.AnexampleofthiscircuitisshowninFigure10.
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FIGURE10.LM78L05FeedingBasicChargePumpFigure11showsasecondpossibilityforbootstrappingtheMOSFETdrivesusingadoubler.ThiscircuitprovidesanequalvoltagedriveofVCC-3VD+VINtoboththehigh-sideandlow-sideMOSFETdrives.Thismethodshouldonlybeusedincircuitsthatuse3.3VforbothVCCandVIN.EvenwithVIN=VCC=3.0V(10%lowertoleranceon3.3V)andVD=0.5Vbothhigh-sideandlow-sidegateswillhaveatleast4.5Vofdrive.Thepowerdissipationofthegatedrivecir-cuitryisdirectlyproportionaltogatedrivevoltage,hencethethermallimitsoftheLM2745/8ICwillquicklybereachedifthiscircuitisusedwithVCCorVINvoltagesover5V.
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FIGURE11.ChargePumpwithAddedGateDriveAllthegatedrivecircuitsshownintheabovefigurestypicallyuse100nFceramiccapacitorsinthebootstraplocations.
FIGURE9.BasicChargePump(Bootstrap)
NotethattheLM2745/8canbepairedwithalowcostlinearregulatorliketheLM78L05torunfromasingleinputrailbetween6.0and14V.The5Voutputofthelinearregulator
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POWERGOODSIGNAL
(Continued)
TheopendrainoutputonthePowerGoodpinneedsapull-upresistortoalowvoltagesource.Thepull-upresistorshouldbechosensothatthecurrentgoingintothePowerGoodpinislessthan1mA.A100kΩresistorisrecom-mendedformostapplications.
ThePowerGoodsignalisanOR-gatedflagwhichtakesintoaccountbothoutputovervoltageandundervoltagecondi-tions.Ifthefeedbackpin(FB)voltageis18%aboveitsnominalvalue(118%xVFB=0.708V)orfalls28%belowthatvalue(72%xVFB=0.42V)thePowerGoodflaggoeslow.ThePowerGoodflagcanbeusedtosignalothercircuitsthattheoutputvoltagehasfallenoutofregulation,howevertheswitchingoftheLM2745/8continuesregardlessofthestateofthePowerGoodsignal.ThePowerGoodflagwillreturntologichighwheneverthefeedbackpinvoltageisbetween72%and118%of0.6V.
UVLO
The2.79Vturn-onthresholdonVCChasabuiltinhysteresisofabout300mV.IfVCCdropsbelow2.42V,thechipdefi-nitelyentersUVLOmode.UVLOconsistsofturningoffthetopandbottomMOSFETSandremaininginthatconditionuntilVCCrisesabove2.79V.Aswithnormalshutdowniniti-atedbytheSDpin,thesoft-startcapacitorisdischargedthroughaninternalMOSFET,ensuringthatthenextstart-upwillbecontrolledbythesoft-startcircuitry.
CURRENTLIMIT
Currentlimitisrealizedbysensingthevoltageacrossthelow-sideMOSFETwhileitison.TheRDSONoftheMOSFETisaknownvalue;hencethecurrentthroughtheMOSFETcanbedeterminedas:
VDS=IOUTxRDSONThecurrentthroughthelow-sideMOSFETwhileitisonisalsothefallingportionoftheinductorcurrent.Thecurrentlimitthresholdisdeterminedbyanexternalresistor,RCS,connectedbetweentheswitchingnodeandtheISENpin.Aconstantcurrent(ISEN-TH)of40µAtypicalisforcedthroughRCS,causingafixedvoltagedrop.ThisfixedvoltageiscomparedagainstVDSandifthelatterishigher,thecurrentlimitofthechiphasbeenreached.ToobtainamoreaccuratevalueforRCSyoumustconsidertheoperatingvaluesofRDSONandISEN-THattheiroperatingtemperaturesinyourapplicationandtheeffectofslightparameterdifferencesfromparttopart.RCScanbefoundbyusingthefollowingequationusingtheRDSONvalueofthelowsideMOSFETatit’sexpectedhottemperatureandtheabsoluteminimumvalueexpectedoverthefulltemperaturerangeforthefortheISEN-THwhichis25µA:
RCS=RDSON-HOTxILIM/ISEN-THForexample,aconservative15Acurrentlimitina10AdesignwithaRDSON-HOTof10mΩwouldrequirea6kΩresistor.TheminimumvalueforRCSinanyapplicationis1kΩ.Becausecurrentsensingisdoneacrossthelow-sideMOSFET,nominimumhigh-sideon-timeisnecessary.TheLM2745/8enterscurrentlimitmodeiftheinductorcurrentexceedsthecurrentlimitthresholdatthepointwherethehigh-sideMOSFETturnsoffandthelow-sideMOSFETturns
13
on.(Thepointofpeakinductorcurrent,seeFigure12).Notethatinnormaloperationmodethehigh-sideMOSFETal-waysturnsonatthebeginningofaclockcycle.Incurrentlimitmode,bycontrast,thehigh-sideMOSFETon-pulseisskipped.Thiscausesinductorcurrenttofall.Unlikeanormaloperationswitchingcycle,however,inacurrentlimitmodeswitchingcyclethehigh-sideMOSFETwillturnonassoonasinductorcurrenthasfallentothecurrentlimitthreshold.TheLM2745/8willcontinuetoskiphigh-sideMOSFETpulsesuntiltheinductorcurrentpeakisbelowthecurrentlimitthreshold,atwhichpointthesystemresumesnormaloperation.
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FIGURE12.CurrentLimitThreshold
Unlikeahigh-sideMOSFETcurrentsensingscheme,whichlimitsthepeaksofinductorcurrent,low-sidecurrentsensingisonlyallowedtolimitthecurrentduringtheconverteroff-time,wheninductorcurrentisfalling.Thereforeinatypi-calcurrentlimitplotthevalleysarenormallywelldefined,butthepeaksarevariable,accordingtothedutycycle.ThePWMerroramplifierandcomparatorcontroltheoff-pulseofthehigh-sideMOSFET,evenduringcurrentlimitmode,meaningthatpeakinductorcurrentcanexceedthecurrentlimitthreshold.Assumingthattheoutputinductordoesnotsaturate,themaximumpeakinductorcurrentduringcurrentlimitmodecanbecalculatedwiththefollowingequation:
WhereTSWistheinverseofswitchingfrequencyfSW.The200nstermrepresentstheminimumoff-timeofthedutycycle,whichensuresenoughtimeforcorrectoperationofthecurrentsensingcircuitry.
Inordertominimizethetimeperiodinwhichpeakinductorcurrentexceedsthecurrentlimitthreshold,theICalsodis-chargesthesoft-startcapacitorthroughafixed90µAsink.TheoutputoftheLM2745/8internalerroramplifierislimitedbythevoltageonthesoft-startcapacitor.Hence,dischargingthesoft-startcapacitorreducesthemaximumdutycycleDofthecontroller.Duringseverecurrentlimitthisreductionindutycyclewillreducetheoutputvoltageifthecurrentlimitconditionslastforanextendedtime.Outputinductorcurrent
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(Continued)
willbereducedinturntoaflatlevelequaltothecurrentlimitthreshold.Thethirdbenefitofthesoft-startcapacitordis-chargeisasmooth,controlledrampofoutputvoltagewhenthecurrentlimitconditioniscleared.SHUTDOWN
Iftheshutdownpinispulledlow,(below0.8V)theLM2745/8entersshutdownmode,anddischargesthesoft-startcapaci-torthroughaMOSFETswitch.Thehighandlow-sideMOS-FETsareturnedoff.TheLM2745/8remainsinthisstateaslongasVSDseesalogiclow(seetheElectricalCharacter-isticstable).ToassureproperICstart-uptheshutdownpinshouldnotbeleftfloating.FornormaloperationthispinshouldbeconnecteddirectlytoVCCortoanothervoltagebetween1.3VtoVCC(seetheElectricalCharacteristicstable).
DESIGNCONSIDERATIONS
ThefollowingisadesignprocedureforallthecomponentsneededtocreatetheTypicalApplicationCircuitshownonthefrontpage.Thisdesignconverts3.3V(VIN)to1.2V(VOUT)atamaximumloadof4Awithanefficiencyof89%andaswitchingfrequencyof300kHz.Thesameprocedurescanbefollowedtocreatemanyotherdesignswithvaryinginputvoltages,outputvoltages,andloadcurrents.
InputCapacitor
TheinputcapacitorsinaBuckconverteraresubjectedtohighstressduetotheinputcurrenttrapezoidalwaveform.Inputcapacitorsareselectedfortheirripplecurrentcapabil-ityandtheirabilitytowithstandtheheatgeneratedsincethatripplecurrentpassesthroughtheirESR.Inputrmsripplecurrentisapproximately:
canmeanincreasinglossintheMOSFETsduetothecharg-inganddischargingofthegates.Generallytheswitchingfrequencyischosensothatconductionlossoutweighsswitchingloss.Theequationforoutputinductorselectionis:
L=1.6µH
Herewehavepluggedinthevaluesforoutputcurrentripple,inputvoltage,outputvoltage,switchingfrequency,andas-sumeda40%peak-to-peakoutputcurrentripple.Thisyieldsaninductanceof1.6µH.Theoutputinductormustberatedtohandlethepeakcurrent(alsoequaltothepeakswitchcurrent),whichis(IOUT+(0.5x∆IOUT))=4.8A,fora4Adesign.
TheCoilcraftDO3316P-222Pis2.2µH,isratedto7.4Apeak,andhasadirectcurrentresistance(DCR)of12mΩ.AfterselectingtheCoilcraftDO3316P-222Pfortheoutputinductor,actualinductorcurrentrippleshouldbere-calculatedwiththeselectedinductancevalue,asthisinfor-mationisneededtoselecttheoutputcapacitor.Re-arrangingtheequationusedtoselectinductanceyieldsthefollowing:
WheredutycycleD=VOUT/VIN.
Thepowerdissipatedbyeachinputcapacitoris:
VIN(MAX)isassumedtobe10%abovethesteadystateinputvoltage,or3.6VatVIN=3.3V.There-calculatedcurrentripplewillthenbe1.2A.Thisgivesapeakinductor/switchcurrentwillbe4.6A.
OutputCapacitor
TheoutputcapacitorformsthesecondhalfofthepowerstageofaBuckswitchingconverter.Itisusedtocontroltheoutputvoltageripple(∆VOUT)andtosupplyloadcurrentduringfastloadtransients.
Inthisexampletheoutputcurrentis4Aandtheexpectedtypeofcapacitorisanaluminumelectrolytic,aswiththeinputcapacitors.Otherpossibilitiesincludeceramic,tanta-lum,andsolidelectrolytecapacitors,howevertheceramictypeoftendonothavethelargecapacitanceneededtosupplycurrentforloadtransients,andtantalumstendtobemoreexpensivethanaluminumelectrolytic.Aluminumca-pacitorstendtohaveveryhighcapacitanceandfairlylowESR,meaningthattheESRzero,whichaffectssystemstability,willbemuchlowerthantheswitchingfrequency.Thelargecapacitancemeansthatattheswitchingfre-quency,theESRisdominant,hencethetypeandnumberofoutputcapacitorsisselectedonthebasisofESR.OnesimpleformulatofindthemaximumESRbasedonthedesiredoutputvoltageripple,∆VOUTandthedesignedout-putcurrentripple,∆IOUT,is:
wherenisthenumberofparalleledcapacitors,andESRistheequivalentseriesresistanceofeachcapacitor.Theequa-tionaboveindicatesthatpowerlossineachcapacitorde-creasesrapidlyasthenumberofinputcapacitorsincreases.Theworst-caserippleforaBuckconverteroccursduringfullloadandwhenthedutycycle(D)is0.5.Forthis3.3Vto1.2Vdesignthedutycycleis0.364.Fora4Amaximumloadtheripplecurrentis1.92A.
OutputInductor
TheoutputinductorformsthefirsthalfofthepowerstageinaBuckconverter.Itisresponsibleforsmoothingthesquarewavecreatedbytheswitchingactionandforcontrollingtheoutputcurrentripple(∆IOUT).Theinductanceischosenbyselectingbetweentradeoffsinefficiencyandresponsetime.Thesmallertheoutputinductor,themorequicklythecon-vertercanrespondtotransientsintheloadcurrent.How-ever,asshownintheefficiencycalculations,asmallerin-ductorrequiresahigherswitchingfrequencytomaintainthesamelevelofoutputcurrentripple.Anincreaseinfrequency
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LM2745/8ApplicationInformation
(Continued)
Inthisexample,inordertomaintaina2%peak-to-peakoutputvoltagerippleanda40%peak-to-peakinductorcur-rentripple,therequiredmaximumESRis20mΩ.TheSanyo4SP560MelectrolyticcapacitorwillgiveanequivalentESRof14mΩ.Thecapacitanceof560µFisenoughtosupplyenergyeventomeetsevereloadtransientdemands.MOSFETs
SelectionofthepowerMOSFETsisgovernedbyatrade-offbetweencost,size,andefficiency.Onemethodistodeter-minethemaximumcostthatcanbeendured,andthenselectthemostefficientdevicethatfitsthatprice.Breakingdownthelossesinthehigh-sideandlow-sideMOSFETsandthencreatingspreadsheetsisonewaytodeterminerelativeefficienciesbetweendifferentMOSFETs.Goodcorrelationbetweenthepredictionandthebenchresultisnotguaran-teed,however.Single-channelbuckregulatorsthatuseacontrollerICanddiscreteMOSFETstendtobemostefficientforoutputcurrentsof2to10A.
Lossesinthehigh-sideMOSFETcanbebrokendownintoconductionloss,gatechargingloss,andswitchingloss.Conduction,orI2Rloss,isapproximately:
PC=D(IO2xRDSON-HIx1.3)
(High-SideMOSFET)
PC=(1-D)x(IO2xRDSON-LOx1.3)
(Low-SideMOSFET)
Intheaboveequationsthefactor1.3accountsforthein-creaseinMOSFETRDSONduetoheating.Alternatively,the1.3canbeignoredandtheRDSONoftheMOSFETestimatedusingtheRDSONVs.TemperaturecurvesintheMOSFETdatasheets.
GatecharginglossresultsfromthecurrentdrivingthegatecapacitanceofthepowerMOSFETs,andisapproximatedas:
PGC=nx(VDD)xQGxfSWwhere‘n’isthenumberofMOSFETs(ifmultipledeviceshavebeenplacedinparallel),VDDisthedrivingvoltage(seeMOSFETGateDriverssection)andQGSisthegatechargeoftheMOSFET.IfdifferenttypesofMOSFETsareused,the‘n’termcanbeignoredandtheirgatechargessimplysummedtoformacumulativeQG.GatechargelossdiffersfromconductionandswitchinglossesinthattheactualdissipationoccursintheLM2745/8,andnotintheMOSFETitself.
Switchinglossoccursduringthebrieftransitionperiodasthehigh-sideMOSFETturnsonandoff,duringwhichbothcur-rentandvoltagearepresentinthechanneloftheMOSFET.Itcanbeapproximatedas:
PSW=0.5xVINxIOx(tr+tf)xfSWwheretrandtfaretheriseandfalltimesoftheMOSFET.Switchinglossoccursinthehigh-sideMOSFETonly.
Forthisexample,themaximumdrain-to-sourcevoltageap-pliedtoeitherMOSFETis3.6V.Themaximumdrivevoltageatthegateofthehigh-sideMOSFETis3.1V,andthemaxi-mumdrivevoltageforthelow-sideMOSFETis3.3V.Duetothelowdrivevoltagesinthisexample,aMOSFETthatturnsonfullywith3.1Vofgatedriveisneeded.Fordesignsof5Aandunder,dualMOSFETsinSO-8provideagoodtrade-offbetweensize,cost,andefficiency.
SupportComponents
CIN2-Asmall(0.1to1µF)ceramiccapacitorshouldbeplacedascloseaspossibletothedrainofthehigh-sideMOSFETandsourceofthelow-sideMOSFET(dualMOS-FETsmakethiseasy).ThiscapacitorshouldbeX5Rtypedielectricorbetter.
RCC,CCC-ThesearestandardfiltercomponentsdesignedtoensuresmoothDCvoltageforthechipsupply.RCCshouldbe1to10Ω.CCCshould1µF,X5Rtypeorbetter.CBOOT-Bootstrapcapacitor,typically100nF.
RPULL-UP–Thisisastandardpull-upresistorfortheopen-drainpowergoodsignal(PWGD).Therecommendedvalueis100kΩconnectedtoVCC.Ifthisfeatureisnotnecessary,theresistorcanbeomitted.
D1-AsmallSchottkydiodeshouldbeusedforthebootstrap.Itallowsforaminimumdropforbothhighandlow-sidedrivers.TheMBR0520orBAT54workwellinmostdesigns.RCS-Resistorusedtosetthecurrentlimit.Sincethedesigncallsforapeakcurrentmagnitude(IOUT+(0.5x∆IOUT))of4.8A,asafesettingwouldbe6A.(Thisisbelowthesatura-tioncurrentoftheoutputinductor,whichis7A.)FollowingtheequationfromtheCurrentLimitsection,a1.3kΩresistorshouldbeused.
RFADJ-Thisresistorisusedtosettheswitchingfrequencyofthechip.TheresistorvalueisapproximatedfromtheFre-quencyvsFrequencyAdjustResistorcurveintheTypicalPerformanceCharacteristicssection.For300kHzoperation,a100kΩresistorshouldbeused.
CSS-Thesoft-startcapacitordependsontheuserrequire-mentsandiscalculatedbasedontheequationgiveninthesectiontitledSTARTUP/SOFT-START.Therefore,fora7msdelay,a12nFcapacitorissuitable.
ControlLoopCompensation
TheLM2745/8usesvoltage-mode(‘VM’)PWMcontroltocorrectchangesinoutputvoltageduetolineandloadtran-sients.VMrequirescarefulsmallsignalcompensationofthecontrolloopforachievinghighbandwidthandgoodphasemargin.
Thecontrolloopiscomprisedoftwoparts.Thefirstisthepowerstage,whichconsistsofthedutycyclemodulator,outputinductor,outputcapacitor,andload.Thesecondpartistheerroramplifier,whichfortheLM2745/8isa9MHzop-ampusedintheclassicinvertingconfiguration.Figure13showstheregulatorandcontrolloopcomponents.
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(Continued)
a=LCO(RO+RC)
b=L+CO(RORL+RORC+RCRL)c=RO+RL20137464
FIGURE13.PowerStageandErrorAmp
Onepopularmethodforselectingthecompensationcompo-nentsistocreateBodeplotsofgainandphaseforthepowerstageanderroramplifier.Combined,theymaketheoverallbandwidthandphasemarginoftheregulatoreasytosee.SoftwaretoolssuchasExcel,MathCAD,andMatlabareusefulforshowinghowchangesincompensationorthepowerstageaffectsystemgainandphase.
ThepowerstagemodulatorprovidesaDCgainADCthatisequaltotheinputvoltagedividedbythepeak-to-peakvalueofthePWMramp.Thisrampis1.0Vpk-pkfortheLM2745/8.TheinductorandoutputcapacitorcreateadoublepoleatfrequencyfDP,andthecapacitorESRandcapacitancecre-ateasinglezeroatfrequencyfESR.Forthisexample,withVIN=3.3V,thesequantitiesare:
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FIGURE14.PowerStageGainandPhase
Thedoublepoleat4.5kHzcausesthephasetodroptoapproximately-130˚ataround10kHz.TheESRzero,at20.3kHz,providesa+90˚boostthatpreventsthephasefromdroppingto-180o.Ifthisloopwereleftuncompensated,thebandwidthwouldbeapproximately10kHzandthephasemargin53˚.Intheory,theloopwouldbestable,butwouldsufferfrompoorDCregulation(duetothelowDCgain)andwouldbeslowtorespondtoloadtransients(duetothelowbandwidth.)Inpractice,theloopcouldeasilybecomeunstableduetotolerancesintheoutputinductor,capacitor,orchangesinoutputcurrent,orinputvoltage.Therefore,theloopiscompensatedusingtheerroramplifierandafewpassivecomponents.
Forthisexample,aTypeIII,orthree-pole-two-zeroapproachgivesoptimalbandwidthandphase.
Inmostvoltagemodecompensationschemes,includingTypeIII,asinglepoleisplacedattheorigintoboostDCgain
16
IntheequationforfDP,thevariableRListhepowerstageresistance,andrepresentstheinductorDCRplustheonresistanceofthetoppowerMOSFET.ROistheoutputvoltagedividedbyoutputcurrent.ThepowerstagetransferfunctionGPSisgivenbythefollowingequation,andFigure14showsBodeplotsofthephaseandgaininthisexample.
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ashighaspossible.TwozeroesfZ1andfZ2areplacedatthedoublepolefrequencytocancelthedoublepolephaselag.Then,apole,fP1isplacedatthefrequencyoftheESRzero.AfinalpolefP2isplacedatone-halfoftheswitchingfre-quency.Thegainoftheerroramplifiertransferfunctionisselectedtogivethebestbandwidthpossiblewithoutviolat-ingtheNyquiststabilitycriteria.Inpractice,agoodcrossoverpointisone-fifthoftheswitchingfrequency,or60kHzforthisexample.Thegenericequationfortheerroramplifiertransferfunctionis:
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InthisequationthevariableAEAisaratioofthevaluesofthecapacitanceandresistanceofthecompensationcompo-nents,arrangedasshowninFigure13.AEAisselectedtoprovidethedesiredbandwidth.Astartingvalueof80,000forAEAshouldgiveaconservativebandwidth.Increasingthevaluewillincreasethebandwidth,butwillalsodecreasephasemargin.Designswith45-60˚areusuallybestbecausetheyrepresentagoodtrade-offbetweenbandwidthandphasemargin.Ingeneral,phasemarginislowestandgainhighest(worst-case)formaximuminputvoltageandmini-mumoutputcurrent.OnemethodtoselectAEAistouseaniterativeprocessbeginningwiththeseworst-caseconditions.1.IncreaseAEA2.Checkoverallbandwidthandphasemargin
3.ChangeVINtominimumandrecheckoverallbandwidth
andphasemargin
4.ChangeIOtomaximumandrecheckoverallbandwidth
andphasemargin
Theprocessendswhenthebothbandwidthandthephasemarginaresufficientlyhigh.Forthisexampleinputvoltagecanvaryfrom3.0to3.6Vandoutputcurrentcanvaryfrom0to4A,andafterafewiterationsamoderategainfactorof101dBisused.
TheerroramplifieroftheLM2745/8hasaunity-gainband-widthof9MHz.Inordertomodeltheeffectofthislimitation,theopen-loopgaincanbecalculatedas:
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FIGURE15.ErrorAmp.GainandPhase
InVMregulators,thetopfeedbackresistorRFB2formsapartofthecompensation.SettingRFB2to10kΩ±1%,usuallygivesvaluesfortheothercompensationresistorsandca-pacitorsthatfallwithinareasonablerange.(Capacitances>1pF,resistances<1MΩ)CC1,CC2,CC3,RC1,andRC2areselectedtoprovidethepolesandzeroesatthedesiredfrequencies,usingthefollowingequations:
Thenewerroramplifiertransferfunctionthattakesintoaccountunity-gainbandwidthis:
ThegainandphaseoftheerroramplifierareshowninFigure15.
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Inpractice,agoodtradeoffbetweenphasemarginandbandwidthcanbeobtainedbyselectingtheclosest±10%capacitorvaluesabovewhataresuggestedforCC1andCC2,theclosest±10%capacitorvaluebelowthesuggestionforCC3,andtheclosest±1%resistorvaluesbelowthesugges-tionsforRC1,RC2.NotethatifthesuggestedvalueforRC2islessthan100Ω,itshouldbereplacedbyashortcircuit.Followingthisguideline,thecompensationcomponentswillbe:
CC1=27pF±10%,CC2=820pF±10%CC3=2.7nF±10%,RC1=39.2kΩ±1%RC2=2.55kΩ±1%
ThetransferfunctionofthecompensationblockcanbederivedbyconsideringthecompensationcomponentsasimpedanceblocksZFandZIaroundaninvertingop-amp:
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FIGURE16.OverallLoopGainandPhase
Thebandwidthofthisexamplecircuitis59kHz,withaphasemarginof60˚.
EFFICIENCYCALCULATIONS
Thefollowingisasamplecalculation.
AreasonableestimationoftheefficiencyofaswitchingbuckcontrollercanbeobtainedbyaddingtogethertheOutputPower(POUT)lossandtheTotalPower(PTOTAL)loss:
Aswiththegenericequation,GEA-ACTUALmustbemodifiedtotakeintoaccountthelimitedbandwidthoftheerrorampli-fier.Theresultis:
ThetotalcontrollooptransferfunctionHisequaltothepowerstagetransferfunctionmultipliedbytheerroramplifiertransferfunction.
H=GPSxHEAThebandwidthandphasemargincanbereadgraphicallyfromBodeplotsofHEAasshowninFigure16.
TheOutputPower(POUT)fortheTypicalApplicationCircuitdesignis(1.2Vx4A)=4.8W.TheTotalPower(PTOTAL),withanefficiencycalculationtocomplementthedesign,isshownbelow.
ThemajorityofthepowerlossesareduetothelowsideandhighsideMOSFET’slosses.ThelossesinanyMOSFETaregroupofswitching(PSW)andconductionlosses(PCND).
PFET=PSW+PCND=61.38mW+270.42mW
PFET=331.8mW
FETSwitchingLoss(PSW)
PSW=PSW(ON)+PSW(OFF)PSW=0.5xVINxIOUTx(tr+tf)xfSWwww.national.com18
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PSW=0.5x3.3Vx4Ax300kHzx31ns
PSW=61.38mW
TheFDS6898Ahasatypicalturn-onrisetimetrandturn-offfalltimetfof15nsand16ns,respectively.TheswitchinglossesforthistypeofdualN-ChannelMOSFETsare0.061W.
FETConductionLoss(PCND)
PCND=PCND1+PCND2PCND1=I2OUTxRDS(ON)xkxDPCND2=I2OUTxRDS(ON)xkx(1-D)
RDS(ON)=13mΩandthefactorisaconstantvalue(k=1.3)toaccountfortheincreasingRDS(ON)ofaFETduetoheat-ing.
PCND1=(4A)2x13mΩx1.3x0.364PCND2=(4A)2x13mΩx1.3x(1-0.364)PCND=98.42mW+172mW=270.42mWTherearefewadditionallossesthataretakenintoaccount:ICOperatingLoss(PIC)PIC=IQ_VCCxVCC,
whereIQ-VCCisthetypicaloperatingVCCcurrent
PIC=1.7mAx3.3V=5.61mW
FETGateChargingLoss(PGATE)
PGATE=nxVCCxQGSxfSWPGATE=2x3.3Vx3nCx300kHz
PGATE=5.94mW
ThevaluenisthetotalnumberofFETsusedandQGSisthetypicalgate-sourcechargevalue,whichis3nC.FortheFDS6898Athegatecharginglossis5.94mW.InputCapacitorLoss(PCAP)
Herenisthenumberofparalleledcapacitors,ESRistheequivalentseriesresistanceofeach,andPCAPisthedissi-pationineach.Soforexampleifweuseonlyoneinputcapacitorof24mΩ.
PCAP=88.8mW
OutputInductorLoss(PIND)
PIND=I2OUTxDCR
whereDCRistheDCresistance.Therefore,forexample
PIND=(4A)2x11mΩPIND=176mWTotalSystemEfficiency
PTOTAL=PFET+PIC+PGATE+PCAP+PINDwhere,
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LM2745/8ExampleCircuits
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FIGURE17.3.3Vto1.8V@2A,fSW=300kHz
PARTU1Q1D1L1CIN1CO1CCC,CBOOT,CIN2,CO2CC3CSSCC2CC1RFB2RFB1RFADJRC2RCSRCCRC1RPULL-UPCCLKPARTNUMBERLM2745FDS6898AMBR0520LTIDO3316P-47216SP100M6SP220M
VJ1206Y104KXXAVJ0805Y332KXXAVJ0805A123KXAAVJ0805A821KXAAVJ0805A220KXAACRCW08051002FCRCW08054991FCRCW08051003FCRCW08052101FCRCW08052101FCRCW080510R0FCRCW08055492FCRCW08051003JVJ0805A560KXAA
TYPESynchronousControllerDualN-MOSFETSchottkyDiodeInductorAluminumElectrolyticAluminumElectrolyticCapacitorCapacitorCapacitorCapacitorCapacitorResistorResistorResistorResistorResistorResistorResistorResistorCapacitor
10mmx6mm10mmx6mm1206805805805805805805805805805805805805805PACKAGETSSOP-14SO-8SOD-123
4.7µH,4.8Arms18mΩ
100µF,16V,2.89Arms220µF,6.3V3.1Arms0.1µF,10%3300pF,10%12nF,10%820pF10%22pF,10%10.0kΩ1%4.99kΩ1%100kΩ1%2.1kΩ1%2.1kΩ1%10.0Ω1%54.9kΩ1%100kΩ5%56pF,10%
CoilcraftSanyoSanyoVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishay
DESCRIPTION
VENDORNSC
20V,10mΩ@4.5V,Fairchild16nC
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LM2745/8ExampleCircuits
(Continued)
20137433
FIGURE18.5Vto2.5V@2A,fSW=300kHz
PARTU1Q1D1L1CIN1CO1CCC,CBOOT,CIN2,CO2CC3CSSCC2CC1RFB2RFB1RFADJRC2RCSRCCRC1RPULL-UPCCLKPARTNUMBERLM2745FDS6898AMBR0520LTIDO3316P-68216SP100M10SP56M
VJ1206Y104KXXAVJ0805Y182KXXAVJ0805A123KXAAVJ0805A821KXAAVJ0805A330KXAACRCW08051002FCRCW08053161FCRCW08051003FCRCW08051301FCRCW08052101FCRCW080510R0FCRCW08053322FCRCW08051003JVJ0805A560KXAA
TYPESynchronousControllerDualN-MOSFETSchottkyDiodeInductorAluminumElectrolyticAluminumElectrolyticCapacitorCapacitorCapacitorCapacitorCapacitorResistorResistorResistorResistorResistorResistorResistorResistorCapacitor
10mmx6mm6.3mmx6mm1206805805805805805805805805805805805805805PACKAGETSSOP-14SO-8SOD-123
6.8µH,4.4Arms,27mΩCoilcraft100µF,16V,2.89Arms56µF,10V1.7Arms0.1µF,10%1800pF,10%12nF,10%820pF10%33pF,10%10.0kΩ1%3.16kΩ1%100kΩ1%1.3kΩ1%2.1kΩ1%10.0Ω1%33.2kΩ1%100kΩ5%56pF,10%
SanyoSanyoVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishay
20V,10mΩ@4.5V,16nC
DESCRIPTION
VENDORNSCFairchild
21www.national.com
元器件交易网www.cecb2b.com
LM2745/8ExampleCircuits
(Continued)
20137434
FIGURE19.12Vto3.3V@4A,fSW=300kHz
PARTU1Q1D1L1CIN1CO1CCC,CBOOT,CIN2,CO2CC3CSSCC2CC1RFB2RFB1RFADJRC2RCSRCCRC1RPULL-UPCCLKPARTNUMBERLM2745FDS6898AMBR0520LTIDO3316P-33216SP100M6SP220M
VJ1206Y104KXXAVJ0805Y222KXXAVJ0805A123KXAAVJ0805Y332KXXAVJ0805A820KXAACRCW08051002FCRCW08052211FCRCW08051003FCRCW08052611FCRCW08054121FCRCW080510R0FCRCW08051272FCRCW08051003JVJ0805A560KXAA
TYPESynchronousControllerDualN-MOSFETSchottkyDiodeInductorAluminumElectrolyticAluminumElectrolyticCapacitorCapacitorCapacitorCapacitorCapacitorResistorResistorResistorResistorResistorResistorResistorResistorCapacitor
10mmx6mm10mmx6mm1206805805805805805805805805805805805805805PACKAGETSSOP-14SO-8SOD-123
3.3µH,5.4Arms15mΩCoilcraft100µF,16V,2.89Arms220µF,6.3V3.1Arms0.1µF,10%2200pF,10%12nF,10%3300pF10%82pF,10%10.0kΩ1%2.21kΩ1%100kΩ1%2.61kΩ1%4.12kΩ1%10.0Ω1%12.7kΩ1%100kΩ5%56pF,10%
SanyoSanyoVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishayVishay
20V,10mΩ@4.5V,16nC
DESCRIPTION
VENDORNSCFairchild
www.national.com22
元器件交易网www.cecb2b.com
LM2745/8SynchronizationPhysicalDimensions
inches(millimeters)unlessotherwisenoted
TSSOP-14PinPackageNSPackageNumberMTC14
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SynchronousBuckControllerwithPre-biasStartup,andOptionalClock
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