LM2655_05资料

LM26552.5AHighEfficiencySynchronousSwitchingRegulatorApril2005

LM2655

2.5AHighEfficiencySynchronousSwitchingRegulator

GeneralDescription

TheLM2655isacurrent-modecontrolledPWMstep-downswitchingregulator.Ithastheuniqueabilitytooperateinsynchronousorasynchronousmode.Thisgivesthede-signerflexibilitytochoosebetweenthehighefficiencyofsynchronousoperation,orthelowsolutioncostofasynchro-nousoperation.Alongwithflexibility,theLM2655offershighpowerdensitywiththesmallfootprintofaTSSOP-16pack-age.

Highefficiency(>90%)isobtainedthroughtheuseofaninternallowON-resistance(33mΩ)MOSFET,andanexter-nalN-ChannelMOSFET.Thisfeature,togetherwithitslowquiescentcurrent,makestheLM2655anidealfitinportableapplications.

IntegratedintheLM2655areallthepower,control,anddrivefunctionsforasynchronousoperation.Inaddition,alow-sidedriveroutputallowseasysynchronousoperation.TheICusespatentedcurrentsensingcircuitrythateliminatestheexternalcurrentsensingresistorrequiredbyothercurrent-modeDC-DCconverters.Aprogrammablesoft-startfeaturelimitsstartupcurrentsurgesandprovidesameansofse-quencingmultiplepowersupplies.

Features

nnnnnnnnnnnnn

Ultra-highefficiencyupto96%4Vto14Vinputvoltagerange

Internalhigh-sideMOSFETwithlowRDS(ON)=0.033Ω300kHzfixedfrequencyinternaloscillatorLow-sidedriveforsynchronousoperation

Guaranteedlessthan12µAshutdowncurrentPatentedcurrentsensingforcurrentmodecontrolProgrammablesoft-startInputundervoltagelockout

OutputovervoltageshutdownprotectionOutputundervoltageshutdownprotectionThermalShutdown

16-pinTSSOPpackage

Applications

nnnnn

HarddiskdrivesInternetappliancesTFTmonitors

ComputerperipheralsBatterypowereddevices

TypicalApplication

10128429

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LM2655ConnectionDiagram

16-LeadTSSOP(MTC)

10128403

TopView

OrderNumberLM2655MTC-ADJSeeNSPackageNumberMTC16

BlockDiagram

10128404

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LM2655PinDescription

Pin1-23-5678

NameSWPVINVCBAVINSD(SS)Function

Switched-nodeconnection,whichisconnectedtothesourceoftheinternalhigh-sideMOSFET.

Mainpowersupplyinputpin.Connectedtothedrainoftheinternalhigh-sideMOSFET.Bootstrapcapacitorconnectionforhigh-sidegatedrive.Inputvoltageforcontrolanddrivecircuits.

ShutdownandSoft-startcontrolpin.Pullingthispinbelow0.3Vshutsofftheregulator.Acapacitorconnectedfromthispintogroundprovidesacontrolrampoftheinputcurrent.Donotdrivethispinwithanexternalsourceorerroneousoperationmayresult.Outputvoltagefeedbackinput.Connectedtotheoutputvoltage.

Compensationnetworkconnection.Connectedtotheoutputofthevoltageerroramplifier.Acapacitorbetweenthispintogroundsetsthedelayfromwhentheoutputvoltagereaches80%ofitsnominaltowhentheundervoltagelatchprotectionisenabled.Low-sideFETgatedrivepin.Powerground.

Mainpowersupplyinputpin.Connectedtothedrainoftheinternalhigh-sideMOSFET.

91011121314-16

FBCOMPLDELAYLDRGNDPVINOrderingInformation

Suppliedas1000units

TapeandReel

LM2655MTC-3.3LM2655MTC-ADJ

Suppliedas3000units,

TapeandReel

LM2655MTCX-3.3LM2655MTCX-ADJ

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LM2655AbsoluteMaximumRatings(Note1)

IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.SupplyVoltage(PVIN)SupplyVoltage(AVIN)FeedbackPinVoltageVCBVoltage,(Note7)CSSVoltageCompVoltageLDELAYVoltageLDRVoltageVSW,(Note8)

PowerDissipation(TA=25˚C),(Note2)

3.8V≤VIN≤14V4.0V≤VIN≤14V-0.4V≤VFB≤5V

7V2.5V2.5V2.5V5V14V

TSSOP-16PackageθJAPowerDissapationLeadTemperatureVaporPhase(60sec.)Infrared(15sec.)ESDSusceptibility(Note3)HumanBodyModel(Note4)MachineModel

140˚C/W3mW215˚C220˚C1kV200V

OperatingRatings(Note1)

StorageTemperatureRangeJunctionTemperatureRange

−65˚C≤TJ≤+150˚C−40˚C≤TJ≤+125˚C

LM2655-3.3ElectricalCharacteristics

SpecificationswithstandardtypefaceareforTJ=25˚C,andthoseinboldfacetypeapplyoverfullOperatingTemperatureRange.VIN=10Vunlessotherwisespecified.SymbolVOUTParameter

OutputVoltage

Conditions

ILOAD=1.5A

Typical(Note5)3.3

3.235/3.1853.392/3.416

VOUTOutputVoltageLineRegulation

OutputVoltageLoadRegulation

VINUVVUV_HYSTICL(Note9)

VINUndervoltageLockoutThresholdVoltageHysteresisfortheInputUndervoltageLockoutAverageOutputCurrentLimit

VIN=5VVOUT=3.3VVIN=5Vto14VILOAD=1.5A

ILOAD=100mAto2.5AVIN=10VRisingEdge

0.5

0.7

0.6

1.7

3.8

3.95

2103.3

Limit(Note6)

UnitsVV(min)V(max)%%(max)%%(max)VV(max)mV

LM2655-ADJElectricalCharacteristics

SpecificationswithstandardtypefaceareforTJ=25˚C,andthoseinboldfacetypeapplyoverfullOperatingTemperatureRange.VIN=10Vunlessotherwisespecified.SymbolVFBParameter

FeedbackVoltage

Conditions

ILOAD=1.5A

Typical(Note5)1.238

1.208/1.1811.260/1.267

VOUTOutputVoltageLineRegulation

OutputVoltageLoadRegulation

VINUVVUV_HYSTICL(Note9)

VINUndervoltageLockoutThresholdVoltageHysteresisfortheInputUndervoltageLockoutAverageOutputCurrentLimit

VIN=5VVOUT=3.3VVIN=5Vto14VILOAD=1.5A

ILOAD=100mAto2.5AVIN=10VRisingEdge

0.5

0.7

0.6

1.7

3.8

3.95

2103.3

Limit(Note6)

UnitsVV(min)V(max)%%(max)%%(max)VV(max)mVA

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LM2655AllOutputVoltageVersionsElectricalCharacteristics

SpecificationswithstandardtypefaceareforTJ=25˚C,andthoseinboldfacetypeapplyoverfullOperatingTemperatureRange.VIN=10Vunlessotherwisespecified.SymbolIQParameter

QuiescentCurrent

Conditions

ShutdownPinFloating(DeviceOn)

DeviceNotSwitchingShutdownPinPulledLowISWITCH=1.5AISWITCH=1.5A

Typical(Note6)1.7

3

7

12/20

33

80

RSW(ON)SwitchOnResistance

(MOSFETONResistance+BondingWireResistance)SwitchLeakageCurrentBootstrapRegulatorVoltage

IBOOT=1mACBOOT=tbd

72

Limit(Note5)

UnitsmAmA(max)µAµA(max)mΩmΩ(max)mΩ

IQSDRDS(ON)QuiescentCurrentinShutdownModeSwitchONResistance

ILVBOOT56.7

6.47.0

1250100

VIN=4V,VFB=.9*VOUT,VCOMP=2V

VIN=4V,VFB=1.1*VOUT,VCOMP=2V

VIN=4V,VFB=.9*VOUT,VCOMP=2V

VIN=4V,VFB=.9*VOUT,VCOMP=2V

MeasuredatSwitchPinVIN=4VVIN=4V

VoltageattheSSPin=1.4V

40

32/10

80

53/30

2.70

2.50/2.40

1.25

1.35/1.50

300

280/255330/345

95

92

11

14

81

7684

5108

106114

55

ShutdownPinPulledLowRisingEdge

2.2

3.7/4.0

0.6

0.250.9

165

nAVV(min)V(max)µmho

GMAVIEA_SOURCEIEA_SINKVEAHVEALFOSCErrorAmplifierTransconductance

ErrorAmplifierVoltageGainErrorAmplifierSourceCurrent

ErrorAmplifierSinkCurrentErrorAmplifierOutputSwingUpperLimit

ErrorAmplifierOutputSwingLowerLimit

OscillatorFrequency

µAµA(min)µAµA(min)VV(min)VV(max)kHzkHz(min)kHz(max)%%(min)µAµA(max)%VOUT%VOUT(min)%VOUT(max)%VOUT%VOUT%VOUT(min)%VOUT(max)%VOUTµAµAµA(max)VV(min)V(max)˚C

DMAXISSVOUTUVMaximumDutyCycleSoft-StartCurrent

VOUTUndervoltageLockoutThresholdVoltageHysteresisforVOUTUVVOUTOVVOUTOvervoltageLockoutThresholdVoltageHysteresisforVOUTOVILDELAY__SOURCELDELAYPinSourceCurrentShutdownPinCurrentShutdownPinThresholdVoltage

ThermalShutdownTemperature

5

ISHUTDOWNVSHUTDOWNTSDwww.national.com

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LM2655AllOutputVoltageVersionsElectricalCharacteristics

(Continued)

SpecificationswithstandardtypefaceareforTJ=25˚C,andthoseinboldfacetypeapplyoverfullOperatingTemperatureRange.VIN=10Vunlessotherwisespecified.SymbolTSD_HYSTParameter

ThermalShutdown

HysteresisTemperature

Conditions

Typical(Note6)25

Limit(Note5)

Units˚C

Low-sideDriver(LDR)Parameters

SpecificationswithstandardtypefaceareforTJ=25˚C,andthoseinboldfacetypeapplyoverfullOperatingTemperatureRange.VIN=10Vunlessotherwisespecified.SymbolVOHParameter

LogicHighLevel

VIN=10VVIN=6.0V

VOLISINKISOURCETRRTFLogicLowLevelLDRSinkCurrentLDRSourceCurrentRiseTimeFallTime

LDRVoltage=1VLDRVoltage=2VCGS=1000pFCGS=1000pF

Conditions

Typical(Note5)6.8

6.6

6

5.8

0

0.05

500180187

Limit(Note6)

UnitsVV(min)VV(min)VV(max)mAmAnsns

Note1:AbsoluteMaximumRatingsindicatelimitsbeyondwhichdamagetothedevicemayoccur.OperatingRatingsindicateconditionsforwhichthedeviceisintendedtobefunctional,butdeviceparameterspecificationsmaynotbeguaranteedundertheseconditions.Forguaranteedspecificationsandtestconditions,seetheElectricalCharacteristics.

Note2:ThemaximumallowablepowerdissipationiscalculatedbyusingPDMAX=(TJMAX−TA)/θJA,whereTJMAXisthemaximumjunctiontemperature,TAistheambienttemperature,andθJAisthejunction-to-ambientthermalresistanceofthespecifiedpackage.The3mWratingresultsfromusing150˚C,25˚C,and140˚C/WforTJMAX,TA,andθJArespectively.AθJAof140˚C/Wrepresentstheworst-caseconditionofnoheatsinkingofthe16-pinTSSOPpackage.Heatsinkingallowsthesafedissipationofmorepower.TheAbsoluteMaximumpowerdissipationmustbederatedby7.14mWper˚Cabove25˚Cambient.TheLM2655activelylimitsitsjunctiontemperaturestoabout165˚C.

Note3:Thehumanbodymodelisa100pFcapacitordischargedthrougha1.5kΩresistorintoeachpin.Themachinemodelisa200pFcapacitordischargeddirectlyintoeachpin.

Note4:ESDsusceptibilityusingthehumanbodymodelis500VforVCB,VSW,LDR,andLDELAY.Note5:Typicalnumbersareat25˚Candrepresentthemostlikelynorm.

Note6:Alllimitsguaranteedatroomtemperature(standardtypeface)andattemperatureextremes(boldtypeface).Allroomtemperaturelimitsare100%productiontested.AlllimitsattemperatureextremesareguaranteedviacorrelationusingstandardStatisticalQualityControl(SQC)methods.AlllimitsareusedtocalculateAverageOutgoingQualityLevel(AOQL).Note7:MeasuredwithrespecttoVSW.

Note8:MeasuredwhileswitchinginclosedloopwithVin=15V.

Note9:Averageoutputcurrentlimitobtainedusingtypicalapplicationcircuit.Thisfigureisdependantonthetheinductorused.Note10:Bondwireresistanceaccountsforapproximately40mΩofRSW(ON).

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LM2655TypicalPerformanceCharacteristics

EfficiencyvsLoadCurrent(VIN=5V,VOUT=3.3V)

EfficiencyvsVIN(ILOAD=0.5A)(Synchronous)

10128405

10128406

lQvsVINIQSDvsVIN1012840710128408

IQSDvsJunctionTemperature

FrequencyvsJunctionTemperature

1012840910128410

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LM2655TypicalPerformanceCharacteristics

RSW(ON)+BondWireResistancevsInputVoltage(Note10)(ILOAD=1.5A)

(Continued)

RSW(ON)+BondWireResistancevs

JunctionTemperature(Note10)(ILOAD=1.5A,VIN=5V)

10128411

10128412

CurrentLimitvsInputVoltage

(Synchronous)CurrentLimitvsInputVoltage

(Asynchronous)

1012841310128414

CurrentLimitvsJunctionTemperature

(VIN=5V,VOUT=3.3V)

ReferenceVoltagevsJunctionTemperature

1012841510128416

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LM2655Operation

TheLM2655isaconstantfrequency(300kHz),current-modePWMswitcherthatcanbeoperatedsynchronouslyorasynchronously.

SYNCHRONOUSOPERATION

AconverterissaidtobeinsynchronousoperationwhenaMOSFETisusedinplaceofthecatchdiode.Inthecaseofthebuckconverter,thisMOSFETisknownasthelow-sideMOSFET(theMOSFETconnectedbetweentheinputsourceandthelow-sideMOSFETisthehigh-sideMOS-FET).ConvertersinsynchronousoperationexhibithigherefficienciescomparedtoasynchronousoperationbecausetheI2RlossesarereducedwiththeuseofaMOSFET.OperationoftheLM2655insynchronousmodeisidenticaltoitsoperationinasynchronousmode,exceptthatinternallogicdrivesthelow-sideMOSFET.Atthebeginningofaswitchingcycle,thehigh-sideMOSFETisonandcurrentfromtheinputsourceflowsthroughtheinductorandtotheload.Thecurrentfromthehigh-sideMOSFETissensedandcomparedwiththeoutputoftheerroramplifier(COMPpin).WhenthesensedcurrentreachestheCOMPpinvoltagelevel,thehigh-sideswitchisturnedoff.Aftera30nsdelay(deadtime),thelow-sidedrivergoeshighandturnsthelow-sideMOSFETon.Thecurrentnowflowsthroughthelow-sideMOSFET,throughtheinductorandontotheload.A30nsdelayisnecessarytoinsurethattheMOSFETsareneveronatthesametime.Duringthe30nsdeadtime,thecurrentisforcedtoflowthroughthelow-sideMOSFET’sbodydiode.Itisrecommendedthatalowforwarddropschottkydiodebeplacedinparalleltothelow-sideMOSFETsothatcurrentwillbemoreefficientlyconductedduringthis30nsdeadtime.ThisSchottkydiodeshouldbeplacedwithin5mmoftheswitchpinsothatcurrentlimitisnoteffected(seeExternalSchottkyDiodesection).Attheendoftheswitchingcycle,thelow-sideswitchisturnedoffandafteranother30nsdelay,thecycleisrepeated.

Currentthroughthehigh-sideMOSFETissensedbypat-entedcircuitrythatdoesnotrequireanexternalsenseresis-tor.Asaresult,systemcostandsizearereduced,efficiencyisincreased,andnoiseimmunityofthesensedcurrentisimproved.Afeedforwardfromtheinputvoltageisaddedtoreducethevariationofthecurrentlimitovertheinputvoltagerange.

ASYNCHRONOUSOPERATION

AuniquefeatureoftheLM2655isthatitcanbeoperatedineithersynchronousorasynchronousmode.Whenoperatinginasynchronousmode,asmallamountofefficiencyissac-rificedforalessexpensivesolution.Anydiodemaybeused,butitisrecommendedthatalowforwarddropschottkydiodebeusetomaximizeefficiency.WhenoperatingtheLM2655inasynchronousmode,theLDRpinshouldbeterminatedwithalargeresistor(>1MegΩ),orleftfloating.Operationinasynchronousmodeissimilartothatofsynchronousmode,excepttheinternallow-sideMOSFETlogicisnotused.Atthebeginningofaswitchingcycle,thehigh-sideMOSFETisonandcurrentfromtheinputsourceflowsthroughtheinductorandtotheload.Thecurrentfromthehigh-sideMOSFETissensedandcomparedwiththeoutputoftheerroramplifier(COMPpin).WhenthesensedcurrentreachestheCOMPpinvoltagelevel,thehigh-sideswitchisturnedoff.Atthisinstant,theloadcurrentiscommutatedthroughthecatchdiode.Thecurrentnowflowsthroughthediodeandtheinductorandontotheload.Attheendoftheswitchingcycle,thehigh-sideswitchisturnedonandthecycleisrepeated.

PROTECTIONS

Thepeakcurrentinthesystemismonitoredbycycle-by-cyclecurrentlimitcircuitry.Thiscircuitrywillturnthehigh-sideMOSFEToffwheneverthecurrentthroughthehigh-sideMOSFETreachesapresetlimit(seeplots).Asecondlevelcurrentlimitisaccomplishedbytheundervoltagepro-tection:iftheloadpullstheoutputvoltagedownbelow80%ofitsnominalvalue,theundervoltagelatchprotectionwillwaitforaperiodoftime(setbythecapacitorattheLDELAYpin,seeLDELAYCAPACITORsectionformoreinformation).Iftheoutputvoltageisstillbelow80%ofitsnominalafterthewaitingperiod,thelatchprotectionwillbeenabled.Inthelatchprotectionmode,thelow-sideMOSFETisonandthehigh-sideMOSFETisoff.Thelatchprotectionwillalsobeenabledimmediatelywhenevertheoutputvoltageexceedstheovervoltagethreshold(110%ofitsnominal).Bothpro-tectionsaredisabledduringstart-up.(SeeSOFT-STARTCA-PACITORsectionandLDELAYCAPACITORsectionformoreinformation.)Togglingtheinputsupplyvoltageortheshutdownpincanresetthedevicefromthelatchedprotec-tionmode.

DesignProcedure

Thissectionpresentsguidelinesforselectingexternalcom-ponents.

INPUTCAPACITOR

AlowESRaluminum,tantalum,ceramic,oranyothertypeofcapacitorisneededbetweentheinputpinandpowerground.Thiscapacitorpreventslargevoltagetransientsfromappearingattheinput.ThecapacitorisselectedbasedontheRMScurrentandvoltagerequirements.TheRMScur-rentisgivenby:

voltage.Thetantalumcapacitorshouldbesurgecurrenttestedbythemanufacturertopreventdamagebytheinrushcurrent.Itisalsorecommendedtoputasmallceramiccapacitor(0.1µF)betweentheinputpinandgroundpintoreducehighfrequencynoise.

INDUCTOR

Themostcriticalparametersfortheinductoraretheinduc-tance,peakcurrentandtheDCresistance.Theinductanceisrelatedtothepeak-to-peakinductorripplecurrent,theinputandtheoutputvoltages:

TheRMScurrentreachesitsmaximum(IOUT/2)whenVINequals2VOUT.Foranaluminumorceramiccapacitor,thevoltageratingshouldbeatleast25%higherthanthemaximuminputvoltage.Ifatantalumcapacitorisused,thevoltageratingrequiredisabouttwicethemaximuminput

9

Ahighervalueofripplecurrentreducesinductance,butincreasestheconductanceloss,coreloss,currentstressfortheinductorandswitchdevices.Italsorequiresabiggeroutputcapacitorforthesameoutputvoltageripplerequire-www.national.com

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LM2655DesignProcedure

(Continued)

ment.Areasonablevalueissettingtheripplecurrenttobe30%oftheDCoutputcurrent.Sincetheripplecurrentin-creaseswiththeinputvoltage,themaximuminputvoltageisalwaysusedtodeterminetheinductance.TheDCresistanceoftheinductorisakeyparameterfortheefficiency.LowerDCresistanceisavailablewithabiggerwindingarea.Agoodtradeoffbetweentheefficiencyandthecoresizeislettingtheinductorcopperlossequal2%oftheoutputpower.OUTPUTCAPACITOR

TheselectionofCOUTisprimarilydeterminedbythemaxi-mumallowableoutputvoltageripple.Theoutputrippleintheconstantfrequency,PWMmodeisapproximatedby:

Sprague593D,594D,AVXTPS,andCDEpolymeralumi-num)isrecommended.Anelectrolyticcapacitorisnotrec-ommendedfortemperaturesbelow−25˚CsinceitsESRrisesdramaticallyatcoldtemperature.AtantalumcapacitorhasamuchbetterESRspecificationatcoldtemperatureandispreferredforlowtemperatureapplications.

Theoutputvoltagerippleinconstantfrequencymodehastobelessthanthesleepmodevoltagehysteresistoavoidenteringthesleepmodeatfullload:

VRIPPLE<20mV*VOUT/VFBTheESRtermusuallyplaysthedominantroleindeterminingthevoltageripple.AlowESRaluminumelectrolyticortanta-lumcapacitor(suchasNichiconPLseries,SanyoOS-CON,

10128421

FIGURE1.Low-side/high-sidedrivertimingdiagram.

TABLE1.MOSFETManufacturers

ManufacturerFairchild

SemiconductorGeneral

SemiconductorInternationalRectifierVishaySiliconixZetex

ModelNumberFDC653NGF4420IRF7807Si4812DYSi4874DYZXMN03X

PackageTypeSuperSOT-6

SO-8SO-8SO-8SO-8SO-8

www.zetex.com

(44)161-622-4422

(44)161-622-4420

wwwAddress

Phone

Fax

207-761-6020631-847-3236310-322-3332408-567-95

www.fairchildsemi.com888-522-5372www.gensemi.comwww.irf.comwww.vishay.com

631-847-3000310-322-3331800-554-5565

LOW-SIDEMOSFETSELECTION

Whenoperatinginsynchronousmode,specialattentionshouldbegiventotheselectionofthelow-sideMOSFET.BesideschoosingaMOSFETwithminimalsizeandonresistance,itiscriticalthattheMOSFETmeetcertainriseandfalltimespecifications.A30nsdeadtimebetweenthelow-sideandhigh-sideMOSFETswitchingtransitionsispro-grammedintotheLM2655,asshowninFigure1.Thepre-ventshoot-throughcurrent,thelow-sideMOSFETmustturnoffbeforethehigh-sideMOSFETturnson.Hence,thelow-

sideMOSFEThas30nstoturnofffromthetimethelow-sidedrivergoeslow.Thefalltimeofthelow-sideMOSFETisgovernedbytheequation:IC=CIN*dVC/dt.

whereICistheLDRsinkcurrentcapability,CINistheequiva-lentcapacitanceseenattheLDRpin,andVCisthegate-to-sourcevoltageoftheMOSFET.ICislimitedbythelow-sidedriveroftheLM2655,butCINisfixedbytheMOSFET.Therefore,itisimportantthatthechosenMOSFEThasasuitableCINsothattheLM2655willbeabletoturnitoff

10

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LM2655DesignProcedure

(Continued)

TSS=CSS*0.6V/2µA+CSS*(2V−0.6V)/10µADuringstart-up,theinternalcircuitismonitoringthesoft-startvoltage.Whenthesoftstartvoltagereaches2V,theunder-voltageandovervoltageprotectionsareenabled.

Iftheoutputvoltagedoesn’triseabove80%ofthenormalvaluebeforethesoft-startreaches2V,undervoltageprotec-tionshutdownthedevice.Youcanavoidthisbyeitherincreasingthevalueofthesoft-startcapacitor,orusingaLDELAYcapacitor.LDELAYCAPACITOR

TheLDELAYcapacitor(CDELAY)providesameanstocon-trolundervoltagelatchprotection.BychangingCDELAY,theusercanadjustthetimedelaybetweentheoutputvoltagedroppingbelow80%ofitsnominalvalueandthepartshut-tingoffduetoundervoltagelatchprotection.TheLDELAYcircuitconsistsofa5µAcurrentsourceinserieswithauserdefinedcapacitor,CDELAY.The5µAcurrentsourceisturnedonwhenevertheoutputvoltageisbelow80%ofitsnominalvalue,otherwisethiscurrentsourceisoff.Withtheoutputvoltagebelow80%ofitsnominalvalue,the5µAcurrentsourcebeginstochargeCDELAY,asshowninFig-ure2.IfthepotentialacrossCDELAYreaches2V,undervolt-agelatchprotectionwillbeenabledandthepartwillshut-down.Iftheoutputvoltagerecoverstoabove80%ofitsnominalvaluebeforethepotentialacrossCDELAYreaches2V,undervoltagelatchprotectionwillremaindisabled.Hence,CDELAYsetsatimedelaybythefollowingequation:

TDELAY(ms)=CDELAY(nF)*2V/5A

UndervoltagelatchprotectioncanbedisabledbytyingtheLDELAYpintotheground.

within30ns.Aninputcapacitanceoflessthan1000pFisrecommended.SeveralsuitableMOSFETsareshowninTable1.

EXTERNALSCHOTTKYDIODE(Syncronous)

ASchottkydiodeisrecommendedtopreventtheintrinsicbodydiodeofthelow-sideMOSFETfromconductingduringthedeadtimeinPWMoperation.Ifthebodydiodeturnson,thereisextrapowerdissipationinthebodydiodebecauseofthereverse-recoverycurrentandhigherforwardvoltagedrop.Inaddition,thehigh-sideMOSFEThasmoreswitchinglossbecausethediodereverse-recoverycurrentaddstothehigh-sideMOSFETturn-oncurrent.Theselossesdegradetheefficiencyby1-2%.TheimprovedefficiencyandnoiseimmunitywiththeSchottkydiodebecomemoreobviouswithincreasinginputvoltageandloadcurrent.

ItisimportanttoplacethediodeveryclosetotheswitchpinoftheLM2655.Extraparasiticimpedanceduetothetracebetweentheswitchpinandthecathodeofthediodewillcausethecurrentlimittodecrease.Thebreakdownvoltageratingofthediodeispreferredtobe25%higherthanthemaximuminputvoltage.Sinceitisonforashortperiodoftime,thediode’saveragecurrentratingneedonlybe30%ofthemaximumoutputcurrent.

EXTERNALSCHOTTKYDIODE(Asyncronous)

Inasyncronousmode,theoutputcurrentcommutatesthroughttheschottkydiodewhenthehigh-sideMOSFETisturnedoff.Usingaschottkydiodewithlowforwardvoltagedropwillminimizetheeffeciencylossinthediode.However,toachievethegreatestefficiency,theLM2655shouldbeoperatedinsyncronousmodeusingalow-sideMOSFET.SincetheSchottkydiodeconductsfortheentiresecondhalfofthedutycycleinasyncronousmode,itshouldberatedhigherthanthefullloadcurrent.

BOOSTCAPACITOR

Theboostcapacitorprovidestheextravotageneededtoturnthehigh-side,n-channelMOSFETon.A0.1µFceramiccapacitorisrecommendedfortheboostcapacitor.Thetypi-calvoltageacrosstheboostcapacitoris6.7V.

SOFT-STARTCAPACITOR

Asoft-startcapacitorisusedtoprovidethesoft-startfeature.Whentheinputvoltageisfirstapplied,orwhentheSD(SS)pinisallowedtogohigh,thesoft-startcapacitorischargedbyacurrentsource(approximately2µA).WhentheSD(SS)pinvoltagereaches0.6V(shutdownthreshold),theinternalregulatorcircuitrystartstooperate.Thecurrentchargingthesoft-startcapacitorincreasesfrom2µAtoapproximately10µA.WiththeSD(SS)pinvoltagebetween0.6Vand1.3V,thelevelofthecurrentlimitiszero,whichmeanstheoutputvoltageisstillzero.WhentheSD(SS)pinvoltageincreasesbeyond1.3V,thecurrentlimitstartstoincrease.Theswitchdutycycle,whichiscontrolledbythelevelofthecurrentlimit,startswithnarrowpulsesandgraduallygetswider.Atthesametime,theoutputvoltageoftheconverterincreasestowardsthenominalvalue,whichbringsdowntheoutputvoltageoftheerroramplifier.Whentheoutputoftheerroramplifierislessthanthecurrentlimitvoltage,ittakesoverthecontrolofthedutycycle.Theconverterentersthenormalcurrent-modePWMoperation.TheSD(SS)pinvoltageiseventuallychargeduptoabout2V.

Thesoft-starttimecanbeestimatedas:

11

10128422

FIGURE2.Undervoltagelatchprotection.

COMPENSATIONCOMPONENTS

Inthecontroltooutputtransferfunction,thefirstpoleFp1canbeestimatedas1/(2πROUTCOUT);TheESRzeroFz1oftheoutputcapacitoris1/(2πESRCOUT);Also,thereisahighfrequencypoleFp2intherangeof45kHzto150kHz:

Fp2=Fs/(πn(1−D))

whereD=VOUT/VIN,n=1+0.348L/(VIN−VOUT)(LisinµHsandVINandVOUTinvolts).

ThetotalloopgainGisapproximately1000/IOUTwhereIOUTisinamperes.

AGmamplifierisusedinsidetheLM2655.Theoutputresis-torRooftheGmamplifierisabout80kΩ.Cc1andRCtogetherwithRogivealagcompensationtorolloffthegain:

Fpc1=1/(2πCc1(Ro+Rc)),Fzc1=1/2πCc1Rc.

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LM2655DesignProcedure

(Continued)

ApplicationCircuits

PROGRAMMABLEOUTPUTVOLTAGE

UsingtheadjustableoutputversionoftheLM2655asshowninFigure3,outputvoltagesbetween1.24Vand13Vcanbeachieved.Usethefollowingformulatoselecttheappropriateresistorvalues:

RFB1=RFB2*(VOUT-VREF)/VREFwhereVREF=1.238V.

Selectresistorsbetween10kΩand100kΩ.(1%orhigheraccuracymetalfilmresistorsforRFB1andRFB2.)

Insomeapplications,theESRzeroFz1cannotbecancelledbyFp2.Then,Cc2isneededtointroduceFpc2tocanceltheESRzero,Fp2=1/(2πCc2Ro\\Rc).

Theruleofthumbistohavemorethan45˚phasemarginatthecrossoverfrequency(G=1).

IfCOUTishigherthan68µF,Cc1=2.2nF,andRc=15KΩaregoodchoicesformostapplications.IftheESRzeroistoolowtobecancelledbyFp2,addCc2.

Ifthetransientresponsetoasteploadisimportant,chooseRCtobehigherthan10kΩ.

10128425

FIGURE3.Programmableoutputvoltage.

EXTENDINGINPUTVOLTAGERANGE

Figure4showsawaytoconfiguretheLM2655sothatinputvoltagesoflessthan4Vcanbeconverted.ThiscircuitmakesuseoftheseparateanalogandpowerVINpins.AllthesupervisorycircuitsoftheLM2655arepoweredthroughtheAVINpin,whilethesourcevoltagethatistobeconvertedisinputtothePVINpins.TheinternalcircuitryoftheLM2655hasanoperatingrangeof4Vcapacitorshouldbeconnectedacrossthissource,andasmallbypasscapacitorshouldbeplacedphysicallyclosetotheAVINpintoground.Withalltheinternalcircuitrybeingpoweredbyaseparatesource,theonlyrequirementofthevoltageatPVINisthatitbeslightlyhigher(∼500mV)thanthedesiredoutputvoltage.ThesourceconnectedtoPVINwillalsoneedaninputcapacitorandbypasscapacitor,buttheinputcapacitormustbeselectedfollowingtheguidelinesexplainedintheINPUTCAPACITORsection.

10128423

FIGURE4.Extendedinputvoltagerange.

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LM2655ApplicationCircuits

(Continued)

OBTAININGOUTPUTVOLTAGESOFLESSTHAN1.25VSomeapplicationsrequireoutputvoltageslessthan1.25V.ThecircuitshowninFigure5willallowtheLM2655todosuchaconversion.Byreferencingthetwofeedbackresis-torstoVADJ(VADJ>1.24V),VOUTcanbeadjustedfrom0VtoVADJbytheequation:

VOUT=(VREF-VADJ)*(RFB1+RFB2)/RFB2+VADJwhereVREF=1.24V.VADJcanbeanyvoltagehigherthanVREF(1.24V).InFigure5,VADJisproducedbyanLMV431adjustablereferencefollowingtheequation:

VADJ=1.24*(RADJ1/RADJ2+1).

10128424

FIGURE5.Obtainingoutputvoltagesoflessthan1.25V

PcbLayoutConsiderations

Layoutiscriticaltoreducenoiseandensurespecifiedper-formance.Theimportantguidelinesarelistedasfollows:1.Minimizetheparasiticinductanceintheloopofinput

capacitorsandtheinternalMOSFETsbyconnectingtheinputcapacitorstoVINandPGNDpinswithshortandwidetraces.Thehighfrequencyceramicbypasscapaci-tor,inparticular,shouldbeplacedasclosetoandnomorethan5mmfromtheVINpin.Thisisimportantbecausetherapidlyswitchingcurrent,togetherwithwir-inginductancecangeneratelargevoltagespikesthatmayresultinnoiseproblems.

2.Minimizethetracefromthecenteroftheoutputresistor

dividertotheFBpinandkeepitawayfromnoise

sourcestoavoidnoisepickup.Forapplicationsthatrequiretightregulationattheoutput,adedicatedsensetrace(separatedfromthepowertrace)isrecommendedtoconnectthetopoftheresistordividertotheoutput.3.IftheSchottkydiodeDisused,minimizethetraces

connectingDtoSWandPGNDpins.Useshortandwidetraces.

4.Ifthelow-sideMOSFETisused,minimizethetrace

connectingtheLDRpintothegateoftheMOSFET,andthetracestoSWandPGNDpins.UseshortandwidetracesforthepowertracesgoingfromtheMOSFETtoSWandPGNDpins.

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LM265510128425SchematicfortheTypicalBoardLayoutTypicalPCBoardLayout:(2XSize)10128426ComponentPlacementGuide10128427ComponentSidePCBoardLayoutwww.national.com14

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LM2655TypicalPCBoardLayout:(2XSize)

(Continued)

10128428

SolderSidePCBoardLayout

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LM26552.5AHighEfficiencySynchronousSwitchingRegulatorPhysicalDimensions

unlessotherwisenoted

inches(millimeters)

16-LeadTSSOP(MTC)NSPackageNumberMTC16OrderNumberLM2655MTC-ADJ

LM2655MTCX-ADJLM2655MTC-3.3LM2655MTCX-3.3

SeeOrderingInformationTableForOrderQuantities

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