LM2716MT资料

LM2716Dual(Step-UpandStep-Down)PWMDC/DCConverterNovember2005

LM2716

Dual(Step-UpandStep-Down)PWMDC/DCConverter

GeneralDescription

TheLM2716iscomposedoftwoPWMDC/DCconverters.Abuck(step-down)converterisusedtogenerateafixedout-putvoltageof3.3V.Aboost(step-up)converterisusedtogenerateanadjustableoutputvoltage.Bothconvertersfea-turelowRDSON(0.16Ωand0.12Ω)internalswitchesformaximumefficiency.Operatingfrequencycanbeadjustedanywherebetween300kHzand600kHzallowingtheuseofsmallexternalcomponents.Externalsoft-startpinsforeachenablestheusertotailorthesoft-starttimestoaspecificapplication.Eachconvertermayalsobeshutdownindepen-dentlywithitsownshutdownpin.TheLM2716isavailableinalowprofile24-leadTSSOPpackage.

Features

nFixed3.3Vbuckconverterwitha1.8A,0.16Ω,internalswitch

nAdjustableboostconverterwitha3.6A,0.12Ω,internalswitch

nAdjustableboostoutputvoltageupto20VnOperatinginputvoltagerangeof4Vto20VnInputundervoltageprotection

n300kHzto600kHzpinadjustableoperatingfrequencynOvertemperatureprotectionnSmall24-LeadTSSOPpackagenPatentedcurrentlimitcircuitry

Applications

nnnn

TFT-LCDDisplaysHandheldDevicesPortableApplications

CellularPhones/DigitalCamers

TypicalApplicationCircuit

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LM2716ConnectionDiagram

TopView

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24-LeadTSSOP

OrderingInformation

OrderNumberLM2716MTLM2716MTX

PackageTypeTSSOP-24TSSOP-24

NSCPackageDrawing

MTC24MTC24

SuppliedAs

61Units,Rail

2500Units,TapeandReel

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LM2716PinDescriptions

Pin123456789101112131415161718192021222324

NamePGNDFB1VC1VBGSS2VC2FB2AGNDAGNDPGNDPGNDPGNDSW2SW2SW2VINVINSHDN2FSLCTSS1SHDN1CB1VINSW1

Buckoutputvoltagefeedbackinput.

Buckcompensationnetworkconnection.Connectedtotheoutputofthevoltageerroramplifier.

Bandgapconnection.Boostsoftstartpin.

Boostcompensationnetworkconnection.Connectedtotheoutputofthevoltageerroramplifier.

Boostoutputvoltagefeedbackinput.

Analogground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.Analogground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.Powerground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.Powerground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.Powerground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.Boostpowerswitchinput.SwitchconnectedbetweenSW2pinsandPGNDpins.SW2pinsshouldbeconnecteddirectlytogetheratthedevice.

Boostpowerswitchinput.SwitchconnectedbetweenSW2pinsandPGNDpins.SW2pinsshouldbeconnecteddirectlytogetheratthedevice.

Boostpowerswitchinput.SwitchconnectedbetweenSW2pinsandPGNDpins.SW2pinsshouldbeconnecteddirectlytogetheratthedevice.

Analogpowerinput.VINpinsmustbeconnectedtogetherdirectlyattheDUT.Analogpowerinput.VINpinsmustbeconnectedtogetherdirectlyattheDUT.ShutdownpinforBoostconverter.Activelow.

Switchingfrequencyselectinput.Usearesistortosetthefrequencyanywherebetween300kHzand600kHz.Bucksoftstartpin.

ShutdownpinforBuckconverter.Activelow.Buckconverterbootstrapcapacitorconnection.

Analogpowerinput.VINpinsmustbeconnectedtogetherdirectlyattheDUT.Buckpowerswitchinput.SwitchconnectedbetweenVINpinsandSW1pin.

Function

Powerground.AGNDandPGNDpinsmustbeconnectedtogetherdirectlyatthepart.

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LM2716BlockDiagram

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LM2716AbsoluteMaximumRatings(Note1)

IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.VINSW1VoltageSW2VoltageFB1VoltageFB2VoltageVC1VoltageVC2VoltageSHDN1VoltageSHDN2VoltageSS1VoltageSS2VoltageFSLCTVoltageCB1Voltage

MaximumJunctionTemperature

−0.3Vto22V−0.3Vto22V−0.3Vto22V−0.3Vto7V−0.3Vto7V

1.75V≤VC1≤2.25V0.965V≤VC2≤1.565V

−0.3Vto7.5V−0.3Vto7.5V−0.3Vto2.1V−0.3Vto0.6VAGNDto5V

VIN+7V(VIN=VSW)

150˚C

PowerDissipation(Note2)LeadTemperatureVaporPhase(60sec.)Infrared(15sec.)ESDSusceptibility(Note3)HumanBodyModelMachineModel

InternallyLimited

300˚C215˚C220˚C2kV200V

OperatingConditions

OperatingJunctionTemperatureRange(Note4)

StorageTemperatureSupplyVoltageSW1VoltageSW2Voltage

−40˚Cto+125˚C−65˚Cto+150˚C

4Vto20V

20V20V

ElectricalCharacteristics

SpecificationsinstandardtypefaceareforTJ=25˚CandthosewithboldfacetypeapplyoverthefullOperatingTempera-tureRange(TJ=−40˚Cto+125˚C)Unlessotherwisespecified.VIN=5VandIL=0A,unlessotherwisespecified.SymbolIQParameter

Conditions

Min(Note4)

Typ(Note5)2.849

1.235

95%DutyCycle(Note7)95%DutyCycle(Note7)VFB1=3.3VVFB2=1.265V

4

∆I=20µA∆I=5µA

1200175100135

90

RF=47.5kΩRF=22.6kΩ

ISHDN1ISHDN2IL1IL2RDSON1RDSON2BuckShutdownPinCurrentBoostShutdownPinCurrentBoostSwitchLeakageCurrent

BuckSwitchRDSONBoostSwitchRDSON0V250500−5−5

0.20.216012095300600

983507005553

1.261.83.66527

755520Max(Note4)3.54.5151.285

UnitsmAmAµAVAAµAnAVµmhoµmhoV/VV/V%kHzkHzµAµAµAµAmΩmΩ

TotalQuiescentCurrent(bothNotSwitchingswitchers)Switching,switchopen

VSHDN=0V

BandgapVoltage

VBGICL1(Note6)BuckSwitchCurrentLimitICL2(Note6)BoostSwitchCurrentLimitIFB1IFB2VINgm1gm2AV1AV2DMAXFSWBuckFBPinBiasCurrent(Note8)

BoostFBPinBiasCurrent(Note8)

InputVoltageRangeBuckErrorAmpTransconductanceBoostErrorAmpTransconductance

BuckErrorAmpVoltageGainBoostErrorAmpVoltageGain

MaximumDutyCycleSwitchingFrequency

BuckSwitchLeakageCurrentVDS=20V

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LM2716ElectricalCharacteristics

SymbolThSHDN1ThSHDN2ISS1ISS2UVPθJAParameter

BuckSHDNThresholdBoostSHDNThresholdBuckSoftStartPinCurrentBoostSoftStartPinCurrentOnThresholdOffThresholdThermalResistance(Note9)

(Continued)

SpecificationsinstandardtypefaceareforTJ=25˚CandthosewithboldfacetypeapplyoverthefullOperatingTempera-tureRange(TJ=−40˚Cto+125˚C)Unlessotherwisespecified.VIN=5VandIL=0A,unlessotherwisespecified.

Conditions

OutputHighOutputLowOutputHighOutputLow

0.86153.353.10

TSSOP,packageonly

0.8Min(Note4)

Typ(Note5)1.371.351.371.359.5193.83.6115

12224.03.92Max(Note4)

2

UnitsVVµAµAV˚C/W

Note1:Absolutemaximumratingsarelimitsbeyondwhichdamagetothedevicemayoccur.OperatingRatingsareconditionsforwhichthedeviceisintendedtobefunctional,butdeviceparameterspecificationsmaynotbeguaranteed.Forguaranteedspecificationsandtestconditions,seetheElectricalCharacteristics.Note2:Themaximumallowablepowerdissipationisafunctionofthemaximumjunctiontemperature,TJ(MAX),thejunction-to-ambientthermalresistance,θJA,andtheambienttemperature,TA.SeetheElectricalCharacteristicstableforthethermalresistance.Themaximumallowablepowerdissipationatanyambienttemperatureiscalculatedusing:PD(MAX)=(TJ(MAX)−TA)/θJA.Exceedingthemaximumallowablepowerdissipationwillcauseexcessivedietemperature,andtheregulatorwillgointothermalshutdown.

Note3:Thehumanbodymodelisa100pFcapacitordischargedthrougha1.5kΩresistorintoeachpin.Themachinemodelisa200pFcapacitordischargeddirectlyintoeachpin.

Note4:Alllimitsguaranteedatroomtemperature(standardtypeface)andattemperatureextremes(boldtypeface).Allroomtemperaturelimitsare100%testedorguaranteedthroughstatisticalanalysis.AlllimitsattemperatureextremesareguaranteedviacorrelationusingstandardStatisticalQualityControl(SQC)methods.AlllimitsareusedtocalculateAverageOutgoingQualityLevel(AOQL).Note5:Typicalnumbersareat25˚Candrepresentthemostlikelynorm.Note6:Dutycycleaffectscurrentlimitduetorampgenerator.Note7:Currentlimitat95%dutycycle.Note8:BiascurrentflowsintoFBpin.

Note9:RefertoNational’spackagingwebsiteformoredetailedthermalinformationandmountingtechniquesfortheTSSOPpackage.

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LM2716TypicalPerformanceCharacteristics

SwitchingFrequencyvs.RFResistor

SwitchingFrequencyvs.InputVoltage

(FSW=300kHz)

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SwitchingFrequencyvs.InputVoltage

(FSW=600kHz)BuckEfficiencyvs.LoadCurrent

(FSW=300kHz)

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BuckEfficiencyvs.LoadCurrent

(FSW=600kHz)BoostEfficiencyvs.LoadCurrent

(FSW=300kHz)

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LM2716TypicalPerformanceCharacteristics

BoostEfficiencyvs.LoadCurrent

(FSW=600kHz)

(Continued)

BoostSwitchRDSONvs.InputVoltage

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LM2716BuckOperation

PROTECTION(BOTHREGULATORS)

TheLM2716hasdedicatedprotectioncircuitryrunningdur-ingnormaloperationtoprotecttheIC.TheThermalShut-downcircuitryturnsoffthepowerdeviceswhenthedietemperaturereachesexcessivelevels.TheUVPcomparatorprotectsthepowerdevicesduringsupplypowerstartupandshutdowntopreventoperationatvoltageslessthantheminimuminputvoltage.TheOVPcomparatorisusedtopreventtheoutputvoltagefromrisingatnoloadsallowingfullPWMoperationoverallloadconditions.TheLM2716alsofeaturesashutdownmodeforeachconverterdecreas-ingthesupplycurrentto9µA(bothinshutdownmode).CONTINUOUSCONDUCTIONMODE

TheLM2716containsacurrent-mode,PWMbuckregulator.Abuckregulatorstepstheinputvoltagedowntoaloweroutputvoltage.Incontinuousconductionmode(whentheinductorcurrentneverreacheszeroatsteadystate),thebuckregulatoroperatesintwocycles.ThepowerswitchisconnectedbetweenVINandSW1.

Inthefirstcycleofoperationthetransistorisclosedandthediodeisreversebiased.EnergyiscollectedintheinductorandtheloadcurrentissuppliedbyCOUTandtherisingcurrentthroughtheinductor.

Duringthesecondcyclethetransistorisopenandthediodeisforwardbiasedduetothefactthattheinductorcurrentcannotinstantaneouslychangedirection.Theenergystoredintheinductoristransferredtotheloadandoutputcapacitor.Theratioofthesetwocyclesdeterminestheoutputvoltage.Theoutputvoltageisdefinedapproximatelyas:

changing)loadconditions.Forhigheroutputcurrentapplica-tionsordynamicloadconditionsa68µFto100µFlowESRcapacitorisrecommended.Itisalsorecommendedtoputasmallceramiccapacitor(0.1µF)betweentheinputpinandgroundpintoreducehighfrequencyspikes.

INDUCTORSELECTION

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

Ahighervalueofripplecurrentreducesinductance,butincreasestheconductanceloss,coreloss,currentstressfortheinductorandswitchdevices.Italsorequiresabiggeroutputcapacitorforthesameoutputvoltageripplerequire-ment.Areasonablevalueissettingtheripplecurrenttobe30%oftheDCoutputcurrent.Sincetheripplecurrentin-creaseswiththeinputvoltage,themaximuminputvoltageisalwaysusedtodeterminetheinductance.TheDCresistanceoftheinductorisakeyparameterfortheefficiency.LowerDCresistanceisavailablewithabiggerwindingarea.Agoodtradeoffbetweentheefficiencyandthecoresizeislettingtheinductorcopperlossequal2%oftheoutputpower.OUTPUTCAPACITOR

TheselectionofCOUTisdrivenbythemaximumallowableoutputvoltageripple.Theoutputrippleintheconstantfre-quency,PWMmodeisapproximatedby:

whereDisthedutycycleoftheswitch,DandD'willberequiredfordesigncalculations.

DESIGNPROCEDURE

Thissectionpresentsguidelinesforselectingexternalcom-ponents.

INPUTCAPACITOR

AlowESRaluminum,tantalum,orceramiccapacitorisneededbetwentheinputpinandpowerground.Thiscapaci-torpreventslargevoltagetransientsfromappearingattheinput.ThecapacitorisselectedbasedontheRMScurrentandvoltagerequirements.TheRMScurrentisgivenby:

TheESRtermusuallyplaysthedominantroleindeterminingthevoltageripple.AlowESRaluminumelectrolyticortanta-lumcapacitor(suchasNichiconPLseries,SanyoOS-CON,Sprague593D,594D,AVXTPS,andCDEpolymeralumi-num)isrecommended.Anelectrolyticcapacitorisnotrec-ommendedfortemperaturesbelow−25˚CsinceitsESRrisesdramaticallyatcoldtemperature.AtantalumcapacitorhasamuchbetterESRspecificationatcoldtemperatureandispreferredforlowtemperatureapplications.

BOOTCAPACITOR

A3.3nForlargerceramiccapacitorisrecommendedforthebootstrapcapacitor.

SOFT-STARTCAPACITOR(BOTHREGULATORS)

TheSSpinsareusedtotailorthesoft-startforaspecificapplication.Acurrentsourcechargestheexternalsoft-startcapacitor,CSS.Thesoft-starttimecanbeestimatedas:

TSS=CSS*0.6V/ISSSoft-starttimesmaybeimplementedusingtheSSpinandacapacitorCSS.

Whenprogrammingthesoftstarttime,simplyusetheequa-tiongivenintheSoft-StartCapacitorsectionabove.Thisequationusesthetypicalroomtemperaturevalueofthesoftstartcurrenttosetthesoftstarttime.

TheRMScurrentreachesitsmaximum(IOUT/2)whenVINequals2VOUT.Thisvalueshouldbeincreasedby50%toaccountfortheripplecurrentincreaseduetotheboostregulator.Foranaluminumorceramiccapacitor,thevoltageratingshouldbeatleast25%higherthanthemaximuminputvoltage.Ifatantalumcapacitorisused,thevoltageratingrequiredisabouttwicethemaximuminputvoltage.Thetantalumcapacitorshouldbesurgecurrenttestedbythemanufacturertopreventbeingshortedbytheinrushcurrent.Theminimumcapacitorvalueshouldbe47µFforloweroutputloadcurrentapplicationsandlessdynamic(quickly

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LM2716BuckOperation

(Continued)

COMPENSATIONCOMPONENTS

Inthecontroltooutputtransferfunction,thefirstpoleFP1canbeestimatedas1/(2πROUTCOUT);TheESRzeroFZ1oftheoutputcapacitoris1/(2πESRCOUT);Also,thereisahighfrequencypoleFP2intherangeof45kHzto150kHz:

FP2=FSW/(πn(1−D))

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

ThetotalloopgainGisapproximately500/IOUTwhereIOUTisinamperes.

AGmamplifierisusedinsidetheLM2716.Theoutputresis-torRooftheGmamplifierisabout85kΩ.CC1andRC1togetherwithRogivealagcompensationtorolloffthegain:

FPC1=1/(2πCC1(Ro+RC1)),FZC1=1/2πCC1RC1.

Insomeapplications,theESRzeroFZ1cannotbecancelledbyFP2.Then,CC3isneededtointroduceFPC2tocanceltheESRzero,FP2=1/(2πCC3Ro\\RC1).

Theruleofthumbistohavemorethan45˚phasemarginatthecrossoverfrequency(G=1).SCHOTTKYDIODE

ThebreakdownvoltageratingofD1ispreferredtobe25%higherthanthemaximuminputvoltage.SinceD1isonlyonforashortperiodoftime,theaveragecurrentratingforD1onlyrequiresbeinghigherthan30%ofthemaximumoutputcurrent.

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LM2716BoostOperation

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FIGURE1.SimplifiedBoostConverterDiagram

(a)FirstCycleofOperation(b)SecondCycleOfOperation

CONTINUOUSCONDUCTIONMODE

TheLM2716containsacurrent-mode,PWMboostregulator.Aboostregulatorstepstheinputvoltageuptoahigheroutputvoltage.Incontinuousconductionmode(whentheinductorcurrentneverreacheszeroatsteadystate),theboostregulatoroperatesintwocycles.

Inthefirstcycleofoperation,showninFigure1(a),thetransistorisclosedandthediodeisreversebiased.EnergyiscollectedintheinductorandtheloadcurrentissuppliedbyCOUT.

ThesecondcycleisshowninFigure1(b).Duringthiscycle,thetransistorisopenandthediodeisforwardbiased.Theenergystoredintheinductoristransferredtotheloadandoutputcapacitor.

Theratioofthesetwocyclesdeterminestheoutputvoltage.Theoutputvoltageisdefinedapproximatelyas:

INTRODUCTIONTOCOMPENSATION

whereDisthedutycycleoftheswitch,DandD'willberequiredfordesigncalculations.

SETTINGTHEOUTPUTVOLTAGE

TheoutputvoltageissetusingthefeedbackpinandaresistordividerconnectedtotheoutputasshowninFigure3.Thefeedbackpinvoltageis1.26V,sotheratioofthefeed-backresistorssetstheoutputvoltageaccordingtothefol-lowingequation:

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FIGURE2.(a)Inductorcurrent.(b)Diodecurrent.

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LM2716BoostOperation

(Continued)

TheLM2716hasacurrentmodePWMboostconverter.Thesignalflowofthiscontrolschemehastwofeedbackloops,onethatsensesswitchcurrentandonethatsensesoutputvoltage.

Tokeepacurrentprogrammedcontrolconverterstableabovedutycyclesof50%,theinductormustmeetcertaincriteria.Theinductor,alongwithinputandoutputvoltage,willdeterminetheslopeofthecurrentthroughtheinductor(seeFigure2(a)).Iftheslopeoftheinductorcurrentistoogreat,thecircuitwillbeunstableabovedutycyclesof50%.Ifthedutycycleisapproachingnear85%uptothemaximumof95%,itmaybenecessarytoincreasetheinductancebyasmuchas2X.SeeInductorandDiodeSelectionformoredetailedinductorsizing.

TheLM2716providesacompensationpin(VC2)tocustom-izethevoltageloopfeedback.ItisrecommendedthataseriescombinationofRC2andCC2beusedforthecompen-sationnetwork,asshowninFigure3.Foranygivenappli-cation,thereexistsauniquecombinationofRC2andCC2thatwilloptimizetheperformanceoftheLM2716circuitintermsofitstransientresponse.TheseriescombinationofRC2andCC2introducesapole-zeropairaccordingtothefollowingequations:

whereFSWistheswitchingfrequency,Disthedutycycle,andRDSONistheONresistanceoftheinternalswitchtakenfromthegraph\"BoostSwitchRDSONvs.InputVoltage\"intheTypicalPerformanceCharacteristicssection.Thisequationisonlygoodfordutycyclesgreaterthan50%(D>0.5),fordutycycleslessthan50%therecommendedvaluesmaybeused.ThecorrespondinginductorcurrentrippleasshowninFigure2(a)isgivenby:

whereROistheoutputimpedanceoftheerroramplifier,approximately850kΩ.Formostapplications,performancecanbeoptimizedbychoosingvalueswithintherange5kΩ≤RC2≤20kΩ(RC2canbeupto200kΩifCC4isused,seeHighOutputCapacitorESRCompensation)and680pF≤CC2≤4.7nF.RefertotheApplicationsInformationsectionforrecommendedvaluesforspecificcircuitsandconditions.RefertotheCompensationsectionforotherdesignrequire-ment.

COMPENSATION

Thissectionwillpresentageneraldesignproceduretohelpinsureastableandoperationalcircuit.Thedesignsinthisdatasheetareoptimizedforparticularrequirements.Ifdiffer-entconversionsarerequired,someofthecomponentsmayneedtobechangedtoensurestability.Belowisasetofgeneralguidelinesindesigningastablecircuitforcontinu-ousconductionoperation(loadsgreaterthanapproximately100mA),inmostallcasesthiswillprovideforstabilityduringdiscontinuousoperationaswell.Thepowercomponentsandtheireffectswillbedeterminedfirst,thenthecompensationcomponentswillbechosentoproducestability.

INDUCTORANDDIODESELECTION

Althoughtheinductorsizesmentionedearlierarefineformostapplications,amoreexactvaluecanbecalculated.Toensurestabilityatdutycyclesabove50%,theinductormusthavesomeminimumvaluedeterminedbytheminimuminputvoltageandthemaximumoutputvoltage.Thisequa-tionis:

Theinductorripplecurrentisimportantforafewreasons.Onereasonisbecausethepeakswitchcurrentwillbetheaverageinductorcurrent(inputcurrentorILOAD/D’)plus∆iL.Asasidenote,discontinuousoperationoccurswhentheinductorcurrentfallstozeroduringaswitchingcycle,or∆iLisgreaterthantheaverageinductorcurrent.Therefore,con-tinuousconductionmodeoccurswhen∆iLislessthantheaverageinductorcurrent.Caremustbetakentomakesurethattheswitchwillnotreachitscurrentlimitduringnormaloperation.Theinductormustalsobesizedaccordingly.Itshouldhaveasaturationcurrentratinghigherthanthepeakinductorcurrentexpected.Theoutputvoltagerippleisalsoaffectedbythetotalripplecurrent.

Theoutputdiodeforaboostregulatormustbechosencorrectlydependingontheoutputvoltageandtheoutputcurrent.Thetypicalcurrentwaveformforthediodeincon-tinuousconductionmodeisshowninFigure2(b).Thediodemustberatedforareversevoltageequaltoorgreaterthantheoutputvoltageused.Theaveragecurrentratingmustbegreaterthanthemaximumloadcurrentexpected,andthepeakcurrentratingmustbegreaterthanthepeakinductorcurrent.Duringshortcircuittesting,orifshortcircuitcondi-tionsarepossibleintheapplication,thediodecurrentratingmustexceedtheswitchcurrentlimit.UsingSchottkydiodeswithlowerforwardvoltagedropwilldecreasepowerdissipa-tionandincreaseefficiency.

DCGAINANDOPEN-LOOPGAIN

Sincethecontrolstageoftheconverterformsacompletefeedbackloopwiththepowercomponents,itformsaclosed-loopsystemthatmustbestabilizedtoavoidpositivefeed-backandinstability.Avalueforopen-loopDCgainwillberequired,fromwhichyoucancalculate,orplace,polesandzerostodeterminethecrossoverfrequencyandthephasemargin.Ahighphasemargin(greaterthan45˚)isdesiredforthebeststabilityandtransientresponse.ForthepurposeofstabilizingtheLM2716,choosingacrossoverpointwellbe-lowwheretherighthalfplanezeroislocatedwillensuresufficientphasemargin.AdiscussionoftherighthalfplanezeroandcheckingthecrossoverusingtheDCgainwillfollow.

OUTPUTCAPACITORSELECTION

Thechoiceofoutputcapacitorsissomewhatarbitraryanddependsonthedesignrequirementsforoutputvoltageripple.ItisrecommendedthatlowESR(EquivalentSeries

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LM2716BoostOperation

(Continued)

Resistance,denotedRESR)capacitorsbeusedsuchasceramic,polymerelectrolytic,orlowESRtantalum.HigherESRcapacitorsmaybeusedbutwillrequiremorecompen-sationwhichwillbeexplainedlateroninthesection.TheESRisalsoimportantbecauseitdeterminesthepeaktopeakoutputvoltagerippleaccordingtotheapproximateequation:

∆VOUT)2∆iLRESR(inVolts)Aminimumvalueof10µFisrecommendedandmaybeincreasedtoalargervalue.Afterchoosingtheoutputcapaci-toryoucandetermineapole-zeropairintroducedintothecontrolloopbythefollowingequations:

zeroisnotexact.DeterminetherangeoffP1overtheex-pectedloadsandthensetthezerofZCtoapointapproxi-matelyinthemiddle.Thefrequencyofthiszeroisdeter-minedby:

WhereRListheminimumloadresistancecorrespondingtothemaximumloadcurrent.ThezerocreatedbytheESRoftheoutputcapacitorisgenerallyveryhighfrequencyiftheESRissmall.IflowESRcapacitorsareuseditcanbeneglected.IfhigherESRcapacitorsareusedseetheHighOutputCapacitorESRCompensationsection.

RIGHTHALFPLANEZERO

Acurrentmodecontrolboostregulatorhasaninherentrighthalfplanezero(RHPzero).Thiszerohastheeffectofazerointhegainplot,causinganimposed+20dB/decadeontherolloff,buthastheeffectofapoleinthephase,subtractinganother90˚inthephaseplot.Thiscancauseundesirableeffectsifthecontrolloopisinfluencedbythiszero.ToensuretheRHPzerodoesnotcauseinstabilityissues,thecontrolloopshouldbedesignedtohaveabandwidthoflessthan1⁄2thefrequencyoftheRHPzero.Thiszerooccursatafre-quencyof:

NowRC2canbechosenwiththeselectedvalueforCC2.ChecktomakesurethatthepolefPCisstillinthe10Hzto500Hzrange,changeeachvalueslightlyifneededtoensurebothcomponentvaluesareintherecommendedrange.Aftercheckingthedesignattheendofthissection,thesevaluescanbechangedalittlemoretooptimizeperformanceifdesired.Thisisbestdoneinthelabonabench,checkingtheloadstepresponsewithdifferentvaluesuntiltheringingandovershootontheoutputvoltageattheedgeoftheloadstepsisminimal.Thisshouldproduceastable,highperformancecircuit.Forimprovedtransientresponse,highervaluesofRC2shouldbechosen.Thiswillimprovetheoverallband-widthwhichmakestheregulatorrespondmorequicklytotransients.Ifmoredetailisrequired,orthemostoptimalperformanceisdesired,refertoamoreindepthdiscussionofcompensatingcurrentmodeDC/DCswitchingregulators.HIGHOUTPUTCAPACITORESRCOMPENSATION

WhenusinganoutputcapacitorwithahighESRvalue,orjusttoimprovetheoverallphasemarginofthecontrolloop,anotherpolemaybeintroducedtocancelthezerocreatedbytheESR.Thisisaccomplishedbyaddinganothercapaci-tor,CC4,directlyfromthecompensationpinVC2toground,inparallelwiththeseriescombinationofRC2andCC2.ThepoleshouldbeplacedatthesamefrequencyasfZ1,theESRzero.Theequationforthispolefollows:

Toensurethisequationisvalid,andthatCC4canbeusedwithoutnegativelyimpactingtheeffectsofRC2andCC2,fPC4mustbegreaterthan10fZC.

CHECKINGTHEDESIGN

Thefinalstepistocheckthedesign.Thisistoensureabandwidthof1⁄2orlessofthefrequencyoftheRHPzero.Thisisdonebycalculatingtheopen-loopDCgain,ADC.Afterthisvalueisknown,youcancalculatethecrossovervisuallybyplacinga−20dB/decadeslopeateachpole,anda+20dB/decadeslopeforeachzero.Thepointatwhichthegainplotcrossesunitygain,or0dB,isthecrossoverfrequency.Ifthecrossoverfrequencyislessthan1⁄2theRHPzero,thephasemarginshouldbehighenoughforstability.Thephasemar-gincanalsobeimprovedbyaddingCC4asdiscussedearlierinthesection.TheequationforADCisgivenbelowwithadditionalequationsrequiredforthecalculation:

whereILOADisthemaximumloadcurrentandD’corre-spondstotheminimuminputvoltage.

SELECTINGTHECOMPENSATIONCOMPONENTS

ThefirststepinselectingthecompensationcomponentsRC2andCC2istosetadominantlowfrequencypoleinthecontrolloop.SimplychoosevaluesforRC2andCC2withintherangesgivenintheIntroductiontoCompensationsectiontosetthispoleintheareaof10Hzto500Hz.Thefrequencyofthepolecreatedisdeterminedbytheequation:

whereROistheoutputimpedanceoftheerroramplifier,approximately850kΩ.SinceRC2isgenerallymuchlessthanRO,itdoesnothavemucheffectontheaboveequationandcanbeneglecteduntilavalueischosentosetthezerofZC.fZCiscreatedtocanceloutthepolecreatedbytheoutputcapacitor,fP1.Theoutputcapacitorpolewillshiftwithdiffer-entloadcurrentsasshownbytheequation,sosettingthe

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LM2716BoostOperation

(Continued)

mc)0.072FSW(inV/s)

whereRListheminimumloadresistance,VINisthemini-muminputvoltage,gmistheerroramplifiertransconduc-tancefoundintheElectricalCharacteristicstable,andRD-SONisthevaluechosenfromthegraph\"RDSON2vs.VIN\"intheTypicalPerformanceCharacteristicssection.

LAYOUTCONSIDERATIONS

TheLM2716usestwoseparategroundconnections,PGNDforthedriversandboostNMOSpowerdeviceandAGNDforthesensitiveanalogcontrolcircuitry.TheAGNDandPGNDpinsshouldbetieddirectlytogetheratthepackage.Thefeedbackandcompensationnetworksshouldbeconnected

directlytoadedicatedanaloggroundplaneandthisgroundplanemustconnecttotheAGNDpin.Ifnoanaloggroundplaneisavailablethenthegroundconnectionsofthefeed-backandcompensationnetworksmusttiedirectlytotheAGNDpin.ConnectingthesenetworkstothePGNDcaninjectnoiseintothesystemandeffectperformance.

TheinputbypasscapacitorCIN,asshowninFigure3,mustbeplacedclosetotheIC.ThiswillreducecoppertraceresistancewhicheffectsinputvoltagerippleoftheIC.Foradditionalinputvoltagefiltering,a100nFbypasscapacitorcanbeplacedinparallelwithCIN,closetotheVINpin,toshuntanyhighfrequencynoisetoground.Theoutputca-pacitors,COUT1andCOUT2,shouldalsobeplacedclosetotheIC.AnycoppertraceconnectionsfortheCOUTXcapaci-torscanincreasetheseriesresistance,whichdirectlyeffectsoutputvoltageripple.Thefeedbacknetwork,resistorsRFB1andRFB2,shouldbekeptclosetotheFBpin,andawayfromtheinductor,tominimizecoppertraceconnectionsthatcaninjectnoiseintothesystem.Traceconnectionsmadetotheinductorsandschottkydiodesshouldbeminimizedtore-ducepowerdissipationandincreaseoverallefficiency.SeeFigure3,Figure4,andFigure5foragoodexampleofproperlayout.FormoredetailonswitchingpowersupplylayoutconsiderationsseeApplicationNoteAN-1149:LayoutGuidelinesforSwitchingPowerSupplies.

ApplicationInformation

SomeRecommendedInductors(othersmaybeused)

ManufacturerCoilcraftCoiltronicsPulseSumida

Inductor

DO3316andDO5022seriesDRQ73andCD1seriesP0751andP0762seriesCDRH8D28andCDRH8D43series

ContactInformationwww.coilcraft.comwww.cooperet.comwww.pulseeng.comwww.sumida.com

SomeRecommendedInputandOutputCapacitors(othersmaybeused)

ManufacturerVishaySpragueTaiyoYudenCornellDubilierPanasonic

Capacitor

293D,592D,and595DseriestantalumHighcapacitanceMLCCceramicESRDseriecPolymerAluminumElectrolytic

SPVandAFKseriesV-chipseries

HighcapacitanceMLCCceramic

EEJ-Lseriestantalum

ContactInformationwww.vishay.comwww.t-yuden.comwww.cde.comwww.panasonic.com

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LM2716ApplicationInformation

(Continued)

20071257

FIGURE3.15V,3.3VOutputApplication

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LM2716ApplicationInformation

(Continued)

20071258

FIGURE4.PCBLayout,Top

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LM2716ApplicationInformation

(Continued)

20071259

FIGURE5.PCBLayout,Bottom

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元器件交易网www.cecb2b.com

LM2716Dual(Step-UpandStep-Down)PWMDC/DCConverterPhysicalDimensions

inches(millimeters)unlessotherwisenoted

TSSOP-24PinPackage(MTC)

ForOrdering,RefertoOrderingInformationTable

NSPackageNumberMTC24

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