TPS40200-EP中文资料

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WIDE-INPUT-RANGENONSYNCHRONOUSVOLTAGE-MODECONTROLLER

FEATURES

ControlledBaseline

–OneAssembly/TestSite,OneFabricationSite

ExtendedTemperaturePerformanceof–55°Cto125°C

EnhancedDiminishingManufacturingSources(DMS)Support

EnhancedProduct-ChangeNotificationQualificationPedigree(1)

InputVoltageRange4.5Vto52VOutputVoltage(700mVto87%Vin)200-mAInternalP-ChannelFETDriverVoltageFeed-ForwardCompensationUndervoltageLockout

ComponentqualificationinaccordancewithJEDECandindustrystandardstoensurereliableoperationoveran

extendedtemperaturerange.Thisincludes,butisnotlimitedto,HighlyAcceleratedStressTest(HAST)orbiased85/85,temperaturecycle,autoclaveorunbiasedHAST,

electromigration,bondintermetalliclife,andmoldcompoundlife.Suchqualificationtestingshouldnotbeviewedasjustifyinguseofthiscomponentbeyondspecifiedperformanceandenvironmentallimits.

(1)

ProgrammableFixed-Frequency(35kHzto500kHz)Operation

ProgrammableShort-CircuitProtectionHiccupOvercurrentFaultRecoveryProgrammableClosed-LoopSoftStart700-mV1%ReferenceVoltageExternalSynchronization

Small8-PinSmall-OutlineIntegratedCircuit(SOIC)(D)Package

APPLICATIONS

IndustrialControl

DistributedPowerSystemsDSL/CableModemsScannersTelecom

DESCRIPTION

TheTPS40200isaflexiblenonsynchronouscontrollerwithabuilt-in200-mAdriverforP-channelFETs.Thecircuitoperateswithinputsupto52V,withapower-savingfeaturethatturnsoffdrivercurrentoncetheexternalFEThasbeenfullyturnedon.Thisfeatureextendstheflexibilityofthedevice,allowingittooperatewithaninputvoltageupto52V,withoutdissipatingexcessivepower.Thecircuitoperateswithvoltage-modefeedbackandhasfeed-forwardinput-voltagecompensationthatrespondsinstantlytoinput-voltagechange.Theintegral700-mVreferenceistrimmedto2%,providingthemeanstoaccuratelycontrollowvoltages.TheTPS40200isavailableinan8-pinSOIC,andsupportsmanyofthefeaturesofmorecomplexcontrollers.Clockfrequency,softstart,andovercurrentlimiteachareeasilyprogrammedbyasingle,externalcomponent.Theparthasundervoltagelockout,andcanbeeasilysynchronizedtoothercontrollersorasystemclocktosatisfysequencingand/ornoise-reductionrequirements.

Pleasebeawarethatanimportantnoticeconcerningavailability,standardwarranty,anduseincriticalapplicationsofTexasInstrumentssemiconductorproductsanddisclaimerstheretoappearsattheendofthisdatasheet.

PRODUCTIONDATAinformationiscurrentasofpublicationdate.ProductsconformtospecificationsperthetermsoftheTexasInstrumentsstandardwarranty.Productionprocessingdoesnotnecessarilyincludetestingofallparameters.

Copyright©2007,TexasInstrumentsIncorporated

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Thesedeviceshavelimitedbuilt-inESDprotection.TheleadsshouldbeshortedtogetherorthedeviceplacedinconductivefoamduringstorageorhandlingtopreventelectrostaticdamagetotheMOSgates.

V100

90

Efficiency - %

80

70

60

50

0.5

1

1.52

Load Current -A

2.5

3

Figure1.12-Vto5-VBuckConverter

With94%EfficiencyFigure2.TypicalEfficiencyofApplicationCircuit1

(DescribedinApplication1ELECTROSTATICDISCHARGE(ESD)PROTECTION

MIN

Human-BodyModelCDM

MAX15001500

UNITVV

ABSOLUTEMAXIMUMRATINGS(1)

overoperatingfree-airtemperaturerange(unlessotherwisenoted)

UNIT

VDD,ISNS

Inputvoltagerange

RC,FBSS

Outputvoltagerange

TJTstg(1)

COMPGDRV

52–0.3to5.5–0.3to9–0.3to9(VIN–10)toVIN

–55to150–55to150

260

V°C°C°CV

OperatingvirtualjunctiontemperaturerangeStoragetemperaturerange

Leadtemperature1,6mm(1/16in)fromcasefor10s

Stressesbeyondthoselistedunderabsolutemaximumratingsmaycausepermanentdamagetothedevice.Thesearestressratingsonlyandfunctionaloperationofthedeviceattheseoranyotherconditionsbeyondthoseindicatedunderrecommendedoperatingconditionsisnotimplied.Exposuretoabsolute-maximum-ratedconditionsforextendedperiodsmayaffectdevicereliability.

RECOMMENDEDOPERATINGCONDITIONS

MIN

VDD

Inputvoltage

4.5

MAX52

UNITV

2

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ELECTRICALCHARACTERISTICS

–55°C<TA=TJ<125°C,VDD=12V,fOSC=100kHz(unlessotherwisenoted)

PARAMETER

VoltageReference

COMP=FB,TJ=25°C

VFB

Feedbackvoltage

TJ=25°C

4.5V<VDD<52V–40°C<TJ<125°C

–55°C<TJ<125°C

GateDriverIsrcIsnkVGATE

GatedriverpullupcurrentGatedriverpulldowncurrentGatedriveroutputvoltage

VGATE=(VDD–VGDRV),for12V<VDD<52VFOSC=300kHz,Drivernotswitching,4.5V<VDD<52V

3.811065190100

0°C<TJ<125°C

VILIMOCDFVILIM(rst)Oscillator

Oscillatorfrequencyrange(1)

FOSC

OscillatorfrequencyFrequencylineregulation

VRMP

Rampamplitude

RRC=200k ,CRC=470pFRRC=68.1k ,CRC=470pF12V<VDD<52V4.5V<VDD<12V4.5V<VDD<52VVDD=12VVDD=30V

FOSC=100kHz,CL=470pFFOSC=300kHz,CL=470pF

938783585255–9–20

VDD÷10

200100959310

12500200

100300

50011534500

%VkHz

OvercurrentthresholdOvercurrentdutycycle(1)Overcurrentresetthreshold

100

150

4.5V<VDD<52V–40°C<TJ<125°C

–55°C<TJ<125°C

6555351252006

3003008

10

mAmAV

689686679675

696696696696

702703710710

mV

TESTCONDITIONS

MIN

TYP

MAX

UNIT

QuiescentCurrentIqq

Devicequiescentcurrent

1.5

3

mA

UndervoltageLockout(UVLO)VUVLO(on)VUVLO(off)VUVLO(HYST)SoftStartRSS(chg)RSS(dchg)VSSRST

Internalsoft-startpullupresistanceInternalsoft-startpulldownresistance

Soft-startresetthreshold

105305150100100100

1704852001401401502200

%mVmVk k mV

TurnonthresholdTurnoffthresholdHysteresis

4.254.05200

2754.5

VmV

OvercurrentProtection

Pulse-WidthModulatortMINDMAXKPWM

MinimumcontrollablepulsewidthMaximumdutycycleModulatorandpower-stagedcgain

InputbiascurrentOpen-loopgain(1)Unitygainbandwidth(1)Outputsourcecurrent

VFB=0.6V,COMP=1V

601.5100

ns%V/V

ErrorAmplifierIIBAOLGBWPICOMP(src)(1)

100803250

250

nAdBMHzµA

Bydesignonly.Nottestedinproduction.

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ELECTRICALCHARACTERISTICS(continued)

–55°C<TA=TJ<125°C,VDD=12V,fOSC=100kHz(unlessotherwisenoted)

PARAMETER

ICOMP(snk)

Outputsinkcurrent

TESTCONDITIONS

VFB=1.2V,COMP=1V

MIN1.0

TYP2.5

MAX

UNITmA

THERMALCHARACTERISTICS

overoperatingfree-airtemperaturerange(unlessotherwisenoted)

PARAMETER

θJCθJAθJA(1)

Thermalresistance,junctiontocaseThermalresistance,junctiontoambientThermalresistance,junctiontoambient

TESTCONDITIONS

(1)(1)HIGH-K(1)LOW-K

TYP4997.5176

UNIT°C/W°C/W°C/W

TIusestestboardsdesignedtoJESD51-3andJESD51-7forthermal-impedancemeasurements.Theparametersoutlinedinthesestandardsalsoareusedtosetupthermalmodels.

ORDERINGINFORMATION

TA

–55°Cto125°C

OUTPUTVOLTAGEAdjustable

PACKAGESOIC–D

ORDERABLEPARTNUMBERTPS40200MDREP

MEDIUMTapeandreel

QUANTITY2500/reel

DEVICEINFORMATION

FunctionalBlockDiagram

COMPFBSS

VDD

ISNSGDRV

GND

RC

D PACKAGE(TOPVIEW)

VDDISNSGDRVGND

4

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DEVICEINFORMATION(continued)

TERMINALFUNCTIONS

TERMINALNAMERC

NO.1

I/O

DESCRIPTION

Switchingfrequency-settingRCnetwork.ConnectcapacitorfromRCpintoGNDpinandresistorfromVINpintoRCpin.Thedevicemaybesynchronizedtoanexternalclockbyconnectinganopen-drainoutputtothispinandpullingittoGND.Thepulsewidthforsynchronizationshouldnotbeexcessive.

Soft-startprogrammingpin.ConnectcapacitorfromSStoGNDtoprogramsoft-starttime.Pullingthispinbelow150mVcausestheoutputswitchingtostop,placingthedeviceinashutdownstate.Thepinalsofunctionsasarestarttimerforovercurrentevents.

Compensation.Erroramplifieroutput.Connectcontrol-loopcompensationnetworkfromCOMPtoFB.Feedback.Erroramplifierinvertinginput.Connectfeedbackresistornetworkcentertaptothispin.Deviceground

OII

DriveroutputforexternalP-channelMOSFET

Current-sensecomparatorinput.ConnectacurrentsenseresistorbetweenISNSandVDDinordertosetdesiredovercurrentthreshold.

Systeminputvoltage.ConnectlocalbypasscapacitorfromVDDtoGND.

I

SSCOMPFBGNDGDRVISNSVDD

2345678

IOI

TYPICALCHARACTERISTICS

QUIESCENTCURRENTvsTEMPERATURE

2.523

QUIESCENTCURRENTvsVDD

IDD- mA

IDD

- mA

1.510.50

510152025

Temperature - °C

3035VDD- V

40455055

Figure3.

SOFT-STARTTHRESHOLDvsTEMPERATURE

156.5156

Figure4.

UVLOTURNONANDTURNOFFvsTEMPERATURE

4.34.25

Reset Threshold - mV

UVLO Turnon - V

155.5155154.5154153.5

4.24.154.14.054

Temperature - °C

Temperature - °C

Figure5.

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Figure6.

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TYPICALCHARACTERISTICS(continued)

OSCILLATORFREQUENCYvsTEMPERATURE

98

9694

CURRENT-LIMITTHRESHOLDvsTEMPERATURE

103102.5102

ILIMThreshold - mV

Frequency-

kHz

92908886848280-50

101.5101100.510099.5

-250255075100125

Temperature (°C)Temperature - °C

Figure7.

OSCILLATORFREQUENCYvsVDD

275270

21.00

Figure8.

POWER-STAGEGAINvsVDD

20.50

Oscillator Frequency-kHz

265260255250245240235230225220

Gain - dB

20.00

19.50

19.00

VDD-VVDD- V

Figure9.

POWER-STAGEGAINvsTEMPERATURE

20.50

20.50

20.30

20.10

20.4020.45

Figure10.

POWER-STAGEGAINvsTEMPERATURE

Gain - dB

19.90

Gain - dB

20.35

20.30

19.70

19.50

Temperature - °C

20.25

Temperature °C

Figure11.Figure12.

6

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TYPICALCHARACTERISTICS(continued)

MODULATORRAMPAMPLITUDEvsTEMPERATURE

32.82.62.42.2

MODULATORRAMPAMPLITUDEvsTEMPERATURE

65.8

5.65.45.254.84.64.44.243.83.63.43.23

Vramp- V

21.81.61.41.21Temperature - °C

Vramp- V

-50-250255075100125

Temperature - °C

Figure13.Figure14.

FEEDBACKAMPLIFIERINPUTBIASCURRENT

vs

TEMPERATURE

160

140120

MODULATORRAMPAMPLITUDEvsVDD

654

100

VRAMP- V

IIB- nA

VDD- V3

2

806040

10

200-50

-25

25

50

75

100

125

Temperature - °C

Figure15.Figure16.

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TYPICALCHARACTERISTICS(continued)

COMPSOURCECURRENT

vs

TEMPERATURE

300250

3.53Output Current - mA

COMPSINKCURRENTvsTEMPERATURE

Output Current -mA

2.521.510.50

200150100500-50

-25

25

50

75

100

125

Temperature - °C

-50-250255075100125

Temperature - °C

Figure17.

GATEDRIVEVOLTAGEvsTEMPERATURE

87.87.6

VGATE- V

Figure18.

GATEDRIVEVOLTAGEvsVIN

8.4

8.287.87.6

7.47.27

7.4

6.86.66.4

Temperature - °C

7.27VDD- V

Figure19.

REFERENCEVOLTAGEvsTEMPERATURE

720718716714VFB- mV

Figure20.

REFERENCEVOLTAGEvsTEMPERATURE

710708706704702700

VFB- mV

712

Temperature - °CTemperature - °C

Figure21.

8

Figure22.

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GENERALINFORMATION

Overview

TheTPS40200isanonsynchronouscontrollerwithabuilt-in200-mAdriver,designedtodrivehigh-speedP-channelFETSupto500kHz.Itssmallsizecombinedwithcompletefunctionalitymakesthepartbothversatileandeasytouse.

Thecontrollerusesalow-valuecurrent-sensingresistorinserieswiththeinputvoltageandthepowerFETsourceconnectiontodetectswitchingcurrent.Whenthevoltagedropacrossthisresistorexceeds100mV,thepartentersahiccupfaultmodeatapproximately2%oftheoperatingfrequency.

Thepartusesvoltagefeedbacktoanerroramplifierthatisbiasedbyaprecision700-mVreference.Feed-forwardcompensationfromtheinputkeepsthepulse-widthmodulator(PWM)gainconstantoverthefullinputvoltagerange,eliminatingtheneedtochangefrequencycompensationfordifferentinputvoltages.Thepartalsoincorporatesasoft-startfeaturewheretheoutputfollowsaslowlyrisingsoft-startvoltage,preventingoutput-voltageovershoot.

ProgrammingtheOperatingFrequency

Theoperatingfrequencyofthecontrollerisdeterminedbyanexternalresistor,RRC,thatisconnectedfromtheRCpintoVDDandacapacitorattachedfromtheRCpintoground.Thisconnection,andthetwooscillatorcomparatorsinsidetheIC,areshowninFigure23Theoscillatorfrequencycanbecalculatedfromthefollowingequation:

1

fSW=

RRC´CRC´0.105(1)Where:

fSW=Clockfrequency

RRC=Timingresistorvalue(in )CRC=Timingcapacitorvalue(inF)

RRCmustbekeptlargeenoughthatthecurrentthroughitdoesnotexceed750µAwhentheinternalswitch

(showninFigure23isdischargingthetimingcapacitor.Thisconditionmaybeexpressedby:VIN

£750mARRC(2)

SynchronizingtheOscillator

Figure23showsthefunctionaldiagramoftheTPS40200oscillator.WhensynchronizingtheoscillatortoanRCmustbepulledbelow150mVfor20nsormore.Theexternalclockfrequencymustbehigherthanthefree-runningfrequencyoftheconverteraswell.Whensynchronizingthecontroller,ifRCisheldlowforanexcessiveamountoftime,erraticoperationmayoccur.ThemaximumamountoftimethatRCshouldbeheldlowis50%ofanominaloutputpulse,or10%oftheperiodofthesynchronizationfrequency.

Undercircumstanceswheretheinputvoltageishighandthedutycycleislessthan50%,aSchottkydiodeconnectedfromRCtoanexternalclockmaybeusedtosynchronizetheoscillator.ThecathodeofthediodeisconnectedtoRC.Thetrippointoftheoscillatorissetbyaninternalvoltagedividertobe1/10oftheinputvoltage.Theclocksignalmusthaveanamplitudehigherthanthistrippoint.Whentheclockgoeslow,itallowstheresetcurrenttorestarttheRCramp,synchronizingtheoscillatortotheexternalclock.Thisprovidesasimple,single-componentmethodforclocksynchronization.

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GENERALINFORMATION(continued)

Figure23.OscillatorFunctionalDiagram

Amplitude >VIN¸10Duty cycle < 50%

Frequency > Controller

Figure24.Diode-ConnectedSynchronization

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GENERALINFORMATION(continued)

Current-LimitResistor

Selection

eatoavoidproblemswithringingsignalsandnuisancetripping.WhentheFETisonandthecontrollersenses100mVormoredropfromtheVDDpintotheISNSpin,anovercurrentconditionisdeclared.Whenthishappens,theFETisturnedoffand,asshowninFigure28thesoft-startcapacitorisdischarged.Whenthesoft-startcapacitorreachesalevelbelow150clearstheovercurrentconditionflagandattemptstorestart.Iftheconditionthatcausedtheovercurrenteventtooccurisstillpresentontheoutputoftheconverter(seeFigure27anotherovercurrentconditionisdeclaredandtheprocessrepeatsindefinitely.Figure27showscapacitorvoltageduringanextendedoutputfaultcondition.Theoveralldutyconductionduringapersistentfaultisapproximately2%.

Figure25.TypicalSoft-StartCapacitorandVOUTDuringOvercurrent

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GENERALINFORMATION(continued)

VFigure26.Current-LimitReset

Ifnecessary,asmallR-Cfiltercanbeaddedtothecurrent-sensingnetworktoreducenuisancetrippingduetonoisepickup.Thisfilteralsocanbeusedtotrimtheovercurrenttrippointtoahigherlevelwiththeadditionofasingleresistor.SeeFigure27ThenominalovercurrenttrippointusingthecircuitofFigure27isdescribedas:

VR+RF2

IOC=ILIM´F1

RILIMRF2(3)Where:

IOC=Overcurrenttrippoint,peakcurrentintheinductor

VILIM=OvercurrentthresholdvoltagefortheTPS40200,typically100mVRILIM=Valueofthecurrentsenseresistor(in )RandR=Valuesofthescalingresistors(in )

ThevalueofthecapacitorisdeterminedbythenominalpulsewidthoftheconverterandthevaluesofthescalingresistorsRF1andRF2.Itisbestnottohavethetimeconstantofthefilterlongerthanthenominalpulsewidthoftheconverter,ingthisconstraint,thecapacitorvaluemaybeboundedby:.

VOR´Rf2

Cf£÷f1

VIN´fSWRf1+Rf2(4)Where:

Cf=Valueofthecurrent-limitfiltercapacitor(inF)VO=OutputvoltageoftheconverterVIN=InputvoltagetotheconverterfSW=Converterswitchingfrequency

Rf1andRf2=Valuesofthescalingresistors(in )

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GENERALINFORMATION(continued)

VNOTE:Thecurrent-limitresistoranditsassociatedcircuitrycanbeeliminatedandpins7and8shorted.However,theresult

ofthismayresultindamagetothepartorPCboardintheeventofanovercurrentevent.

Figure27.Current-LimitAdjustment

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GENERALINFORMATION(continued)

MOSFETGateDrive

Theoutputdriversinkingcurrentisapproximately200mAandisdesignedtodriveP-channelpowerFETs.WhenthedriverpullsthegatechargeoftheFET,itiscontrollingto–8V,thedrivecurrentfoldsbacktoalowlevelsothathighpowerdissipationonlyoccursduringtheturnonperiodoftheFET.ThisfeatureisparticularlyvaluablewhenturningonaFETathighinputvoltages,whereleavingthegatedrivecurrentonwouldotherwisecauseunacceptablepowerdissipation.

UndervoltageLockout(UVLO)Protection

UVLOprotectionensuresproperstartupofthedeviceonlywhentheinputvoltagehasexceededminimumoperatingvoltage.Undervoltageprotectionincorporateshysteresis,whicheliminateshiccupstartingincaseswhereinputsupplyimpedanceishigh.

Figure28.UndervoltageLockout

Undervoltageprotectionensuresproperstartupofthedeviceonlywhentheinputvoltagehasexceededminimumoperatingvoltage.TheUVLOlevelismeasuredattheVDDpinwithrespecttoGND.Startupvoltageistypically4.3V,withapproximately200mVofhysteresis.Thepartshutsoffatanominal4.1V.AsshowninFigure28whentheinputVDDvoltagerisesto4.3V,the1.3-Vcomparator’sthresholdvoltageisexceededandoccurs.Feedbackfromtheoutputclosestheswitchandshuntsthe200-k resistorsothatanapproximate200-mVlowervoltage,or4.1V,isrequiredbeforethepartshutsdown.

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GENERALINFORMATION(continued)

ProgrammingtheSoft-StartTime

Anexternalcapacitor,CSS,connectedfromthesoft-start(SS)pintogroundcontrolstheTPS40200soft-startinterval.AninternalchargingresistorconnectedtoVDDproducesarisingreferencevoltage,whichisconnectedthougha700-mVoffsettothereferenceinputoftheTPS40200erroramplifier.Whenthesoft-startcapacitorvoltage(VCSS)isbelow150mV,thereisnoswitchingactivity.WhenVCSSrisesabovethe700-mVoffset,theerroramplifierstartstofollowVSST–700mV,andusesthisrisingvoltageasareference.WhenVCSSreaches1.4V,theinternalreferencetakesover,andfurtherincreaseshavenoeffect.Anadvantageofinitiatingaslowstartinthisfashionisthatthecontrollercannotovershootbecauseitsoutputfollowsascaledversionofthecontroller'sreferencevoltage.AconceptualdrawingofthecircuitthatproducestheseresultsisshowninFigure29Aconsequenceofthe700-mVoffsetisthatthecontrollerdoesnotstartswitchinguntiltheVCSShasto700mV.Theoutputremainsat0Vduringtheresultingdelay.WhenVCCSexceedsthe700-mVoffset,theTPS40200outputfollowsthesoft-starttimeconstant.Onceabove1.4V,the700-mVinternalreferencetakesover,andnormaloperationbegins.

Css

Figure29.Soft-StartCircuit

Theslow-starttimeshouldbemore(slower)thanthetimeconstantoftheoutputLCfilter.Thistimeconstraintmaybeexpressedas:

tS³2p´O´CO

(5)

Thecalculationofthesoft-startintervalissimplythetimeittakestheRCnetworktoexponentiallychargefrom0Vto1.4V.Aninternal105-k chargingresistorisconnectedfromtheSSpintoVSST.Forapplicationswherethevoltageisabove8V,aninternalregulatorclampsthemaximumchargingvoltageto8V.Theresultofthisisaformulaforthestart-uptime,asgivenby:

æVSSTö

÷tSS=Rc´CSS´lnççV÷-1.4èSSTøWhere:

tSS=Requiredsoft-starttime(inseconds)CSS=Soft-startcapacitorvalue(inF)

Rc=Internalsoft-startchargingresistor(105k nominal)VSST=Inputvoltageuptoamaximumof8V

(6)

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GENERALINFORMATION(continued)

VoltageSettingandModulatorGain

Sincetheinputcurrenttotheerroramplifierisnegligible,thefeedbackimpedancecanbeselectedoverawiderange.Knowingthatthereferencevoltageis708mV,pickaconvenientvalueforR1andthencalculatethevalueofR2fromthefollowingformula:

æR2ö

÷+VOUT=0.708ç1çR÷1øè(7)

Figure30.SystemGainElements

TheerroramplifierhasaDCopen-loopgainofatleast60dB,withaminimumofa1.5-MHzgainbandwidth

product,whichgivestheuserflexibilitywithrespecttothetypeoffeedbackcompensationusedforthisparticularapplication.Thegainselectedbytheuseratthecrossoverfrequencyissettoprovideanoverallunitygainforthesystem.Thecrossoverfrequencyshouldbeselectedsothattheerroramplifieropen-loopgainishighwithrespecttotherequiredclosed-loopgain.Thisensuresthattheamplifierresponseisdeterminedbythepassivefeedbackelements.

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EXAMPLEAPPLICATIONS

Application1:BuckRegulator,8-Vto12-VInput,3.3Vor5Vat2.5-AOutput

Overview

ThebuckregulatordesignshowninFigure31illustratestheuseoftheTPS40200.Itdelivers2.5Aateither3.3Vor5Vasselectedbyasingleresistor.Itachievesapproximately90%efficiencyat3.3Vand94%at5V.AdiscussionofdesigntradeoffsandmethodologyisincludedtoserveasaguidetothesuccessfuldesignofforwardconvertersusingtheTPS40200.

TheBillofMaterials(BOM)forthisapplicationisgiveninTable2TheefficiencyandloadregulationfromboardsbuiltfromthisdesignareshowninFigure32andFigure33Gerberfilesandadditionalapplicationinformationareavailablefromthefactory.

Figure31.8-Vto16-VVINStep-DownBuckConverter

100

100

90

90

Efficiency - %

70

Efficiency - %

80

80

70

60

60

3

50

0.5

1

1.52

Load Current -A

2.5

3

50

0.5

1

1.52

Load Current -A

2.5

Figure32.Full-LoadEfficiencyat5-VVOUTFigure33.Full-LoadEfficiencyat3.3-VVOUT

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EXAMPLEAPPLICATIONS(continued)

ComponentSelection

Table1.DesignParameters

SYMBOLVINVOUT

PARAMETER

InputvoltageOutputvoltageLineregulationLoadregulation

VOUT

OutputvoltageLineregulationLoadregulation

VRIPPLEVOVERVUNDERIOUTISCP

OutputripplevoltageOutputovershootOutputundershootOutputcurrent

Short-circuitcurrenttrippointEfficiency

FS(1)

Switchingfrequency

AtnominalinputvoltageandmaximumoutputcurrentIOUTat2.5A±0.2%VOUT±0.2%VOUTIOUTat2.5A±0.2%VOUT±0.2%VOUT

Atmaximumoutputcurrent

For2.5-Aloadtransientfrom2.5Ato0.25AFor2.5-Aloadtransientfrom0.25Ato2.5A

0.1253.75

90300

TESTCONDITION

MIN83.2003.2933.2934.854.9904.990

NOM123.33.33.35556010060

2.55.00MAX163.400(1)3.3073.3075.150(1)5.0105.010

UNITVVVVVVVmVmVmVAA%kHz

Set-pointaccuracyisdependentonexternalresistortoleranceandtheICreferencevoltage.Lineandloadregulationvaluesarereferencedtothenominaldesignoutputvoltage.

FETSelectionCriteria

Themaximuminputvoltageforthisapplicationis16V.Switchingtheinductorcausesovershootvoltagesthatcanequaltheinputvoltage.SincetheRDSONoftheFETriseswithbreakdownvoltage,selectaFETwithaslowabreakdownvoltageaspossible.Inthiscase,a30-VFETwasselected.

TheselectionofapowerFET’ssizerequiresknowingboththeswitchinglossesanddclossesintheapplication.AClossesareallfrequencydependentanddirectlyrelatedtodevicecapacitancesanddevicesize.Conversely,dclossesareinverselyrelatedtodevicesize.Theresultisanoptimumwherethetwotypesoflossesareequal.SincedevicesizeisproportionaltoRDSON,astartingpointistoselectadevicewithanRDSONthatresultsinasmalllossofpowerrelativetopackagethermalcapabilityandoverallefficiencyobjectives.

Inthisapplication,theefficiencytargetis90%andtheoutputpower8.25W.Thisgivesatotalpower-lossbudgetof0.916W.TotalFETlossesmustbesmall,relativetothisnumber.

2

ThedcconductionlossintheFETisgivenby:

PDC=Irms´RDSON

(8)

12öù

Thermscurrentisgivenby:

éæDIpp÷2úIrms=êD´çIOUT+

ç÷ê12ú

øûëè

2

(9)

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SGLS371–JANUARY2007

Where:

DIpp=DV´D´

tSLl

DV=VIN-VOUT-(DCR+RDSON)´IOUT

RDSON=FETon-stateresistanceDCR=InductordcresistanceD=Dutycycle

tS=Reciprocaloftheswitchingfrequency

Usingthevaluesinthisexample,thedcpowerlossis129mW.TheremainingFETlossesare: PSW–PowerdissipatedwhileswitchingtheFETonandoff Pgate–PowerdissipateddrivingtheFETgatecapacitance PCOSS–PowerswitchingtheFEToutputcapacitance

ThetotalpowerdissipatedbytheFETisthesumofthesecontributions:PFET=PSW+Pgate+PCOSS+PRDSON

TheP-channelFETusedinthisapplicationisanFDC654P,withthefollowingcharacteristics:trise=13×10–9tfall=6×10–9RDSON=0.1 Qgd=1.2×10–9

COSS=83×10–12Qg=9nCVgate=1.9VQgs=1.0×10–9

Usingthesedevicecharacteristicsandthefollowingformulasproduces:

öfSæf

÷PSW=S´ç=10mWVt´´VIN´Ipk´tCHOFFIpkINCHON÷+

2ç2øè

()

(10)

Where:

tCHON=and

QGD´RG

VIN-VTHQGD´RG

VIN

tCHOFF=

aretheswitchingtimesforthepowerFET.

PGATE=QG´VGATE´fS=22mW

PCOSS=

COSS´VIN_MAX´fS

2

2

=2mW

IG=QG×fS=2.7mAisthegatecurrent

Thesumoftheswitchinglossesis34mWandiscomparabletothe129-mWdclosses.Ataddedexpense,aslightlylargerFETwouldbebetterbecausethedclosswoulddropandtheaclosseswouldincrease,withbothmovingtowardtheoptimumpointofequallosses.

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RectifierSelectionCriteria Rectifierbreakdownvoltage

Therectifierhastowithstandthemaximuminputvoltagewhich,inthiscase,is16V.Toallowforswitchingtransientsthatcanapproachtheswitchingvoltage,a30-Vrectifierwasselected. Diodesize

TheimportanceofpowerlossesfromtheSchottkyrectifier(D2)isdeterminedbythedutycycle.Foralowduty-cycleapplication,therectifierisconductingmostofthetime,andthecurrentthatflowsthroughittimesitsforwarddropcanbethelargestcomponentoflossintheentirecontroller.Inthisapplication,thedutycyclerangesfrom20%to40%,whichintheworstcasemeansthatthediodeisconducting80%ofthetime.Whereefficiencyisofmajorimportance,chooseadiodewithaslowaforwarddropaspossible.Inmorecost-sensitiveapplications,sizemaybereducedtothepointofthethermallimitationsofthediodepackage.Thedeviceinthisapplicationislarge,relativetothecurrentrequiredbytheapplication.Inamorecost-sensitiveapplication,asmallerdiodeinaless-expensivepackageprovidesaless-efficient,butappropriate,solution.

Thedeviceusedhasthefollowingcharacteristics: Vf=0.3Vat3A

Ct=300pF(Ct=theeffectivereverse-voltagecapacitanceofthesynchronousrectifier,D2)ThetwocomponentsofthelossesfromthediodeD2are:

IRIPPLEöæ

÷+1-D)= 653 mWPCOND=Vf´çIçOUT÷´(4èø(11)Where:

D=DutycycleIRIPPLE=RipplecurrentIOUT=OutputcurrentVF=Forwardvoltage

PCOND=Conductionpowerloss

TheswitchingcapacitanceofthisdiodeaddsanACloss,givenby:

PSW+1[C (VIN)Vf)2 f]+6.8mW

Thisadditionallossraisesthetotallossto:660mW.

Atanoutputvoltageof3.3V,theapplicationrunsatanominaldutycycleof27%,andthediodeisconducting72.5%ofthetime.Astheoutputvoltageismovedupto5V,theontimeincreasesto46%,andthediodeisconductingonly54%ofthetimeduringeachclockcycle.Thischangeindutycycleproportionatelyreducestheconductionpowerlossesinthediode.Thisreductionmaybeexpressedas:

æ0.54ö660ç÷=491mW

è0.725ø(13)forasavingsinpowerof660–491=169mW.

Toillustratetherelevanceofthispowersavings,thefull-loadmoduleefficiencywasmeasuredforthisapplicationat3.3Vand5V.The5-Voutputefficiencyis92%versus89%forthe3.3-Vdesign.Thisdifferenceinefficiencyrepresentsa456-mWreductioninlossesbetweenthetwoconditions.This169-mWpower-lossreductionintherectifierrepresents37%ofthedifference.

(12)

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InductorSelectionCriteria

TheTPS40200P-channelFETdriverfacilitatesswitchingthepowerFETatahighfrequency.This,inturn,enablestheuseofsmaller,less-expensiveinductorsasshowninthis300-kHzapplication.Ferrite,withitsgoodhigh-frequencyproperties,isthematerialofchoice.Severalmanufacturersprovidecatalogswithinductorsaturationcurrents,inductancevalues,andLSRs(internalresistance)fortheirvarious-sizedferrites.

Inthisapplication,thepartmustdeliveramaximumcurrentof2.5A.Thisrequiresthattheoutputinductorsaturationcurrentbeabove2.5Aplusone-halftheripplecurrentcausedduringinductorswitching.Thevalueoftheinductordeterminesthisripplecurrent.Alowvalueofinductancehasahigherripplecurrentthatcontributestoripplevoltageacrosstheresistanceoftheoutputcapacitors.Theadvantagesofalowinductanceareahighertransientresponse,lowerDCR,highersaturationcurrent,andasmaller,less-expensivepart.Toolowaninductor,however,leadstohigherpeakcurrentsthatultimatelyareboundedbytheovercurrentlimitsettoprotecttheoutputFETorbyoutputripplevoltage.Fortunately,withlow-ESRceramiccapacitorsontheoutput,theresultingripplevoltageforrelativelyhighripplecurrentscanbesmall.

Forexample,asingle1-µF1206-sized6.3-Vceramiccapacitorhasaninternalresistanceof2 at1MHz.Forthis2.5-Aapplication,a10%ripplecurrentof0.25Aproducesa50-mVripplevoltage.Thisripplevoltagemaybefurtherreducedbyadditionalparallelcapacitors.

Theotherboundoninductanceistheminimumcurrentatwhichthecontrollerentersdiscontinuousconduction.Atthispoint,inductorcurrentiszero.Theminimumoutputcurrentforthisapplicationisspecifiedat0.125A.Thisaveragecurrentisone-halfthepeakcurrentthatmustdevelopduringaminimumontime.Theconditionsforminimumontimearehighlineandlowload,using:

V-VOUT

LMAX=IN´tON= 32mH

IPEAK(14)Where:VIN=16VVOUT=3.3VIPEAK=0.25AtON=0.686µst3.3V1

´

isgivenby300kHz16V

Theinductorusedinthecircuitisthecloseststandardvalueof33µH.Thisisthemaximuminductancethatcanbeusedintheconvertertodelivertheminimumcurrent,whilemaintainingcontinuousconduction.

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