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LM2743www.
ti.
comSNVS276G–APRIL2004–REVISEDMARCH2013LM2743LowVoltageN-ChannelMOSFETSynchronousBuckRegulatorControllerCheckforSamples:LM27431FEATURESDESCRIPTIONTheLM2743isahigh-speedsynchronousbuck2PowerStageInputVoltagefrom1Vto16VregulatorcontrollerwithanaccuratefeedbackvoltageControlStageInputVoltagefrom3Vto6Vaccuracyof±2%.
ItcanprovidesimpledownOutputVoltageAdjustabledownto0.
6Vconversiontooutputvoltagesaslowas0.
6V.
ThoughthecontrolsectionsoftheICareratedfor3to6V,PowerGoodFlagandShutdownthedriversectionsaredesignedtoacceptinputOutputOver-VoltageandUnder-Voltagesupplyrailsashighas16V.
Theuseofadaptivenon-DetectionoverlappingMOSFETgatedrivershelpsavoid±2%FeedbackVoltageAccuracyOverpotentialshoot-throughproblemswhilemaintainingTemperaturehighefficiency.
TheICisdesignedforthemorecost-effectiveoptionofdrivingonlyN-channelMOSFETsLow-SideAdjustableCurrentSensinginboththehigh-sideandlow-sidepositions.
ItsensesAdjustableSoft-Startthelow-sideswitchvoltagedropforprovidingaTrackingandSequencingwithShutdownandsimple,adjustablecurrentlimit.
Soft-StartPinsThefixed-frequencyvoltage-modePWMcontrolSwitchingFrequencyfrom50kHzto1MHzarchitectureisadjustablefrom50kHzto1MHzwithoneexternalresistor.
ThiswiderangeofswitchingTSSOP-14PackagefrequencygivesthepowersupplydesignertheflexibilitytomakebettertradeoffsbetweenAPPLICATIONScomponentsize,costandefficiency.
3.
3VBuckRegulationFeaturesincludesoft-start,inputunder-voltageCableModem,DSLandADSLlockout(UVLO)andPowerGood(basedonbothLaserJetandInkJetPrintersunder-voltageandover-voltagedetection).
Inaddition,theshutdownpinoftheICcanbeusedforLowVoltagePowerModulesprovidingstartupdelay,andthesoft-startpincanbeDSP,ASIC,CoreandI/Ousedforimplementingprecisetracking,forthepurposeofsequencingwithrespecttoanexternalrail.
TypicalApplicationFigure1.
TypicalApplicationCircuit1Pleasebeawarethatanimportantnoticeconcerningavailability,standardwarranty,anduseincriticalapplicationsofTexasInstrumentssemiconductorproductsanddisclaimerstheretoappearsattheendofthisdatasheet.
2Alltrademarksarethepropertyoftheirrespectiveowners.
PRODUCTIONDATAinformationiscurrentasofpublicationdate.
Copyright2004–2013,TexasInstrumentsIncorporatedProductsconformtospecificationsperthetermsoftheTexasInstrumentsstandardwarranty.
Productionprocessingdoesnotnecessarilyincludetestingofallparameters.
LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
ti.
comConnectionDiagramFigure2.
14-LeadPlasticTSSOPθJA=155°C/WPinDescriptionsBOOT(Pin1)-Bootstrappin.
Thisisthesupplyrailforthegatedrivers.
Whenthehigh-sideMOSFETturnson,thevoltageonthispinshouldbeatleastonegatethresholdabovetheregulatorinputvoltageVINtoproperlyturnontheMOSFET.
SeeMOSFETGATEDRIVERSintheApplicationInformationsectionformoredetailsonhowtoselectMOSFETs.
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.
Formostapplicationsarecommendedvalueforthepull-upresistoris100k.
Thevoltageonthispinisthuspulledlowunderoutputunder-voltageorover-voltagefaultconditionsandalsounderinputUVLO.
ISEN(Pin7)-Currentlimitthresholdsettingpin.
Thissourcesafixed40Acurrent.
Aresistorofappropriatevalueshouldbeconnectedbetweenthispinandthedrainofthelow-sideMOSFET(switchnode).
EAO(Pin8)-Outputoftheerroramplifier.
Thevoltagelevelonthispiniscomparedwithaninternallygeneratedrampsignaltodeterminethedutycycle.
Thispinisnecessaryforcompensatingthecontrolloop.
SS/TRACK(Pin9)-Soft-startandtrackingpin.
Thispinisinternallyconnectedtothenon-invertinginputoftheerroramplifierduringsoft-start,andinfactanytimetheSS/TRACKpinvoltagehappenstobebelowtheinternalreferencevoltage.
Forthebasicsoft-startfunction,acapacitorofminimumvalue1nFisconnectedfromthispintoground.
Totracktherisingrampofanotherpowersupply'soutput,connectaresistordividerfromtheoutputofthatsupplytothispinasdescribedinApplicationInformation.
FB(Pin10)-Feedbackpin.
Thisistheinvertinginputoftheerroramplifier,whichisusedforsensingtheoutputvoltageandcompensatingthecontrolloop.
FREQ(Pin11)-Frequencyadjustpin.
Theswitchingfrequencyissetbyconnectingaresistorofsuitablevaluebetweenthispinandground.
TheequationforcalculatingtheexactvalueisprovidedinApplicationInformation,butsometypicalvalues(roundeduptotheneareststandardvalues)are324kfor100kHz,97.
6kfor300kHz,56.
2kfor500kHz,24.
9kfor1MHz.
SD(Pin12)-ICshutdownpin.
PullthispintoVCCtoensuretheICisenabled.
ConnecttogroundtodisabletheIC.
Undershutdown,bothhigh-sideandlow-sidedrivesareoff.
Thispinalsofeaturesaprecisionthresholdforpowersupplysequencingpurposes,aswellasalowthresholdtoensureminimalquiescentcurrent.
HG(Pin14)-High-gatedrivepin.
Thisisthegatedriveforthehigh-sideN-channelMOSFET.
ThissignalisinterlockedwithLG(Pin2)toavoidshoot-through.
Thesedeviceshavelimitedbuilt-inESDprotection.
TheleadsshouldbeshortedtogetherorthedeviceplacedinconductivefoamduringstorageorhandlingtopreventelectrostaticdamagetotheMOSgates.
2SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013AbsoluteMaximumRatings(1)(2)VCC-0.
3to7VBOOTVoltage-0.
3to21VISEN-0.
3to9.
5VAllotherpins-0.
3toVCC+0.
3VJunctionTemperature150°CStorageTemperature65°Cto150°CLeadTemperature(soldering,10sec)260°CSolderingInformationInfraredorConvection(20sec)235°CESDRating(3)2kV(1)Absolutemaximumratingsindicatelimitsbeyondwhichdamagetothedevicemayoccur.
Operatingratingsindicateconditionsforwhichthedeviceoperatescorrectly.
OperatingRatingsdonotimplyspecifiedperformancelimits.
(2)IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheTexasInstrumentsSalesOffice/Distributorsforavailabilityandspecifications.
(3)Thehumanbodymodelisa100pFcapacitordischargedthrougha1.
5kresistorintoeachpin.
OperatingRatingsSupplyVoltageRange(VCC)3Vto6VJunctionTemperatureRange(TJ)40°Cto+125°CThermalResistance(θJA)155°C/WElectricalCharacteristics(1)VCC=3.
3Vunlessotherwiseindicated.
TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=25°C.
LimitsappearinginboldfacetypeapplyoverfullOperatingTemperatureRange.
Datasheetmin/maxspecificationlimitsarespecifiedbydesign,test,orstatisticalanalysis.
SymbolParameterConditionsMinTypMaxUnitsVFBFBPinVoltageVCC=3Vto6V0.
5880.
60.
612VVONUVLOThresholdsRising2.
76VFalling2.
42VCC=3.
3V,VSD=3.
3V1.
01.
52.
1Fsw=600kHzOperatingVCCCurrentmAIQ_VCCVCC=5V,VSD=3.
3V1.
01.
72.
1Fsw=600kHzShutdownVCCCurrentVCC=3.
3V,VSD=0V110185AtPWGD1PWGDPinResponseTimeVFBRising6stPWGD2PWGDPinResponseTimeVFBFalling6sISS-ONSSPinSourceCurrentVSS=0V71014AISS-OCSSPinSinkCurrentDuringOverVSS=2.
5V90ACurrentISEN-THISENPinSourceCurrentTripPoint254055AERRORAMPLIFIERGBWErrorAmplifierUnityGain9MHzBandwidthGErrorAmplifierDCGain106dBSRErrorAmplifierSlewRate3.
2V/sIEAOEAOPinCurrentSourcingandVEAO=1.
5,FB=0.
55V2.
6mASinkingCapabilityVEAO=1.
5,FB=0.
65V9.
2VEAErrorAmplifierOutputVoltageMinimum1VMaximum2V(1)ThepowerMOSFETscanrunonaseparate1Vto16Vrail(Inputvoltage,VIN).
PracticallowerlimitofVINdependsonselectionoftheexternalMOSFET.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback3ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comElectricalCharacteristics(1)(continued)VCC=3.
3Vunlessotherwiseindicated.
TypicalsandlimitsappearinginplaintypeapplyforTA=TJ=25°C.
LimitsappearinginboldfacetypeapplyoverfullOperatingTemperatureRange.
Datasheetmin/maxspecificationlimitsarespecifiedbydesign,test,orstatisticalanalysis.
SymbolParameterConditionsMinTypMaxUnitsGATEDRIVEIQ-BOOTBOOTPinQuiescentCurrentVBOOT=12V,VSD=01890ARHG_UPHigh-SideMOSFETDriverPull-UpVBOOT=5V@350mASourcing3ONresistanceRHG_DNHigh-SideMOSFETDriverPull-HG=5V@350mASourcing2DownONresistanceRLG_UPLow-SideMOSFETDriverPull-UpVBOOT=5V@350mASourcing3ONresistanceRLG_DNLow-SideMOSFETDriverPull-LG=5V@350mASourcing2DownONresistanceOSCILLATORRFADJ=702.
1k50RFADJ=98.
74k300fSWPWMFrequencykHzRFADJ=45.
74k475600725RFADJ=24.
91k1000DMaxHigh-SideDutyCyclefSW=300kHz80%fSW=600kHz76fSW=1MHz73LOGICINPUTSANDOUTPUTSVSTBY-IHStandbyHighTripPointVFB=0.
575V,VBOOT=3.
3V,VSD1.
1VRisingVSTBY-ILStandbyLowTripPointVFB=0.
575V,VBOOT=3.
3V,VSD0.
232VFallingVSD-IHSDPinLogicHighTripPointVSDRising1.
3VVSD-ILSDPinLogicLowTripPointVSDFalling0.
8VVPWGD-TH-LOPWGDPinTripPointsFBFalling0.
4080.
4340.
457VVPWGD-TH-HIPWGDPinTripPointsFBRising0.
6770.
7100.
742VVPWGD-HYSPWGDHysteresisFBFalling60mVFBRising904SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013TypicalPerformanceCharacteristicsEfficiency(VOUT=1.
2V)Efficiency(VOUT=2.
5V)VCC=3.
3V,fSW=300kHzVCC=3.
3V,fSW=300kHzFigure3.
Figure4.
VCCOperatingCurrentplusBOOTCurrentvsEfficiency(VOUT=3.
3V)FrequencyVCC=5V,fSW=300kHzFDS6898AFET(TA=25°C)Figure5.
Figure6.
BOOTPinCurrentBOOTPinCurrentvsvsTemperatureforBOOTVoltage=3.
3VTemperatureforBOOTVoltage=5VfSW=300kHz,FDS6898AFET,No-LoadfSW=300kHz,FDS6898AFET,No-LoadFigure7.
Figure8.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback5ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comTypicalPerformanceCharacteristics(continued)BOOTPinCurrentvsInternalReferenceVoltageTemperatureforBOOTVoltage=12VvsfSW=300kHz,FDS6898AFET,No-LoadTemperatureFigure9.
Figure10.
FrequencyOutputVoltagevsvsTemperatureOutputCurrentFigure11.
Figure12.
SwitchWaveforms(HGRising)SwitchWaveforms(HGFalling)VCC=3.
3V,VIN=5V,VOUT=1.
2VVCC=3.
3V,VIN=5V,VOUT=1.
2VIOUT=4A,CSS=12nF,fSW=300kHzIOUT=4A,CSS=12nF,fSW=300kHzFigure13.
Figure14.
6SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013TypicalPerformanceCharacteristics(continued)Start-Up(No-Load)Start-Up(Full-Load)VCC=3.
3V,VIN=5V,VOUT=1.
2VVCC=3.
3V,VIN=5V,VOUT=1.
2VCSS=12nF,fSW=300kHzIOUT=4A,CSS=12nF,fSW=300kHzFigure15.
Figure16.
Shutdown(Full-Load)LoadTransientResponse(IOUT=0Ato4A)VCC=3.
3V,VIN=5V,VOUT=1.
2VVCC=3.
3V,VIN=5V,VOUT=1.
2VIOUT=4A,CSS=12nF,fSW=300kHzCSS=12nF,fSW=300kHzFigure17.
Figure18.
LoadTransientResponse(IOUT=4Ato0A)LoadTransientResponseVCC=3.
3V,VIN=5V,VOUT=1.
2VVCC=3.
3V,VIN=5V,VOUT=1.
2VCSS=12nF,fSW=300kHzCSS=12nF,fSW=300kHzFigure19.
Figure20.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback7ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comTypicalPerformanceCharacteristics(continued)LineTransientResponse(VIN=3Vto9V)LineTransientResponse(VIN=9Vto3V)VCC=3.
3V,VOUT=1.
2VVCC=3.
3V,VOUT=1.
2VIOUT=2A,fSW=300kHzIOUT=2A,fSW=300kHzFigure21.
Figure22.
8SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013BlockDiagramCopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback9ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
ti.
comAPPLICATIONINFORMATIONTHEORYOFOPERATIONTheLM2743isavoltage-mode,high-speedsynchronousbuckregulatorwithaPWMcontrolscheme.
Itisdesignedforuseinset-topboxes,thinclients,DSL/Cablemodems,andotherapplicationsthatrequirehighefficiencybuckconverters.
Ithasoutputshutdown(SD),inputunder-voltagelock-out(UVLO)modeandpowergood(PWGD)flag(basedonover-voltageandunder-voltagedetection).
Theover-voltageandunder-voltagesignalsarelogicallyOR'edtodrivethepowergoodsignalandprovidealogicsignaltothesystemiftheoutputvoltagegoesoutofregulation.
CurrentlimitisachievedbysensingthevoltageVDSacrossthelowsideMOSFET.
STARTUP/SOFT-STARTWhenVCCexceeds2.
76Vandtheshutdownpin(SD)seesalogichigh,thesoft-startperiodbegins.
Thenaninternal,fixed10Asourcebeginschargingthesoft-startcapacitor.
Duringsoft-startthevoltageonthesoft-startcapacitorCSSisconnectedinternallytothenon-invertinginputoftheerroramplifier.
Thesoft-startperiodlastsuntilthevoltageonthesoft-startcapacitorexceedstheLM2743referencevoltageof0.
6V.
Atthispointthereferencevoltagetakesoveratthenon-invertingerroramplifierinput.
ThecapacitanceofCSSdeterminesthelengthofthesoft-startperiod,andcanbeapproximatedby:WhereCSSisinFandtSSisinms.
DuringsoftstartthePowerGoodflagisforcedlowanditisreleasedwhentheFBpinvoltagereaches70%of0.
6V.
Atthispointthechipentersnormaloperationmode,andtheoutputovervoltageandundervoltagemonitoringstarts.
NORMALOPERATIONWhileinnormaloperationmode,theLM2743regulatestheoutputvoltagebycontrollingthedutycycleofthehighsideandlowsideMOSFETs(seeFigure1).
Theequationgoverningoutputvoltageis:ThePWMfrequencyisadjustablebetween50kHzand1MHzandissetbyanexternalresistor,RFADJ,betweentheFREQpinandground.
Theresistanceneededforadesiredfrequencyisapproximately:WherefSWisinHzandRFADJisink.
TRACKINGAVOLTAGELEVELTheLM2743cantracktheoutputofamasterpowersupplyduringsoft-startbyconnectingaresistordividertotheSS/TRACKpin.
Inthisway,theoutputvoltageslewrateoftheLM2743willbecontrolledbythemastersupplyforloadsthatrequireprecisesequencing.
Whenthetrackingfunctionisusednosoft-startcapacitorshouldbeconnectedtotheSS/TRACKpin.
Otherwise,aCSSvalueofatleast1nFbetweenthesoft-startpinandgroundshouldbeused.
10SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Figure23.
TrackingCircuitOnewaytousethetrackingfeatureistodesignthetrackingresistordividersothatthemastersupply'soutputvoltage(VOUT1)andtheLM2743'soutputvoltage(representedsymbolicallyinFigure23asVOUT2,i.
e.
withoutexplicitlyshowingthepowercomponents)bothrisetogetherandreachtheirtargetvaluesatthesametime.
Forthiscase,theequationgoverningthevaluesofthetrackingdividerresistorsRT1andRT2is:ThecurrentthroughRT1shouldbeabout3mAto4mAforprecisetracking.
ThefinalvoltageoftheSS/TRACKpinshouldbesethigherthanthefeedbackvoltageof0.
6V(sayabout0.
65Vasintheaboveequation).
Ifthemastersupplyvoltagewas5VandtheLM2743outputvoltagewas1.
8V,forexample,thenthevalueofRT1neededtogivethetwosuppliesidenticalsoft-starttimeswouldbe150.
Atimingdiagramfortheequalsoft-starttimecaseisshowninFigure24.
Figure24.
TrackingwithEqualSoft-StartTimeTRACKINGAVOLTAGESLEWRATEThetrackingfeaturecanalternativelybeusednottomakebothrailsreachregulationatthesametimebutrathertohavesimilarriserates(intermsofoutputdV/dt).
ThismethodensuresthattheoutputvoltageoftheLM2743alwaysreachesregulationbeforetheoutputvoltageofthemastersupply.
Becausetheoutputofthemastersupplyisdivideddown,inordertotrackproperlytheoutputvoltageoftheLM2743mustbelowerthanthevoltageofthemastersupply.
Inthiscase,thetrackingresistorscanbedeterminedbasedonthefollowingequation:Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback11ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comFortheexamplecaseofVOUT1=5VandVOUT2=1.
8V,withRT1setto150asbefore,RT2iscalculatedfromtheaboveequationtobe265.
AtimingdiagramforthecaseofequalslewratesisshowninFigure25.
Figure25.
TrackingwithEqualSlewRatesSEQUENCINGThestartup/soft-startoftheLM2743canbedelayedforthepurposeofsequencingbyconnectingaresistordividerfromtheoutputofamasterpowersupplytotheSDpin,asshowninFigure26.
Figure26.
SequencingCircuitAdesireddelaytimetDELAYbetweenthestartupofthemastersupplyoutputvoltageandtheLM2743outputvoltagecanbesetbasedontheSDpinlow-to-highthresholdVSD-IHandtheslewrateofthevoltageattheSDpin,SRSD:tDELAY=VSD-IH/SRSDNoteagain,thatinFigure26,theLM2743'soutputvoltagehasbeenrepresentedsymbolicallyasVOUT2,i.
e.
withoutexplicitlyshowingthepowercomponents.
VSD-IHistypically1.
08VandSRSDistheslewrateoftheSDpinvoltage.
ThevaluesofthesequencingdividerresistorsRS1andRS2settheSRSDbasedonthemastersupplyoutputvoltageslewrate,SROUT1,usingthefollowingequation:12SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
ti.
comSNVS276G–APRIL2004–REVISEDMARCH2013Forexample,ifthemastersupplyoutputvoltageslewratewas1V/msandthedesireddelaytimebetweenthestartupofthemastersupplyandLM2743outputvoltagewas5ms,thenthedesiredSDpinslewratewouldbe(1.
08V/5ms)=0.
216V/ms.
DuetotheinternalimpedanceoftheSDpin,themaximumrecommendedvalueforRS2is1k.
Toachievethedesiredslewrate,RS1wouldthenbe274.
AtimingdiagramforthisexampleisshowninFigure27.
Figure27.
DelayforSequencingSDPINIMPEDANCEWhenconnectingaresistordividertotheSDpinoftheLM2743somecarehastobetaken.
OncetheSDvoltagegoesaboveVSD-IH,a17Apull-upcurrentisactivatedasshowninFigure28.
Thiscurrentisusedtocreatetheinternalhysteresis(170mV);however,highexternalimpedanceswillaffecttheSDpinlogicthresholdsaswell.
TheexternalimpedanceusedforthesequencingdividernetworkshouldpreferablybeasmallfractionoftheimpedanceoftheSDpinforgoodperformance(around1k).
Figure28.
SDPinLogicCopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback13ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comMOSFETGATEDRIVERSTheLM2743hastwogatedriversdesignedfordrivingN-channelMOSFETsinasynchronousmode.
Notethatunlikemostothersynchronouscontrollers,thebootstrapcapacitoroftheLM2743providespowernotonlytothedriveroftheupperMOSFET,butthelowerMOSFETdrivertoo(bothdriversaregroundreferenced,i.
e.
nofloatingdriver).
TofullyturnthetopMOSFETon,theBOOTvoltagemustbeatleastonegatethresholdgreaterthanVINwhenthehigh-sidedrivegoeshigh.
Thisbootstrapvoltageisusuallysuppliedfromalocalchargepumpstructure.
ButlookingattheTypicalApplicationschematic,thisalsomeansthatthedifferencevoltageVCC-VD1,whichisthevoltagethebootstrapcapacitorchargesupto,mustbealwaysgreaterthanthemaximumtolerancelimitofthethresholdvoltageoftheupperMOSFET.
HereVD1istheforwardvoltagedropacrossthebootstrapdiodeD1.
Thisthereforemayplacerestrictionsontheminimuminputvoltageand/ortypeofMOSFETused.
ThemostbasicchargebootstrappumpcircuitcanbebuiltusingoneSchottkydiodeandasmallcapacitor,asshowninFigure29.
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.
LogiclevelMOSFETsarenotspecifiedtoturnon,ormayhavemuchhigheron-resistanceat2.
5V.
Sub-logiclevelMOSFETs,usuallyspecifiedatVGS=2.
5V,willwork,butaremoreexpensive,andtendtohavehigheron-resistance.
ThecircuitinFigure29workswellforinputvoltagesrangingfrom1Vupto16VandVCC=5V±10%,becausethedrivevoltagedependsonlyonVCC.
Figure29.
BasicChargePump(Bootstrap)NotethattheLM2743canbepairedwithalowcostlinearregulatorliketheLM78L05torunfromasingleinputrailbetween6.
0and14V.
The5VoutputofthelinearregulatorpowersboththeVCCandthebootstrapcircuit,providingefficientdriveforlogiclevelMOSFETs.
AnexampleofthiscircuitisshowninFigure30.
14SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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LM78L05FeedingBasicChargePumpFigure31showsasecondpossibilityforbootstrappingtheMOSFETdrivesusingadoubler.
ThiscircuitprovidesanequalvoltagedriveofVCC-3VD+VINtoboththehigh-sideandlow-sideMOSFETdrives.
Thismethodshouldonlybeusedincircuitsthatuse3.
3VforbothVCCandVIN.
EvenwithVIN=VCC=3.
0V(10%lowertoleranceon3.
3V)andVD=0.
5Vbothhigh-sideandlow-sidegateswillhaveatleast4.
5Vofdrive.
Thepowerdissipationofthegatedrivecircuitryisdirectlyproportionaltogatedrivevoltage,hencethethermallimitsoftheLM2743ICwillquicklybereachedifthiscircuitisusedwithVCCorVINvoltagesover5V.
Figure31.
ChargePumpwithAddedGateDriveAllthegatedrivecircuitsshownintheabovefigurestypicallyuse100nFceramiccapacitorsinthebootstraplocations.
POWERGOODSIGNALTheopendrainoutputonthePowerGoodpinneedsapull-upresistortoalowvoltagesource.
Thepull-upresistorshouldbechosensothatthecurrentgoingintothePowerGoodpinislessthan1mA.
A100kresistorisrecommendedformostapplications.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback15ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
ti.
comThePowerGoodsignalisanOR-gatedflagwhichtakesintoaccountbothoutputover-voltageandunder-voltageconditions.
Ifthefeedbackpin(FB)voltageis18%aboveitsnominalvalue(118%xVFB=0.
708V)orfalls28%belowthatvalue(72%xVFB=0.
42V)thePowerGoodflaggoeslow.
ThePowerGoodflagcanbeusedtosignalothercircuitsthattheoutputvoltagehasfallenoutofregulation,howevertheswitchingoftheLM2743continuesregardlessofthestateofthePowerGoodsignal.
ThePowerGoodflagwillreturntologichighwheneverthefeedbackpinvoltageisbetween72%and118%of0.
6V.
UVLOThe2.
76Vturn-onthresholdonVCChasabuiltinhysteresisofabout300mV.
IfVCCdropsbelow2.
42V,thechipentersUVLOmode.
UVLOconsistsofturningoffthetopandbottomMOSFETSandremaininginthatconditionuntilVCCrisesabove2.
76V.
Aswithshutdown,thesoft-startcapacitorisdischargedthroughaninternalMOSFET,ensuringthatthenextstart-upwillbecontrolledbythesoft-startcircuitry.
CURRENTLIMITCurrentlimitisrealizedbysensingthevoltageacrossthelow-sideMOSFETwhileitison.
TheRDS(ON)oftheMOSFETisaknownvalue;hencethecurrentthroughtheMOSFETcanbedeterminedas:VDS=IOUTxRDS(ON)Thecurrentthroughthelow-sideMOSFETwhileitisonisalsothefallingportionoftheinductorcurrent.
Thecurrentlimitthresholdisdeterminedbyanexternalresistor,RCS,connectedbetweentheswitchingnodeandtheISENpin.
Aconstantcurrentof40AisforcedthroughRCS,causingafixedvoltagedrop.
ThisfixedvoltageiscomparedagainstVDSandifthelatterishigher,thecurrentlimitofthechiphasbeenreached.
ToobtainamoreaccuratevalueforRCSyoumustconsidertheoperatingvaluesofRDS(ON)andISEN-THattheiroperatingtemperaturesinyourapplicationandtheeffectofslightparameterdifferencesfromparttopart.
RCScanbefoundbyusingthefollowingequationusingtheRDS(ON)valueofthelowsideMOSFETatit'sexpectedhottemperatureandtheabsoluteminimumvalueexpectedoverthefulltemperaturerangeforthefortheISEN-THwhichis25A:RCS=RDSON-HOTxILIM/40AForexample,aconservative15Acurrentlimitina10AdesignwithaminimumRDS(ON)of10mwouldrequirea6kresistor.
TopreventtheISENpinfromsinkingtoomuchcurrentwhentheswitchnodegoesabove9.
5V,thevalueofthecurrentlimitsettingresistorRCSshouldnotbetoolow.
Thecriterionisasfollows,wherethe10mAisthemaximumcurrentISENpinisallowedtosink.
ForexampleifVIN=13.
2V,theminimumvalueofRCSis370.
Becausecurrentsensingisdoneacrossthelow-sideMOSFET,nominimumhigh-sideon-timeisnecessary.
TheLM2743enterscurrentlimitmodeiftheinductorcurrentexceedsthecurrentlimitthresholdatthepointwherethehigh-sideMOSFETturnsoffandthelow-sideMOSFETturnson.
(Thepointofpeakinductorcurrent,seeFigure32).
Notethatinnormaloperationmodethehigh-sideMOSFETalwaysturnsonatthebeginningofaclockcycle.
Incurrentlimitmode,bycontrast,thehigh-sideMOSFETon-pulseisskipped.
Thiscausesinductorcurrenttofall.
Unlikeanormaloperationswitchingcycle,however,inacurrentlimitmodeswitchingcyclethehigh-sideMOSFETwillturnonassoonasinductorcurrenthasfallentothecurrentlimitthreshold.
TheLM2743willcontinuetoskiphigh-sideMOSFETpulsesuntiltheinductorcurrentpeakisbelowthecurrentlimitthreshold,atwhichpointthesystemresumesnormaloperation.
16SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Figure32.
CurrentLimitThresholdUnlikeahigh-sideMOSFETcurrentsensingscheme,whichlimitsthepeaksofinductorcurrent,low-sidecurrentsensingisonlyallowedtolimitthecurrentduringtheconverteroff-time,wheninductorcurrentisfalling.
Thereforeinatypicalcurrentlimitplotthevalleysarenormallywelldefined,butthepeaksarevariable,accordingtothedutycycle.
ThePWMerroramplifierandcomparatorcontroltheoff-pulseofthehigh-sideMOSFET,evenduringcurrentlimitmode,meaningthatpeakinductorcurrentcanexceedthecurrentlimitthreshold.
Assumingthattheoutputinductordoesnotsaturate,themaximumpeakinductorcurrentduringcurrentlimitmodecanbecalculatedwiththefollowingequation:WhereTSWistheinverseofswitchingfrequencyfSW.
The200nstermrepresentstheminimumoff-timeofthedutycycle,whichensuresenoughtimeforcorrectoperationofthecurrentsensingcircuitry.
Inordertominimizethetimeperiodinwhichpeakinductorcurrentexceedsthecurrentlimitthreshold,theICalsodischargesthesoft-startcapacitorthroughafixed90Asink.
TheoutputoftheLM2743internalerroramplifierislimitedbythevoltageonthesoft-startcapacitor.
Hence,dischargingthesoft-startcapacitorreducesthemaximumdutycycleDofthecontroller.
Duringseverecurrentlimitthisreductionindutycyclewillreducetheoutputvoltageifthecurrentlimitconditionslastforanextendedtime.
Outputinductorcurrentwillbereducedinturntoaflatlevelequaltothecurrentlimitthreshold.
Thethirdbenefitofthesoft-startcapacitordischargeisasmooth,controlledrampofoutputvoltagewhenthecurrentlimitconditioniscleared.
FOLDBACKCURRENTLIMITInthecasewhereextraprotectionisusedtohelpanoutputshortcondition,acurrentfoldbackresistor(RCLF)shouldbeconsidered,seeFigure33.
Firstselectthepercentageofcurrentlimitfoldback(PLIM):PLIM=ILIMxPwherePisaratiobetween0and1.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback17ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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FoldbackCurrentLimitCircuitObtaintheRCSwiththefollowingequation:whereISEN=40μA.
Iftheswitchnodegoesabove9.
5Vthefollowingcriterionmustbesatisfied:Theequationforcalculatingthefoldbackresistancevalueis:SHUTDOWNIftheshutdownpinispulledlow,(below0.
8V)theLM2743entersshutdownmode,anddischargesthesoft-startcapacitorthroughaMOSFETswitch.
Thehighandlow-sideMOSFETsareturnedoff.
TheLM2743remainsinthisstateaslongasVSDseesalogiclow(seetheElectricalCharacteristicstable).
ToassureproperICstart-uptheshutdownpinshouldnotbeleftfloating.
FornormaloperationthispinshouldbeconnecteddirectlytoVCCortoanothervoltagebetween1.
3VtoVCC(seetheElectricalCharacteristicstable).
DESIGNCONSIDERATIONSThefollowingisadesignprocedureforallthecomponentsneededtocreatetheFigure1.
Thisdesignconverts3.
3V(VIN)to1.
2V(VOUT)atamaximumloadof4Awithanefficiencyof89%andaswitchingfrequencyof300kHz.
Thesameprocedurescanbefollowedtocreatemanyotherdesignswithvaryinginputvoltages,outputvoltages,andloadcurrents.
DutyCycleCalculationThecompletedutycycleforabuckconverterisdefinedwiththefollowingequation:whereVSWLandVSWHaretherespectiveforwardvoltagedropsthatdevelopacrossthelowsideandhighsideMOSFETs.
Assumingtheinductorripplecurrentis20%to30%oftheoutputcurrent,therefore:VSWL=IOUTxRDS(ON)LOW(Low-SideMOSFET)VSWH=IOUTxRDS(ON)HIGH(High-SideMOSFET)18SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Tocalculatethemaximumdutycycleusetheestimated'hot'RDS(on)valueoftheMOSFETs,theminimuminputvoltage,andmaximumload.
AsshowninFigure34,theworstcasemaximumdutycyclesoftheLM2743occursat125°CjunctiontemperaturevsVCC(ICcontrolsectionvoltage).
EnsurethattheoperatingdutycycleisbelowthecurveinFigure34,ifthisconditionisnotsatisfied,thesystemwillbeunabletodeveloptherequireddutycycletoderivethenecessarysystempowerandsotheoutputvoltagewillfalloutofregulation.
Figure34.
MaximumDutyCyclevsVCCTJ=125°CInputCapacitorTheinputcapacitorsinaBuckconverteraresubjectedtohighstressduetotheinputcurrenttrapezoidalwaveform.
InputcapacitorsareselectedfortheirripplecurrentcapabilityandtheirabilitytowithstandtheheatgeneratedsincethatripplecurrentpassesthroughtheirESR.
Inputrmsripplecurrentisapproximately:Thepowerdissipatedbyeachinputcapacitoris:wherenisthenumberofcapacitors,andESRistheequivalentseriesresistanceofeachcapacitor.
Theequationaboveindicatesthatpowerlossineachcapacitordecreasesrapidlyasthenumberofinputcapacitorsincreases.
Theworst-caserippleforaBuckconverteroccursduringfullloadandwhenthedutycycle(D)is0.
5.
Forthis3.
3Vto1.
2Vdesignthedutycycleis0.
364.
Fora4Amaximumloadtheripplecurrentis1.
92A.
OutputInductorTheoutputinductorformsthefirsthalfofthepowerstageinaBuckconverter.
Itisresponsibleforsmoothingthesquarewavecreatedbytheswitchingactionandforcontrollingtheoutputcurrentripple(ΔIOUT).
Theinductanceischosenbyselectingbetweentradeoffsinefficiencyandresponsetime.
Thesmallertheoutputinductor,themorequicklytheconvertercanrespondtotransientsintheloadcurrent.
However,asshownintheefficiencycalculations,asmallerinductorrequiresahigherswitchingfrequencytomaintainthesamelevelofoutputcurrentripple.
AnincreaseinfrequencycanmeanincreasinglossintheMOSFETsduetothecharginganddischargingofthegates.
Generallytheswitchingfrequencyischosensothatconductionlossoutweighsswitchingloss.
Theequationforoutputinductorselectionis:L=1.
6HCopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback19ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comHerewehavepluggedinthevaluesforoutputcurrentripple,inputvoltage,outputvoltage,switchingfrequency,andassumeda40%peak-to-peakoutputcurrentripple.
Thisyieldsaninductanceof1.
6H.
Theoutputinductormustberatedtohandlethepeakcurrent(alsoequaltothepeakswitchcurrent),whichis(IOUT+(0.
5xΔIOUT))=4.
8A,fora4Adesign.
TheCoilcraftDO3316P-222Pis2.
2H,isratedto7.
4Apeak,andhasadirectcurrentresistance(DCR)of12m.
Afterselectinganoutputinductor,inductorcurrentrippleshouldbere-calculatedwiththenewinductancevalue,asthisinformationisneededtoselecttheoutputcapacitor.
Re-arrangingtheequationusedtoselectinductanceyieldsthefollowing:VIN(MAX)isassumedtobe10%abovethesteadystateinputvoltage,or3.
6V.
Theactualcurrentripplewillthenbe1.
2A.
Peakinductor/switchcurrentwillbe4.
6A.
OutputCapacitorTheoutputcapacitorformsthesecondhalfofthepowerstageofaBuckswitchingconverter.
Itisusedtocontroltheoutputvoltageripple(ΔVOUT)andtosupplyloadcurrentduringfastloadtransients.
Inthisexampletheoutputcurrentis4Aandtheexpectedtypeofcapacitorisanaluminumelectrolytic,aswiththeinputcapacitors.
Otherpossibilitiesincludeceramic,tantalum,andsolidelectrolytecapacitors,howevertheceramictypeoftendonothavethelargecapacitanceneededtosupplycurrentforloadtransients,andtantalumstendtobemoreexpensivethanaluminumelectrolytic.
AluminumcapacitorstendtohaveveryhighcapacitanceandfairlylowESR,meaningthattheESRzero,whichaffectssystemstability,willbemuchlowerthantheswitchingfrequency.
Thelargecapacitancemeansthatattheswitchingfrequency,theESRisdominant,hencethetypeandnumberofoutputcapacitorsisselectedonthebasisofESR.
OnesimpleformulatofindthemaximumESRbasedonthedesiredoutputvoltageripple,ΔVOUTandthedesignedoutputcurrentripple,ΔIOUT,is:Inthisexample,inordertomaintaina2%peak-to-peakoutputvoltagerippleanda40%peak-to-peakinductorcurrentripple,therequiredmaximumESRis20m.
TheSanyo4SP560MelectrolyticcapacitorwillgiveanequivalentESRof14m.
Thecapacitanceof560Fisenoughtosupplyenergyeventomeetsevereloadtransientdemands.
MOSFETsSelectionofthepowerMOSFETsisgovernedbyatradeoffbetweencost,size,andefficiency.
Onemethodistodeterminethemaximumcostthatcanbeendured,andthenselectthemostefficientdevicethatfitsthatprice.
Breakingdownthelossesinthehigh-sideandlow-sideMOSFETsandthencreatingspreadsheetsisonewaytodeterminerelativeefficienciesbetweendifferentMOSFETs.
Goodcorrelationbetweenthepredictionandthebenchresultisnotspecified,however.
Single-channelbuckregulatorsthatuseacontrollerICanddiscreteMOSFETstendtobemostefficientforoutputcurrentsof2Ato10A.
Lossesinthehigh-sideMOSFETcanbebrokendownintoconductionloss,gatechargingloss,andswitchingloss.
Conductionloss,orI2Rloss,isapproximately:PC=D((IO)2xRDSON-HIx1.
3)(High-SideMOSFET)PC=(1-D)x((IO)2xRDSON-LOx1.
3)(Low-SideMOSFET)Intheaboveequationsthefactor1.
3accountsfortheincreaseinMOSFETRDSONduetoheating.
Alternatively,the1.
3canbeignoredandtheRDSONoftheMOSFETestimatedusingtheRDSONVs.
TemperaturecurvesintheMOSFETdatasheets.
GatecharginglossresultsfromthecurrentdrivingthegatecapacitanceofthepowerMOSFETs,andisapproximatedas:PGC=nx(VDD)xQGxfSW20SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013where'n'isthenumberofMOSFETs(ifmultipledeviceshavebeenplacedinparallel),VDDisthedrivingvoltage(seeMOSFETGATEDRIVERSsection)andQGSisthegatechargeoftheMOSFET.
IfdifferenttypesofMOSFETsareused,the'n'termcanbeignoredandtheirgatechargessimplysummedtoformacumulativeQG.
GatechargelossdiffersfromconductionandswitchinglossesinthattheactualdissipationoccursintheLM2743,andnotintheMOSFETitself.
Switchinglossoccursduringthebrieftransitionperiodasthehigh-sideMOSFETturnsonandoff,duringwhichbothcurrentandvoltagearepresentinthechanneloftheMOSFET.
Itcanbeapproximatedas:PSW=0.
5xVINxIOx(tr+tf)xfSWwheretRandtFaretheriseandfalltimesoftheMOSFET.
Switchinglossoccursinthehigh-sideMOSFETonly.
Forthisexample,themaximumdrain-to-sourcevoltageappliedtoeitherMOSFETis3.
6V.
Themaximumdrivevoltageatthegateofthehigh-sideMOSFETis3.
1V,andthemaximumdrivevoltageforthelow-sideMOSFETis3.
3V.
Duetothelowdrivevoltagesinthisexample,aMOSFETthatturnsonfullywith3.
1Vofgatedriveisneeded.
Fordesignsof5Aandunder,dualMOSFETsinSOIC-8packageprovideagoodtrade-offbetweensize,cost,andefficiency.
SupportComponentsCIN2-Asmallvalue(0.
1Fto1F)ceramiccapacitorshouldbeplacedascloseaspossibletothedrainofthehigh-sideMOSFETandsourceofthelow-sideMOSFET(dualMOSFETsmakethiseasy).
ThiscapacitorshouldbeX5Rtypedielectricorbetter.
RCC,CCC-ThesearestandardfiltercomponentsdesignedtoensuresmoothDCvoltageforthechipsupply.
RCCshouldbe1to10.
CCCshould1F,X5Rtypeorbetter.
CBOOT-Bootstrapcapacitor,typically100nF.
RPULL-UP–Thisisastandardpull-upresistorfortheopen-drainpowergoodsignal(PWGD).
Therecommendedvalueis10kconnectedtoVCC.
Ifthisfeatureisnotnecessary,theresistorcanbeomitted.
D1-AsmallSchottkydiodeshouldbeusedforthebootstrap.
Itallowsforaminimumdropforbothhighandlow-sidedrivers.
TheMBR0520orBAT54workwellinmostdesigns.
RCS-Resistorusedtosetthecurrentlimit.
Sincethedesigncallsforapeakcurrentmagnitude(IOUT+(0.
5xΔIOUT))of4.
8A,asafesettingwouldbe6A.
(Thisisbelowthesaturationcurrentoftheoutputinductor,whichis7A.
)FollowingtheequationfromtheCURRENTLIMITsection,a1.
3kresistorshouldbeused.
RFADJ-Thisresistorisusedtosettheswitchingfrequencyofthechip.
TheresistorvalueiscalculatedfromequationinNORMALOPERATIONsection.
For300kHzoperation,a97.
6kresistorshouldbeused.
CSS-Thesoft-startcapacitordependsontheuserrequirementsandiscalculatedbasedontheequationgiveninthesectiontitledSTARTUP/SOFT-START.
Therefore,fora700μsdelay,a12nFcapacitorissuitable.
ControlLoopCompensationTheLM2743usesvoltage-mode('VM')PWMcontroltocorrectchangesinoutputvoltageduetolineandloadtransients.
Oneoftheattractiveadvantagesofvoltagemodecontrolisitsrelativeimmunitytonoiseandlayout.
HoweverVMrequirescarefulsmallsignalcompensationofthecontrolloopforachievinghighbandwidthandgoodphasemargin.
Thecontrolloopiscomprisedoftwoparts.
Thefirstisthepowerstage,whichconsistsofthedutycyclemodulator,outputinductor,outputcapacitor,andload.
Thesecondpartistheerroramplifier,whichfortheLM2743isa9MHzop-ampusedintheclassicinvertingconfiguration.
Figure35showstheregulatorandcontrolloopcomponents.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback21ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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PowerStageandErrorAmpOnepopularmethodforselectingthecompensationcomponentsistocreateBodeplotsofgainandphaseforthepowerstageanderroramplifier.
Combined,theymaketheoverallbandwidthandphasemarginoftheregulatoreasytosee.
SoftwaretoolssuchasExcel,MathCAD,andMatlabareusefulforshowinghowchangesincompensationorthepowerstageaffectsystemgainandphase.
ThepowerstagemodulatorprovidesaDCgainADCthatisequaltotheinputvoltagedividedbythepeak-to-peakvalueofthePWMramp.
Thisrampis1.
0VP-PfortheLM2743.
TheinductorandoutputcapacitorcreateadoublepoleatfrequencyfDP,andthecapacitorESRandcapacitancecreateasinglezeroatfrequencyfESR.
Forthisexample,withVIN=3.
3V,thesequantitiesare:IntheequationforfDP,thevariableRListhepowerstageresistance,andrepresentstheinductorDCRplustheonresistanceofthetoppowerMOSFET.
ROistheoutputvoltagedividedbyoutputcurrent.
ThepowerstagetransferfunctionGPSisgivenbythefollowingequation,andFigure36showsBodeplotsofthephaseandgaininthisexample.
a=LCO(RO+RC)b=L+CO(RORL+RORC+RCRL)c=RO+RL22SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Figure36.
PowerStageGainandPhaseThedoublepoleat4.
5kHzcausesthephasetodroptoapproximately-130°ataround10kHz.
TheESRzero,at20.
3kHz,providesa+90°boostthatpreventsthephasefromdroppingto-180.
Ifthisloopwereleftuncompensated,thebandwidthwouldbeapproximately10kHzandthephasemargin53°.
Intheory,theloopwouldbestable,butwouldsufferfrompoorDCregulation(duetothelowDCgain)andwouldbeslowtorespondtoloadtransients(duetothelowbandwidth.
)Inpractice,theloopcouldeasilybecomeunstableduetotolerancesintheoutputinductor,capacitor,orchangesinoutputcurrent,orinputvoltage.
Therefore,theloopiscompensatedusingtheerroramplifierandafewpassivecomponents.
Forthisexample,aTypeIII,orthree-pole-two-zeroapproachgivesoptimalbandwidthandphase.
Inmostvoltagemodecompensationschemes,includingTypeIII,asinglepoleisplacedattheorigintoboostDCgainashighaspossible.
TwozeroesfZ1andfZ2areplacedatthedoublepolefrequencytocancelthedoublepolephaselag.
Then,apole,fP1isplacedatthefrequencyoftheESRzero.
AfinalpolefP2isplacedatone-halfoftheswitchingfrequency.
ThegainoftheerroramplifiertransferfunctionisselectedtogivethebestbandwidthpossiblewithoutviolatingtheNyquiststabilitycriteria.
Inpractice,agoodcrossoverpointisone-fifthoftheswitchingfrequency,or60kHzforthisexample.
Thegenericequationfortheerroramplifiertransferfunctionis:InthisequationthevariableAEAisaratioofthevaluesofthecapacitanceandresistanceofthecompensationcomponents,arrangedasshowninFigure35.
AEAisselectedtoprovidethedesiredbandwidth.
Astartingvalueof80,000forAEAshouldgiveaconservativebandwidth.
Increasingthevaluewillincreasethebandwidth,butwillalsodecreasephasemargin.
Designswith45°to60°areusuallybestbecausetheyrepresentagoodtrade-offbetweenbandwidthandphasemargin.
Ingeneral,phasemarginislowestandgainhighest(worst-case)formaximuminputvoltageandminimumoutputcurrent.
OnemethodtoselectAEAistouseaniterativeprocessbeginningwiththeseworst-caseconditions.
Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback23ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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IncreaseAEA2.
Checkoverallbandwidthandphasemargin3.
ChangeVINtominimumandrecheckoverallbandwidthandphasemargin4.
ChangeIOtomaximumandrecheckoverallbandwidthandphasemarginTheprocessendswhenthebothbandwidthandthephasemarginaresufficientlyhigh.
Forthisexampleinputvoltagecanvaryfrom3.
0to3.
6Vandoutputcurrentcanvaryfrom0to4A,andafterafewiterationsamoderategainfactorof101dBisused.
TheerroramplifieroftheLM2743hasaunity-gainbandwidthof9MHz.
Inordertomodeltheeffectofthislimitation,theopen-loopgaincanbecalculatedas:Thenewerroramplifiertransferfunctionthattakesintoaccountunity-gainbandwidthis:ThegainandphaseoftheerroramplifierareshowninFigure37.
Figure37.
ErrorAmp.
GainandPhaseInVMregulators,thetopfeedbackresistorRFB2formsapartofthecompensation.
SettingRFB2to10k,±1%usuallygivesvaluesfortheothercompensationresistorsandcapacitorsthatfallwithinareasonablerange.
(Capacitances>1pF,resistances<1M)CC1,CC2,CC3,RC1,andRC2areselectedtoprovidethepolesandzeroesatthedesiredfrequencies,usingthefollowingequations:24SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Inpractice,agoodtradeoffbetweenphasemarginandbandwidthcanbeobtainedbyselectingtheclosest±10%capacitorvaluesabovewhataresuggestedforCC1andCC2,theclosest±10%capacitorvaluebelowthesuggestionforCC3,andtheclosest±1%resistorvaluesbelowthesuggestionsforRC1,RC2.
NotethatifthesuggestedvalueforRC2islessthan100,itshouldbereplacedbyashortcircuit.
Followingthisguideline,thecompensationcomponentswillbe:CC1=27pF±10%CC2=820pF±10%CC3=2.
7nF±10%RC1=39.
2k±1%RC2=2.
55k±1%ThetransferfunctionofthecompensationblockcanbederivedbyconsideringthecompensationcomponentsasimpedanceblocksZFandZIaroundaninvertingop-amp:Aswiththegenericequation,GEA-ACTUALmustbemodifiedtotakeintoaccountthelimitedbandwidthoftheerroramplifier.
Theresultis:ThetotalcontrollooptransferfunctionHisequaltothepowerstagetransferfunctionmultipliedbytheerroramplifiertransferfunction.
H=GPSxHEACopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback25ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comThebandwidthandphasemargincanbereadgraphicallyfromBodeplotsofHEAareshowninFigure38.
Figure38.
OverallLoopGainandPhaseThebandwidthofthisexamplecircuitis59kHz,withaphasemarginof60°.
EFFICIENCYCALCULATIONSThefollowingisasamplecalculation.
AreasonableestimationoftheefficiencyofaswitchingbuckcontrollercanbeobtainedbyaddingtogethertheOutputPower(POUT)lossandtheTotalPower(PTOTAL)loss:TheOutputPower(POUT)fortheFigure1designis(1.
2Vx4A)=4.
8W.
TheTotalPower(PTOTAL),withanefficiencycalculationtocomplementthedesign,isshownbelow.
ThemajorityofthepowerlossesareduetolowandhighsideofMOSFET'slosses.
ThelossesinanyMOSFETaregroupofswitching(PSW)andconductionlosses(PCND).
PFET=PSW+PCND=61.
38mW+270.
42mWPFET=331.
8mWFETSwitchingLoss(PSW)PSW=PSW(ON)+PSW(OFF)PSW=0.
5xVINxIOUTx(tr+tf)xfSWPSW=0.
5x3.
3Vx4Ax300kHzx31nsPSW=61.
38mW26SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013TheFDS6898Ahasatypicalturn-onrisetimetrandturn-offfalltimetfof15nsand16ns,respectively.
TheswitchinglossesforthistypeofdualN-ChannelMOSFETsare0.
061W.
FETConductionLoss(PCND)PCND=PCND1+PCND2PCND1=(IOUT)2xRDS(ON)xkxDPCND2=(IOUT)2xRDS(ON)xkx(1-D)RDS(ON)=13mandthefactorisaconstantvalue(k=1.
3)toaccountfortheincreasingRDS(ON)ofaFETduetoheating.
PCND1=(4A)2x13mx1.
3x0.
364PCND2=(4A)2x13mx1.
3x(1-0.
364)PCND=98.
42mW+172mW=270.
42mWTherearefewadditionallossesthataretakenintoaccount:ICOperatingLoss(PIC)PIC=IQ_VCCxVCC,whereIQ-VCCisthetypicaloperatingVCCcurrentPIC=1.
5mAx3.
3V=4.
95mWFETGateChargingLoss(PGATE)PGATE=nxVCCxQGSxfSWPGATE=2x3.
3Vx3nCx300kHzPGATE=5.
94mWThevaluenisthetotalnumberofFETsusedandQGSisthetypicalgate-sourcechargevalue,whichis3nC.
FortheFDS6898Athegatecharginglossis5.
94mW.
InputCapacitorLoss(PCAP)where,Herenisthenumberofparalleledcapacitors,ESRistheequivalentseriesresistanceofeach,andPCAPisthedissipationineach.
Soforexampleifweuseonlyoneinputcapacitorof24m.
PCAP=88.
8mWOutputInductorLoss(PIND)PIND=I2OUTxDCRwhereDCRistheDCresistance.
Therefore,forexamplePIND=(4A)2x11mPIND=176mWCopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback27ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
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comTotalSystemEfficiencyPTOTAL=PFET+PIC+PGATE+PCAP+PINDExampleCircuitsFigure39.
3.
3Vto1.
8V@2A,fSW=300kHzTable1.
BillofMaterialsPARTPARTNUMBERTYPEPACKAGEDESCRIPTIONVENDORU1LM2743SynchronousTSSOP-14TIControllerQ1FDS6898ADualN-MOSFETSOIC-820V,10m@4.
5V,Fairchild16nCD1MBR0520LTISchottkyDiodeSOD-123L1DO3316P-472Inductor4.
7H,4.
8Arms18mCoilcraftCIN116SP100MAluminumElectrolytic10mmx6mm100F,16V,2.
89ArmsSanyoCO16SP220MAluminumElectrolytic10mmx6mm220F,6.
3V3.
1ArmsSanyoCCC,CBOOT,VJ1206Y104KXXACapacitor12060.
1F,10%VishayCIN2,CO2CC3VJ0805Y332KXXACapacitor8053300pF,10%VishayCSSVJ0805A123KXAACapacitor80512nF,10%VishayCC2VJ0805A821KXAACapacitor805820pF10%VishayCC1VJ0805A220KXAACapacitor80522pF,10%VishayRFB2CRCW08051002FResistor80510.
0k1%VishayRFB1CRCW08054991FResistor8054.
99k1%VishayRFADJCRCW08051103FResistor805110k1%VishayRC2CRCW08052101FResistor8052.
1k1%VishayRCSCRCW08052101FResistor8052.
1k1%Vishay28SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
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comSNVS276G–APRIL2004–REVISEDMARCH2013Table1.
BillofMaterials(continued)PARTPARTNUMBERTYPEPACKAGEDESCRIPTIONVENDORRCCCRCW080510R0FResistor80510.
01%VishayRC1CRCW08055492FResistor80554.
9k1%VishayRPULL-UPCRCW08051003JResistor805100k5%VishayFigure40.
5Vto2.
5V@2A,fSW=300kHzTable2.
BillofMaterialsPARTPARTNUMBERTYPEPACKAGEDESCRIPTIONVENDORU1LM2743SynchronousTSSOP-14TIControllerQ1FDS6898ADualN-MOSFETSOIC-820V,10m@4.
5V,16nCFairchildD1MBR0520LTISchottkyDiodeSOD-123L1DO3316P-682Inductor6.
8H,4.
4Arms,27mCoilcraftCIN116SP100MAluminum10mmx6mm100F,16V,2.
89ArmsSanyoElectrolyticCO110SP56MAluminum6.
3mmx6mm56F,10V1.
7ArmsSanyoElectrolyticCCC,CBOOT,VJ1206Y104KXXACapacitor12060.
1F,10%VishayCIN2,CO2CC3VJ0805Y182KXXACapacitor8051800pF,10%VishayCSSVJ0805A123KXAACapacitor80512nF,10%VishayCC2VJ0805A821KXAACapacitor805820pF10%VishayCC1VJ0805A330KXAACapacitor80533pF,10%VishayRFB2CRCW08051002FResistor80510.
0k1%VishayRFB1CRCW08053161FResistor8053.
16k1%VishayRFADJCRCW08051103FResistor805110k1%VishayRC2CRCW08051301FResistor8051.
3k1%VishayRCSCRCW08052101FResistor8052.
1k1%VishayRCCCRCW080510R0FResistor80510.
01%VishayRC1CRCW08053322FResistor80533.
2k1%VishayRPULL-UPCRCW08051003JResistor805100k5%VishayCopyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback29ProductFolderLinks:LM2743LM2743SNVS276G–APRIL2004–REVISEDMARCH2013www.
ti.
comFigure41.
12Vto3.
3V@4A,fSW=300kHzTable3.
BillofMaterialsPARTPARTNUMBERTYPEPACKAGEDESCRIPTIONVENDORU1LM2743SynchronousTSSOP-14TIControllerQ1FDS6898ADualN-MOSFETSOIC-820V,10m@4.
5V,16nCFairchildD1MBR0520LTISchottkyDiodeSOD-123L1DO3316P-332Inductor3.
3H,5.
4Arms15mCoilcraftCIN116SP100MAluminumElectrolytic10mmx6mm100F,16V,2.
89ArmsSanyoCO16SP220MAluminumElectrolytic10mmx6mm220F,6.
3V3.
1ArmsSanyoCCC,CBOOT,VJ1206Y104KXXACapacitor12060.
1F,10%VishayCIN2,CO2CC3VJ0805Y222KXXACapacitor8052200pF,10%VishayCSSVJ0805A123KXAACapacitor80512nF,10%VishayCC2VJ0805Y332KXXACapacitor8053300pF10%VishayCC1VJ0805A820KXAACapacitor80582pF,10%VishayRFB2CRCW08051002FResistor80510.
0k1%VishayRFB1CRCW08052211FResistor8052.
21k1%VishayRFADJCRCW08051103FResistor805110k1%VishayRC2CRCW08052611FResistor8052.
61k1%VishayRCSCRCW08054121FResistor8054.
12k1%VishayRCCCRCW080510R0FResistor80510.
01%VishayRC1CRCW08051272FResistor80512.
7k1%VishayRPULL-UPCRCW08051003JResistor805100k5%Vishay30SubmitDocumentationFeedbackCopyright2004–2013,TexasInstrumentsIncorporatedProductFolderLinks:LM2743LM2743www.
ti.
comSNVS276G–APRIL2004–REVISEDMARCH2013REVISIONHISTORYChangesfromRevisionF(March2013)toRevisionGPageChangedlayoutofNationalDataSheettoTIformat30Copyright2004–2013,TexasInstrumentsIncorporatedSubmitDocumentationFeedback31ProductFolderLinks:LM2743PACKAGEOPTIONADDENDUMwww.
ti.
com21-Feb-2014Addendum-Page1PACKAGINGINFORMATIONOrderableDeviceStatus(1)PackageTypePackageDrawingPinsPackageQtyEcoPlan(2)Lead/BallFinish(6)MSLPeakTemp(3)OpTemp(°C)DeviceMarking(4/5)SamplesLM2743MTCNRNDTSSOPPW1494TBDCallTICallTI-40to1252743MTCLM2743MTC/NOPBACTIVETSSOPPW1494Green(RoHS&noSb/Br)CUSNLevel-1-260C-UNLIM-40to1252743MTCLM2743MTCXNRNDTSSOPPW142500TBDCallTICallTI-40to1252743MTCLM2743MTCX/NOPBACTIVETSSOPPW142500Green(RoHS&noSb/Br)CUNIPDAU|CUSNLevel-1-260C-UNLIM-40to1252743MTC(1)Themarketingstatusvaluesaredefinedasfollows:ACTIVE:Productdevicerecommendedfornewdesigns.
LIFEBUY:TIhasannouncedthatthedevicewillbediscontinued,andalifetime-buyperiodisineffect.
NRND:Notrecommendedfornewdesigns.
Deviceisinproductiontosupportexistingcustomers,butTIdoesnotrecommendusingthispartinanewdesign.
PREVIEW:Devicehasbeenannouncedbutisnotinproduction.
Samplesmayormaynotbeavailable.
OBSOLETE:TIhasdiscontinuedtheproductionofthedevice.
(2)EcoPlan-Theplannedeco-friendlyclassification:Pb-Free(RoHS),Pb-Free(RoHSExempt),orGreen(RoHS&noSb/Br)-pleasecheckhttp://www.
ti.
com/productcontentforthelatestavailabilityinformationandadditionalproductcontentdetails.
TBD:ThePb-Free/Greenconversionplanhasnotbeendefined.
Pb-Free(RoHS):TI'sterms"Lead-Free"or"Pb-Free"meansemiconductorproductsthatarecompatiblewiththecurrentRoHSrequirementsforall6substances,includingtherequirementthatleadnotexceed0.
1%byweightinhomogeneousmaterials.
Wheredesignedtobesolderedathightemperatures,TIPb-Freeproductsaresuitableforuseinspecifiedlead-freeprocesses.
Pb-Free(RoHSExempt):ThiscomponenthasaRoHSexemptionforeither1)lead-basedflip-chipsolderbumpsusedbetweenthedieandpackage,or2)lead-baseddieadhesiveusedbetweenthedieandleadframe.
ThecomponentisotherwiseconsideredPb-Free(RoHScompatible)asdefinedabove.
Green(RoHS&noSb/Br):TIdefines"Green"tomeanPb-Free(RoHScompatible),andfreeofBromine(Br)andAntimony(Sb)basedflameretardants(BrorSbdonotexceed0.
1%byweightinhomogeneousmaterial)(3)MSL,PeakTemp.
-TheMoistureSensitivityLevelratingaccordingtotheJEDECindustrystandardclassifications,andpeaksoldertemperature.
(4)Theremaybeadditionalmarking,whichrelatestothelogo,thelottracecodeinformation,ortheenvironmentalcategoryonthedevice.
(5)MultipleDeviceMarkingswillbeinsideparentheses.
OnlyoneDeviceMarkingcontainedinparenthesesandseparatedbya"~"willappearonadevice.
IfalineisindentedthenitisacontinuationofthepreviouslineandthetwocombinedrepresenttheentireDeviceMarkingforthatdevice.
(6)Lead/BallFinish-OrderableDevicesmayhavemultiplematerialfinishoptions.
Finishoptionsareseparatedbyaverticalruledline.
Lead/BallFinishvaluesmaywraptotwolinesifthefinishvalueexceedsthemaximumcolumnwidth.
PACKAGEOPTIONADDENDUMwww.
ti.
com21-Feb-2014Addendum-Page2ImportantInformationandDisclaimer:TheinformationprovidedonthispagerepresentsTI'sknowledgeandbeliefasofthedatethatitisprovided.
TIbasesitsknowledgeandbeliefoninformationprovidedbythirdparties,andmakesnorepresentationorwarrantyastotheaccuracyofsuchinformation.
Effortsareunderwaytobetterintegrateinformationfromthirdparties.
TIhastakenandcontinuestotakereasonablestepstoproviderepresentativeandaccurateinformationbutmaynothaveconducteddestructivetestingorchemicalanalysisonincomingmaterialsandchemicals.
TIandTIsuppliersconsidercertaininformationtobeproprietary,andthusCASnumbersandotherlimitedinformationmaynotbeavailableforrelease.
InnoeventshallTI'sliabilityarisingoutofsuchinformationexceedthetotalpurchasepriceoftheTIpart(s)atissueinthisdocumentsoldbyTItoCustomeronanannualbasis.
OTHERQUALIFIEDVERSIONSOFLM2743,LM2743-Q1:Catalog:LM2743Automotive:LM2743-Q1NOTE:QualifiedVersionDefinitions:Catalog-TI'sstandardcatalogproductAutomotive-Q100devicesqualifiedforhigh-reliabilityautomotiveapplicationstargetingzerodefectsTAPEANDREELINFORMATION*AlldimensionsarenominalDevicePackageTypePackageDrawingPinsSPQReelDiameter(mm)ReelWidthW1(mm)A0(mm)B0(mm)K0(mm)P1(mm)W(mm)Pin1QuadrantLM2743MTCXTSSOPPW142500330.
012.
46.
958.
31.
68.
012.
0Q1LM2743MTCX/NOPBTSSOPPW142500330.
012.
46.
958.
31.
68.
012.
0Q1PACKAGEMATERIALSINFORMATIONwww.
ti.
com22-Feb-2014PackMaterials-Page1*AlldimensionsarenominalDevicePackageTypePackageDrawingPinsSPQLength(mm)Width(mm)Height(mm)LM2743MTCXTSSOPPW142500367.
0367.
035.
0LM2743MTCX/NOPBTSSOPPW142500367.
0367.
035.
0PACKAGEMATERIALSINFORMATIONwww.
ti.
com22-Feb-2014PackMaterials-Page2IMPORTANTNOTICETexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,enhancements,improvementsandotherchangestoitssemiconductorproductsandservicesperJESD46,latestissue,andtodiscontinueanyproductorserviceperJESD48,latestissue.
Buyersshouldobtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentandcomplete.
Allsemiconductorproducts(alsoreferredtohereinas"components")aresoldsubjecttoTI'stermsandconditionsofsalesuppliedatthetimeoforderacknowledgment.
TIwarrantsperformanceofitscomponentstothespecificationsapplicableatthetimeofsale,inaccordancewiththewarrantyinTI'stermsandconditionsofsaleofsemiconductorproducts.
TestingandotherqualitycontroltechniquesareusedtotheextentTIdeemsnecessarytosupportthiswarranty.
Exceptwheremandatedbyapplicablelaw,testingofallparametersofeachcomponentisnotnecessarilyperformed.
TIassumesnoliabilityforapplicationsassistanceorthedesignofBuyers'products.
BuyersareresponsiblefortheirproductsandapplicationsusingTIcomponents.
TominimizetherisksassociatedwithBuyers'productsandapplications,Buyersshouldprovideadequatedesignandoperatingsafeguards.
TIdoesnotwarrantorrepresentthatanylicense,eitherexpressorimplied,isgrantedunderanypatentright,copyright,maskworkright,orotherintellectualpropertyrightrelatingtoanycombination,machine,orprocessinwhichTIcomponentsorservicesareused.
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Informationofthirdpartiesmaybesubjecttoadditionalrestrictions.
ResaleofTIcomponentsorserviceswithstatementsdifferentfromorbeyondtheparametersstatedbyTIforthatcomponentorservicevoidsallexpressandanyimpliedwarrantiesfortheassociatedTIcomponentorserviceandisanunfairanddeceptivebusinesspractice.
TIisnotresponsibleorliableforanysuchstatements.
Buyeracknowledgesandagreesthatitissolelyresponsibleforcompliancewithalllegal,regulatoryandsafety-relatedrequirementsconcerningitsproducts,andanyuseofTIcomponentsinitsapplications,notwithstandinganyapplications-relatedinformationorsupportthatmaybeprovidedbyTI.
Buyerrepresentsandagreesthatithasallthenecessaryexpertisetocreateandimplementsafeguardswhichanticipatedangerousconsequencesoffailures,monitorfailuresandtheirconsequences,lessenthelikelihoodoffailuresthatmightcauseharmandtakeappropriateremedialactions.
BuyerwillfullyindemnifyTIanditsrepresentativesagainstanydamagesarisingoutoftheuseofanyTIcomponentsinsafety-criticalapplications.
Insomecases,TIcomponentsmaybepromotedspecificallytofacilitatesafety-relatedapplications.
Withsuchcomponents,TI'sgoalistohelpenablecustomerstodesignandcreatetheirownend-productsolutionsthatmeetapplicablefunctionalsafetystandardsandrequirements.
Nonetheless,suchcomponentsaresubjecttotheseterms.
NoTIcomponentsareauthorizedforuseinFDAClassIII(orsimilarlife-criticalmedicalequipment)unlessauthorizedofficersofthepartieshaveexecutedaspecialagreementspecificallygoverningsuchuse.
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TIhasspecificallydesignatedcertaincomponentsasmeetingISO/TS16949requirements,mainlyforautomotiveuse.
Inanycaseofuseofnon-designatedproducts,TIwillnotberesponsibleforanyfailuretomeetISO/TS16949.
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