ieee802.3at高功率供電器件pd接口帶有外部柵極驅(qū)動器

ZHCS845–MARCH查詢樣品特 應(yīng)用范具有狀態(tài)標(biāo)志的IEEE802.3at類型2硬件分 IEEE802.3at兼容器 應(yīng)晶體管 微PowerPad0.85A的電流。通過使用一個外 Figure1.TypicalFromSparePairsorDC/DC

SO-8

Pleasebeawarethatanimportantnoticeconcerningavailability,standardwarranty,anduseincriticalapplicationsofTexasInstrumentssemiconductorproductsanddierstheretoappearsattheendofthisdatasheet.Copyright?2012,TexasInstrumentsPowerPad,Copyright?2012,TexasInstrumentsPRODUCTIONDATAinformationiscurrentasofpublicationdate.ProductsconformtospecificationsperthetermsoftheInstrumentsstandardwarranty.Productionprocessingdoes EnglishDataSheet:necessarilyincludetestingofallThesedeviceshavelimitedbuilt-inESDprotection.TheleadsshouldbeshortedtogetherorthedeviceplacedinconductivefoamduringstorageorhandlingtopreventelectrostaticdamagetotheMOSgates.PRODUCT–40°Cto(SO-8Forthemostcurrentpackageandorderinginformation,seethePackageOptionAddendumattheendofthis,orseetheTIwebsiteat UM mendedTJrange;voltageswithrespecttoVVSS(unlessotherwiseInputVDD,V[CDB,T2P]toSinkingInternallyCDB,51SourcingHumanbody2ChargeddeviceVSystemlevel8InternallyStressesthoselistedunder“absoluteumratings”maycausepermanentdamagetothedevice.Thesearestressratingsonlyandfunctionaloperationofthedeviceattheseoranyotherconditionsthoseindicatedunder“ mendedoperatingconditions”isnotimplied.Exposuretoabsolute- um-ratedconditionsforextendedperiodsmayaffectdevicereliability.WithI(RTN)=DonotapplyvoltagestotheseSOAlimitedtoRTN=80Vat1.2DischargesappliedtocircuitofFigure1betweenRJ-45,adapter,andoutputvoltagerailsperEN61000-4-2,THERMALTHERMALSO-88Junction-to-ambientthermalJunction-to-case(top)thermalJunction-to-boardthermalJunction-to-topcharacterizationJunction-to-boardcharacterizationJunction-to-case(bottom)thermal有關(guān)傳統(tǒng)和新的熱度量的信息，請參閱IC封裝熱度量應(yīng)用報告SPRA953MENDEDOPERATINGVoltageswithrespecttoVSS(unlessotherwiseInputvoltageRTN,0VT2PorCDBto0SinkingACDB,2ΩJunction(1)VoltageshouldnotbeexternallyappliedtothisELECTRICAL40V≤VVDD≤57V,RDEN=24.9k?,CDB,CLS,GATE,T2Popen;–40°C≤TJ≤125°C.Positivecurrentsareintopins.Typicalvaluesareat25°C.AllvoltagesarewithrespecttoVVSS(unlessotherwisenoted)TESTDETECTIONDetectionMeasureISUPPLY(VDD,RTN,VDD=1.4VDD=10.1V,NotinBiasDENopen,VVDD=10.1V,MeasureISUPPLY,Notin3DisableDEN35VCLASSIFICATIONClassification13V≤VDD≤21V,MeasureIVDD+IDEN+RCLS=1270RCLS=243RCLS=137RCLS=90.9RCLS=63.4ClasslowerVDDrising,VCLSVClassupperVDDrising,VMarkresetVVDD35Markstate2-pointmeasurementat5Vand10.16LeakageVDD=57V,VCLS=0V,measure1GATE(AuxiliaryGateOutputhigh8VSourcingVGATE=0SinkingVGATE=4V,VDD=48→25VDD=25V,VGATE=0→45CurrentlimitPASSDEVICEOnΩInputbiasVDD=VRTN=30V,measureCurrentVRTN=1.51AInrushcurrentVRTN=2V,VDD:20V→48ELECTRICALCHARACTERISTICS40V≤VVDD≤57V,RDEN=24.9k?,CDB,CLS,GATE,T2Popen;–40°C≤TJ≤125°C.Positivecurrentsareintopins.Typicalvaluesareat25°C.AllvoltagesarewithrespecttoVVSS(unlessotherwisenoted)TESTInrushPercentageofinrushFoldbackVRTNVFoldbackdeglitchVRTNrisingtowhencurrentlimitchangestoinrushcurrentCONVERTERDISABLEOutputlowICDB=2mA,VRTN=2V,VDD:20V→48VMinimumvoltage,–RTN),forCDBtobeVCDB=VDD,ICDB=1mA,in3VLeakageVCDB=57V,VRTN=0TYPE2PSEINDICATIONOutputlowIT2P=2mA,after2-eventclassificationandinrushiscomplete,VRTN=0VVLeakageVT2P=57V,VRTN=0UVLOrisingVVDDVUVLOfallingVVDDUVLOVTHERMALVDDBIASOperating40V≤VVDD≤57TYPE1HardwarePoEStartup UVLO T2PlowTYPE2HardwareFigure2.PDClassState182736PIN1IConnecttopositivePoEinputpowerrail.Bypasswith0.1μFto2Connect24.9kΩtoVDDfordetection.PulltoVSSdisablepass3OConnectresistorfromCLStoVSStoprogramclassification4ConnecttonegativepowerrailderivedfromPoE5ODrainofPoEpass6OOpen-drainconverterdisableoutput,activelow,referencedto7OActivelowindicatestype2PSE8OAuxiliarygatedriverThePowerPad?mustbeconnectedtoVSS.Alargefillareaisrequiredtoassistinheat

12V12V22V 2.5V5V& V MarkCompUVLOComp1=0=current6Inrush38.1VCurrentlimit0GATEHighifoverISignalsreferencedtoVunlessotherwiseIsense,1if<90%ofinrushcurrentType2QFigure3.FunctionalBlockDETAILEDPINThefollowingdescriptionsrefertotheschematicofFigure1orFigure4andthefunctionalblockCDB(ConverterDisbleBar):ThisactivelowoutputispulledtoRTNwhenthedeviceisininrushcurrentlimiting,goingopenwheninrushperiodhascompletedoncetheGATEoutputhas ehigherthan6V.Thisensuresthattheexternalpasstransistorisenhancedbeforetheloadisenabled.Itremainsinahighimpedancestateatallload.CDBmaybeleftopenifitisnotused.CLS:Anexternalresistor(RCLSinFigure1)connectedbetweentheCLSpinandVSSprovidesaclassificationsignaturetothePSE.Thecontrollerplacesavoltageofapproximay2.5VacrosstheexternalresistorwheneverthevoltagedifferentialbetweenVDDandVSSliesbetweenabout10.9Vand22V.Thecurrentdrawnbythisresistor,combinedwiththeinternalcurrentdrainofthecontrollerandanyleakagethroughtheinternalpassMOSFET,createstheclassificationcurrent.Table1liststheexternalresistorvaluesrequiredforeachofthePDpowerrangesdefinedbyIEEE802.3at.The umaveragepowerdrawnbythePD,plusthepowerdtothedownstreamload,shouldnotexceedthe umpowerindicatedinTable1.High-powerPSEsmayperformtwoclassificationcyclesifClass4ispresentedonthefirstTable1.ClassResistorATPD(W)ATPD(W)RCLS(Ω)01234DEN(DetectionandEnable):Thispinimplementstwoseparatefunctions.Aresistor(RDENinFigure1)connectedbetweenVDDandDENgeneratesadetectionsignaturewheneverthevoltagedifferentialbetweenVDDandVSSliesbetweenapproximay1.4and10.9V.thisrange,thecontrollerdisconnectsthisresistortosavepower.ForapplicationsthatwishtocomplywiththerequirementsofIEEE802.3at,theexternalshouldequal24.9Ifthe connectedbetweenVDDandDENisdividedintotworoughlyequalportions,thentheapplicationcircuitcandisablethePDbygroundingthetappointbetweenthetwos.ThisactionsimultaneouslyspoilsthedetectionsignatureandtherebysignalsthePSEthatthePDnolongerrequirespower.GATE(AuxiliaryGateDriver):ThispinallowstheconnectionofanexternalpassMOSFETinparallelwiththeinternalpasstransistor.TheGATEpinenablestheexternaltransistorafterinrushhascompleted.CurrentisdividedbetweentheexternalMOSFETandtheinternaltransistorasafunctionoftheirrespectives.Theadditionofabalancingresistor(RBLSTinFigure1)inserieswithRTNandtheexternalMOSFETcanensuredesireddistributionofthetwocurrents.WhenevertheRTNcurrentexceedsthecurrentlimitthreshold,theGATEpinwillpulllowaftera365μsdelay.TheGATEpinispulledlowinthermalshutdown.Afterthecontrollercoolsdown,andtheinrushcycleiscomplete,theGATEpinrisesagain.RTN:Thispinprovidesthenegativepowerreturnpathfortheload.OnceVDDexceedstheUVLOthreshold,theinternalpassMOSFETpullsRTNtoVSS.InrushlimitingpreventstheRTNcurrentfromexceedingabout140mAuntilthebulkcapacitance(CBULKinFigure1)isfullycharged.InrushendswhentheRTNcurrentdropsbelowabout125mA.TheRTNcurrentissubsequentlylimitedtoabout1A.CDBpullslowtosignalthedownstreamloadthatthebulkcapacitanceisfullycharged.IfRTNeverexceedsabout12Vforlongerthan800μs,thentheTPS2379returnstoinrushlimiting.T2P(Type-2PSEIndicator):ThecontrollerpullsthispintoRTNwhenevertype-2hardwareclassificationhasbeenobserved.TheT2Poutputwillreturntoahigh-impedancestateifthepartentersthermalshutdown,thepassMOSFETentersinrushlimiting,orifatype-2PSEwasnotdetected.Thecircuitrythatwatchesfortype-2hardwareclassificationlatchesitsresultwhentheVDD-to-VSSvoltagedifferentialrisesabovetheupperclassificationthreshold.ThiscircuitresetswhentheVDD-to-VSSvoltagedifferentialdropsbelowthemarkthreshold.TheT2Ppincanbeleftunconnectedifitisnotused.VDD:Thispinconnectstothepositivesideoftheinputsupply.ItprovidesoperatingpowertothePDcontrollerandallowsmonitoringoftheinputlinevoltage.VSS:Thisistheinputsupplynegativerailthatservesasalocalground.ThePowerPad?mustbeconnectedtothispintoensureproperoperation.ThePowerPadisinternallyconnectedtoVSS.ItshouldbetiedtoalargeVSScopperareaonthePCBtoprovidealowthermalpathtothecircuitboard.Itis mendedthataclearanceof0.025”bemaintainedbetweenVSSandhigh-voltagesignalssuchasVDD.TYPICAL 6II 210

0 VVDDVSS VVDDVSS Figure4.DetectionBiasCurrentvsPoE Figure5.DetectionvsPoEVVVDDVSS

VVVDDVSSClassClassUpperThreshold,ClassUpperThreshold,

JunctionTemperature JunctionTemperature Figure6.ClassificationUpperThresholdvs Figure7.ClassificationLowerThresholdvs II

4VVVDDVSS

3 VVDDVSS JunctionTemperature Figure8.IVDDBiasCurrentvs Figure9.MarkResetThresholdvsTYPICALTYPICALCHARACTERISTICS

9

PassPass

JunctionTemperature JunctionTemperature Figure10.Markvs Figure11.PassFETvs CurrentInrushLimitCurrentInrushLimitnrushCurrentTermination

JunctionTemperature JunctionTemperature Figure12.PoEInrushCurrentLimitvs Figure13.InrushTerminationThresholdvs1CurrentLimitCurrentLimit10

38VVDDVVDDVSS3838 JunctionTemperature JunctionTemperature Figure14.PoECurrentLimitvs Figure15.UVLORisingThresholdvs VVVDDVSS

VGATEVGATEVSS3232

3200JunctionTemperature JunctionTemperature Figure16.UVLOFallingThresholdvs Figure17.AuxiliaryGateVoltagevsDETAILEDPoEThefollowingtextisintendedasanaidinunderstandingtheoperationoftheTPS2379butnotasasubstitutefortheIEEE802.3atstandard.TheIEEE802.3atstandardisanupdatetoIEEE802.3-2008clause33(PoE),addinghigh-poweroptionsandenhancedclassification.Generallyspeaking,adevicecomplianttoIEEE802.3-2008isreferredtoasatype1device,anddeviceswithhighpowerandenhancedclassificationwillbereferredtoastype2devices.Standardschangeandshouldalwaysbereferencedwhenmakingdesigndecisions.TheIEEE802.3atstandarddefinesamethodofsafelypoweringaPD(powereddevice)overacablebypowersourcingequipment(PSE),andthenremovingpowerifaPDisdisconnected.Theprocessproceedsthroughanidlestateandthreeoperationalstatesofdetection,classification,andoperation.ThePSEleavesthecableunpowered(idlestate)whileitperiodicallylookstoseeifsomethinghasbeenpluggedin;thisisreferredtoasdetection.ThelowpowerlevelsusedduringdetectionareunlikelytodamagedevicesnotdesignedforPoE.IfavalidPDsignatureispresent,thePSEmayinquirehowmuchpowerthePDrequires;thisisreferredtoasclassification.ThePSEmaythenpowerthePDifithasadequatecapacity.Type2PSEsarerequiredtodotype1hardwareclassificationplusa(new)data-layerclassification,oranenhancedtype2hardwareclassification.Type1PSEsarenotrequiredtodohardwareordatalinklayer(DLL)classification.Atype2PDmustdotype2hardwareclassificationaswellasDLLclassification.ThePDmayreturnthedefault,13Wcurrent-encodedclass,oroneoffourotherchoices.DLLclassificationoccursafterpower-onandtheEthernetdatalinkhasbeenestablished.MustTurnOffby-VoltageFallingLowerLimit-OperatingRangeOncestarted,thePDmustpresentamaintainpowersignature(MPS)toassurethePSEthatitisstillpresent.ThePSEmonitorsitsoutputforavalidMPS,andturnstheportoffifitlosestheMPS.LossoftheMustTurnOffby-VoltageFallingLowerLimit-OperatingRangeNormal10.110.157PIVoltageLowerLimitUpperLimitUpperLimitIEEELowerLimitIEEELowerLimit-13WOp.ThePDinput,typicallyanRJ-45eight-leadconnector,isreferredtoasthepowerinterface(PI).PDinputrequirementsdifferfromPSEoutputrequirementstoaccountforvoltagedropsandoperatingmargin.Thestandardallotsthe umlosstothecableregardlessoftheactualinstallationtosimplifyimplementation.IEEE802.3-2008wasdesignedtorunoverinfrastructureincludingISO/IEC11801classC(CAT3perTIA/EIA-568)thatmayhavehadAWG26conductors.IEEE802.3attype2cablingpowerlossallotmentsandvoltagedropshavebeenadjustedfor12.5ΩpowerloopsperISO/IEC11801classD(CAT5orhigherperTIA/EIA-568,typicallyAWG#24conductors).Table2showskeyoperationallimitsbrokenoutforthetworevisionsofthestandard.Table2.ComparisonofOperationalPOWERPSESTATICSTATICPDINPUTPowerPowerIEEE802.3at-802.3at(Type37V–802.3at(Type37V–42.5V–ThePSEcanapplyvoltageeitherbetweentheRXandTXpairs(pins1-2and3-6for10baseTor100baseT),orbetweenthetwosparepairs(4-5and7-8).Powerapplicationtothesamepincombinationsin1000baseTsystemsisrecognizedinIEEE802.3at.1000baseTsystemscanhandledataonallpairs,eliminatingthesparepairterminology.ThePSEmayonlyapplyvoltagetoonesetofpairsatatime.ThePDusesinputdiodebridgestoacceptpowerfromanyofthepossiblePSEconfigurations.ThevoltagedropsassociatedwiththeinputbridgescreateadifferencebetweenthestandardlimitsatthePIandtheTPS2379specifications.Acomplianttype2PDhaspowermanagementrequirementsnotpresentwithatype1PD.Theserequirementsincludethefollowing:Mustinterprettype2hardwareMustpresenthardwareclassMustimplementDLLMustbehavelikeatype1PDduringinrushandMustnotdrawmorethan13Wfor80msafterthePSEappliesoperatingvoltage(power-Mustnotdrawmorethan13Wifithasnotreceivedatype2hardwareclassificationorreceivedpermissionthroughDLL,Mustmeetvariousoperatingandtransienttemplates,Optionallymonitorforthepresenceorabsenceofanadapter(assumehighAsaresultoftheserequirements,thePDmustbeabletodynamicallycontrolitsloading,andmonitorT2Pforchanges.Incaseswherethedesignneedstoknowspecificallyifanadapterispluggedinandoperational,theadaptershouldbeindividuallymonitored,typicallywithanoptocoupler.ThresholdTheTPS2379hasanumberofinternalcomparatorswithhysteresisforstableswitchingbetweenthevariousstates.Figure19relatestheparametersintheElectricalCharacteristicssectiontothePoEstates.ThemodelabeledIdlebetweenClassificationandOperationimpliesthattheDEN,CLS,andRTNpinsareallhighimpedance.ThestatelabeledMark,whichisdrawnindashedlines,ispartofthenewtype2hardwareclassstatemachine.VDD-Type VCL VCUVDD-TypeNote:VariablenamesrefertoElectricalCharacteristicTableFigure19.ThresholdPoEStartupCurrent:100ThewaveformsofFigure20demonstratedetection,classification,andstartupfromaPSEwithtype2hardwareclassification.ThekeywaveformsshownareV(VDD-VSS),V(RTN-VSS),andIPI.IEEE802.3atrequiresaminimumoftwodetectionlevels,twoclassandmarkcycles,andstartupfromthesecondmarkevent.VRTNtoVSSfallsastheTPS2379chargesCBULKfollowingapplicationoffullvoltage.InFigure20,deassertionoftheCDBsignalisdelayedandusedtoenableloadcurrentasseenintheICurrent:100VRTN-Time:50Figure20.TheTPS2379pullsDENtoVSSwheneverV(VDD-VSS)isbelowthelowerclassificationthreshold.WhentheinputvoltagerisesaboveVCL-ON,theDENpingoestoanopen-drainconditiontoconservepower.Whileindetection,RTNishighimpedance,andalmostalltheinternalcircuitsaredisabled.AnRDENof24.9kΩ(±1%),presentsthecorrectsignature.Itmaybeasmall,low-powerresistorsinceitonlyseesastressofabout5mW.AvalidPDdetectionsignatureisanincremental(ΔV/ΔI)between23.75kΩand26.25kΩatthePI.Thedetection seenbythePSEatthePIistheresultoftheinputbridge inserieswiththeparallelcombinationofRDENandinternalVDDloading.Theinputdiodebridge’sincremental maybehundredsofohmsattheverylowcurrentsdrawnwhen2.7VisappliedtothePI.Theinputbridge partiallycompensatedbytheTPS2379'seffective duringdetection.Thetype2hardwareclassificationprotocolofIEEE802.3atspecifiesthatatype2PSEdropsitsoutputvoltageintothedetectionrangeduringtheclassificationsequence.ThePDisrequiredtohaveanincorrectdetectionsignatureinthiscondition,whichisreferredtoasamarkevent(seeFigure20).Afterthefirstmarkevent,theTPS2379willpresentasignaturelessthan12kΩuntilithasexperiencedaV(VDD-VSS)voltagebelowthemarkresetthreshold(VMSR).ThisisexplainedmorefullyunderHardwareClassification.HardwareHardwareclassificationallowsaPSEtodetermineaPD’spowerrequirementsbeforepowering,andhelpswithpowermanagementoncepowerisapplied.Type2hardwareclassificationpermitshighpowerPSEsandPDstodeterminewhethertheconnecteddevicecansupporthigh-poweroperation.Atype2PDpresentsclass4inhardwaretoindicatethatitisahigh-powerdevice.Atype1PSEwilltreataclass4devicelikeaclass0device,allotting13WifitchoosestopowerthePD.APDthatreceivesa2-eventclassunderstandsthatitispoweredfromahigh-powerPSEanditmaydrawupto25.5Wimmediayafterthe80msstartupperiodcompletes.Atype2PDthatdoesnotreceivea2-eventhardwareclassificationmaychoosetonotstart,ormuststartina13WconditionandrequestmorepowerthroughtheDLLafterstartup.Thestandardrequiresatype2PDtoindicatethatitisunderpoweredifthisoccurs.Startupofahigh-powerPDunder13Wimplicitlyrequiressomeformofpoweringdownsectionsoftheapplicationcircuits. umpowerentriesinTable1determinetheclassthePDmustadvertise.ThePSEmaydisconnectaPDifitdrawsmorethanitsstatedClasspower,whichmaybethehardwareclassoralowerDLL-derivedpowerlevel.ThestandardpermitsthePDtodrawlimitedcurrentpeaksthatincreasetheinstantaneouspowerabovetheTable1limit,howevertheaveragepowerrequirementalwaysapplies.TheTPS2379implementstwo-eventclassification.SelectinganRCLSof63.4Ωprovidesavalidtype2signature.TPS2379maybeusedasacompatibletype1devicesimplybyprogrammingclass0–3perTable1.DLLcommunicationisimplementedbytheEthernetcommunicationsysteminthePDandisnotimplementedbytheTheTPS2379disablesclassificationaboveVCU_OFFtoavoidexcessivepowerdissipation.CLSvoltageisturnedoffduringPDthermallimitingorwhenDENisactive.TheCLSoutputisinherentlycurrentlimited,butshouldnotbeshortedtoVSSforlongperiodsoftime.Figure21showshowclassificationworksfortheTPS2379.Transitionfromstate-to-stateoccurswhencomparatorthresholdsarecrossed(seeFigure18andFigure19).Thesecomparatorshavehysteresis,whichaddsinherentmemorytothemachine.Operationbeginsatidle(unpoweredbyPSE)andproceedswithincreasingvoltagefromlefttoright.A2-eventclassificationfollowsthe(heavylined)pathtowardsthebottom,endingupwithalatchedtype2decodealongthelowerbranchthatishighlighted.ThisstateresultsinalowT2Pduringnormaloperation.Oncethevalidpathtotype2PSEdetectionisbroken,theinputvoltagemusttransitionbelowthemarkresetthresholdtostartanew.openTYPE1openTYPE1 PoEPoEStartupTYPE2T2PlowFigure21.Two-EventClassInternalInrushandIEEE802.3athasastartupcurrentandtimelimitation,providingtype2PSEcompatibilityfortype1PDs.Atype2PSElimitsoutputcurrenttobetween400mAand450mAforupto75msafterpower-up(applying“48V”tothePI)inordertomirrortype1PSEfunctionality.Thetype2PSEwillsupporthigheroutputcurrentafter75ms.TheTPS2379implementsa140mAinrushcurrent,whichiscompatiblewithallPSEtypes.Ahigh-powerPDmustlimititsconverterstartuppeakcurrent.Theoperationalcurrentcannotexceed400mAforaperiodof80msorlonger.Thisrequirementimplicitlyrequiressomeformofpoweringdownsectionsoftheapplicationcircuits.MaintainPowerTheMPSisanelectricalsignaturepresentedbythePDtoassurethePSEthatitisstillpresentafteroperatingvoltageisapplied.AvalidMPSconsistsofaminimumdccurrentof10mA(ora10mApulsedcurrentforatleast75msevery325ms)andanacimpedancelowerthan26.3kΩinparallelwith0.05μF.Theacimpedanceis plishedbytheminimumoperatingCBULKrequirementof5μF.WhenDENisusedtoforcethehotswapswitchoff,thedcMPSwillnotbemet.APSEthatmonitorsthedcMPSwillremovepowerfromthePDwhenthisoccurs.APSEthatmonitorsonlytheacMPSmayremovepowerfromthePD.StartupandTheinternalPoEUVLO(UnderVoltageLockOut)circuitholdsthehotswapswitchoffbeforethePSEprovidesfullvoltagetothePD.ThispreventsthedownstreamconvertercircuitsfromloadingthePoEinputduringdetectionandclassification.TheconvertercircuitswilldischargeCBULKwhilethePDisunpowered.ThusV(VDD-RTN)willbeasmallvoltagejustafterfullvoltageisappliedtothePD,asseeninFigure20.ThePSEdrivesthePIvoltagetotheoperatingrangeonceithasdecidedtopowerupthePD.WhenVVDDrisesabovetheUVLOturn-onthreshold(VUVLO-R,~38V)withRTNhigh,theTPS2379enablesthehotswapMOSFETwitha~140mA(inrush)currentlimitasseeninFigure22.TheCDBpinisactivewhileCBULKchargesandVRTNfallsfromVVDDtonearlyVVSS.Oncetheinrushcurrentfallsabout10%belowtheinrushcurrentlimit,thePDcurrentlimitswitchestotheoperationallevel(~1000mA)andCDBisdeassertedtoallowdownstreamconvertercircuitrytostart.TheTPS2379assertsGATEafterinrushiscompletetoenableanexternalpassMOSFETifused.InFigure22,T2Pisactivebecauseatype2PSEispluggedin.Type1Type1Type2PILoadenabledusingCDBplusdelayFigure22.PowerUpandPDHotswapIEEE802.3athastakenanewapproachtoPSEoutputlimiting.Atype2PSEmustmeetanoutputcurrentversustimetemplatewithspecifiedminimumand umsourcingboundaries.Thepeakoutputcurrentmaybeashighas50Afor10μsor1.75Afor75ms.ThismakesrobustprotectionofthePDdeviceevenmoreimportantthanitwasinIEEE802.3-2008.TheinternalhotswapMOSFETisprotectedagainstoutputfaultsandinputvoltagestepswithacurrentlimitanddeglitched(time-delayfiltered)foldback.AnoverloadonthepassMOSFETengagesthecurrentlimit,withVRTN-VVSSrisingasaresult.GATEispulleddownabout300μsafterRTNcurrentreachesthecurrentlimitlevel.IfVRTNrisesabove~12Vforlongerthan~800μs,thecurrentlimitrevertstotheinrushvalue.The800μsdeglitchfeaturepreventsmomentarytransientsfromcausingaPDreset,providedthatrecoverylieswithintheboundsofthehotswapandPSEprotection.Figure23showsanexampleoftheRTNcurrentprofileduringVDDtoRTNshortcircuitwhenonlytheinternalhotswapMOSFETisused.ThehotswapMOSFETgoesintocurrentlimit,causingtheRTNvoltagetoincrease.OnceVRTNexceeds12V,IRTNwhichwasclampedtothecurrentlimitdropstothelevelofinrushcurrentlimitafter800μs.VRTNVRTNVSS>Figure23.ResponsetoPDOutputShortCircuitWithoutAUXFigure24showsanexampleoftheRTNcurrentprofileduringVDDtoRTNshortcircuitwhentheexternalMOSFETisused.ThecircuitisdepictedinFigure1.ThecurrentwilldividebetweentheinternalandexternalMOSFETs.Duringtheshortcircuit,thehotswapMOSFETgoesintocurrentlimit,causingtheRTNvoltagetoincrease.WhentheinternalMOSFETexceedscurrentlimitfor~300μs,GATEwillde-assertandshutofftheauxiliaryMOSFET.VRTNwillrisequicklyandtheinternalMOSFETwillgointocurrentlimitfor~800μs(afterVRTN>~12V)andthenIRTNwhichwasclampedtothecurrentlimitdropsintotheinrushcurrentlimit.

Figure24.ResponsetoPDOutputShortCircuitwithAUXThePDcontrolhasathermalsensorthatprotectstheinternalhotswapMOSFET.ConditionslikestartuporoperationintoaVDD-to-RTNshortcausehighpowerdissipationintheMOSFET.Anover-temperatureshutdown(OTSD)turnsoffthehotswapMOSFETandclassregulator,whicharerestartedafterthedevicecools.ThehotswapMOSFETwillbere-enabledwiththeinrushcurrentlimitwhenexitingfromanover-temperatureevent.PullingDENtoVSSduringpoweredoperationcausestheinternalhotswapMOSFETtoturnoff.ThehotswapswitchwillbeforcedoffunderthefollowingV(DEN–VSS)<VPD_DISwhenVVDD–VVSSisintheoperationalPDisover-temperature,V(DEN–VSS)PoEUVLOfallingthreshold(~32CDBandCDB(converterdisable)isanactive-lowpinthatindicateswhentheinternalhotswapMOSFETisinrushlimiting.CDBde-assertswheninrushisoverandcanbeusedtoenableadownstreamconvertertostartup.Commoninterfacestotheconvertercontrollerincludethesoftstartorenablepins.T2P(type2PSE)isanactive-lowmultifunctionpinthatindicatesif(PSE=Type_2)and(PDcurrentlimit≠TheusageofT2PisdemonstratedinFigure27.WhenPSEappliesandPDobservesatype2hardwareclassification,T2PpinispulledtoRTNasaindicationofthetypeofPSE.AuxiliaryPassMOSFETTheTPS2379canbeusedinnon-standardapplicationsrequiringpowersignificantlyabovetheIEEE802.3at,type2levels.ThisimplementationcanbeachievedbyutilizingallfourEthernetwirepairsandboostingtheTPS2379hotswapMOSFEToperatingcurrentlimit.BoostingtheTPS2379operatingcurrentlimitisachievedbyaddinganexternalpassMOSFETtosharethetotalloadcurrentwiththeinternalhotswapMOSFET.TheexternalpassMOSFETisenabledbytheGATEpinaftertheinternalhotswapMOSFETinrushiscomplete.TheGATEpinwillde-assertiftheTPS2379internalcurrentlimitisexceededinexcessof300μs.AcomprehensivehighpowerPOEdesignexampleisdiscussedinapplicationreportImplementinga60-W,End-to-EndPoESystem(literaturenumberSLVA498).UsingDENtoDisableTheDENpinmaybeusedtoturnthePoEhotswapswitchoffbypullingittoVSSwhileintheoperationalstate,ortopreventdetectionwhenintheidlestate.AlowvoltageonDENforcesthehotswapMOSFEToffduringnormalAPPLICATIONINPUTBRIDGESANDSCHOTTKYUsingSchottkydiodesinsteadofPNjunctiondiodesforthePoEinputbridgeswillreducethepowerdissipationinthesedevicesbyabout30%.Thereare,however,somethingstoconsiderwhenusingthem.TheIEEEstandardspecifiesaumbackfeedvoltageof2.8V.A100kΩresistorisplacedbetweentheunpoweredpairsandthevoltageismeasuredacrosstheresistor.SchottkydiodesoftenhaveahigherreverseleakagecurrentthanPNdiodes,makingthisaharderrequirementtomeet.Tocompensate,useconservativedesignfordiodeoperatingtemperature,selectlower-leakagedeviceswherepossible,andmatchleakageandtemperaturesbyusingpackagedbridges.Schottkydiodeleakagecurrentsandlowerdynamic scanimpactthedetectionsignature.Settingreasonableexpectationsforthetemperaturerangeoverwhichthedetectionsignatureisaccurateisthesimplestsolution.IncreasingRDENslightlymayalsohelpmeettherequirement.SchottkydiodeshaveprovenlessrobusttothestressesofESDtransientsthanPNjunctiondiodes.AfterexposuretoESD,Schottkydiodesmay eshortedorleak.Caremustbetakentoprovideadequateprot