MaximMAX5974APoE应用参考设计关键词：电源管理,PoE,DC/DC转换器,通信技术,网络通信Maxim公司的MAX5974A/MAX5974B/MAX5974C/MAX5974D是宽输入电压的电流模式正向电源转换器，主要用于PoE的受电设备（PD）。MAX5974A/MAX5974C适合通用或电信输入电压，而MAX5974B/MAX5974D输入电压低到10.5V，器件内部提供1%的误差放大器，可编开关频率从100kHz到600kHz，主要用在PoEIEEE®802.3af/at受电设备，大功率PD(Beyondthe802.3af/atStandard)，有源箝位的正向DC/DC转换器，IP电话，无线接入点，安检照相机。本文介绍MAX5974A/B/C/D主要特性，方块图，典型应用电路图，以及PoE应用参考设计指标和电路图。TheMAX5974_providecontrolforwide-input-voltage,active-clamped,current-modePWM,forwardconvertersinPower-over-Ethernet(PoE)powereddevice(PD)applications.TheMAX5974A/MAX5974Carewell-suitedforuniversalortelecominputrange,whiletheMAX5974B/MAX5974Dalsoaccommodatelowinputvoltagedownto10.5V.Thedevicesincludeseveralfeaturestoenhancesupplyefficiency.TheAUXdriverrecyclesmagnetizingcurrentinsteadofwastingitinadissipativeclampcircuit.ProgrammabledeadtimebetweentheAUXandmaindriverallowsforzero-voltageswitching(ZVS).Underlightloadconditions,thedevicesreducetheswitchingfrequency(frequencyfoldback)toreduceswitchinglosses.TheMAX5974A/MAX5974Bfeatureuniquecircuitrytoachieveoutputregulationwithoutusinganoptocoupler,whiletheMAX5974C/MAX5974Dutilizethetraditionaloptocouplerfeedbackmethod.Aninternalerroramplifierwitha1%referenceisveryusefulinnonisolateddesign,eliminatingtheneedforanexternalshuntregulator.Thedevicesfeatureauniquefeed-forwardmaximumduty-cycleclampthatmakesthemaximumclampvoltageduringtransientconditionsindependentofthelinevoltage,allowingtheuseofapowerMOSFETwithlowerbreakdownvoltage.Theprogrammablefrequencyditheringfeatureprovideslow-EMI,spread-spectrumoperation.TheMAX5974_areavailablein16-pinTQFN-EPpackagesandareratedforoperationoverthe-40℃to+85℃temperaturerange.MAX5974A/B/C/D主要特性：PeakCurrent-ModeControl,Active-ClampedForwardPWMControllerRegulationWithoutOptocoupler(MAX5974A/MAX5974B)Internal1%ErrorAmplifier100kHzto600kHzProgrammableQ8%SwitchingFrequency,SynchronizationUpto1.2MHzProgrammableFrequencyDitheringforLow-EMI,Spread-SpectrumOperationProgrammableDeadTime,PWMSoft-Start,CurrentSlopeCompensationProgrammableFeed-ForwardMaximumDuty-CycleClamp,80%MaximumLimitFrequencyFoldbackforHigh-EfficiencyLight-LoadOperationInternalBootstrapUVLOwithLargeHysteresis100μA(typ)StartupSupplyCurrentFastCycle-by-CyclePeakCurrent-Limit,35nsTypicalPropagationDelay115nsCurrent-SenseInternalLeading-EdgeBlankingOutputShort-CircuitProtectionwithHiccupModeReverseCurrentLimittoPreventTransformerSaturationDuetoReverseCurrent3mmx3mm,Lead-Free,16-PinTQFN-EPMAX5974A/B/C/D应用：PoEIEEE®802.3af/atPoweredDevicesHigh-PowerPD(Beyondthe802.3af/atStandard)Active-ClampedForwardDC-DCConvertersIPPhonesWirelessAccessNodesSecurityCameras图1。MAX5974A/MAX5974B方框图图2。MAX5974C/MAX5974D方框图图3。MAX5974C/MAX5974D典型应用电路图图4。MAX5974A/MAX5974B典型应用电路图图5。MAX5974C/MAX5974D典型应用电路图（2）PoE应用参考设计Active-ClampedForward,Dual-OutputPDProvidesHigh-PerformanceSolutionforPoEApplicationsThisreferencedesignisforahighlyefficient,active-clampedforward,dual3.3V/8.8Voutputpowereddevice(PD).ThedesignfeaturestheMAX5969AandMAX5900asitscontrollers.ThedesignalsousestheMAX5974C,whichcontrolscurrent-modePWMconvertersandprovideszero-voltageswitching(ZVS)andfrequencyfoldbacktoenhancesupplyefficiency.Usingthesedevices,thisRDisIEEE®802.3atcompliantandisahigh-performance,compact,andcost-efficientsolutionforPoEandnonstandardhigh-powerPDs.Thedesigncanalsosupporttheauxiliaryinputvoltagetoprovidemaximum55Woutputpower.ThedesignfeaturestheMAX5969A,MAX5900,andMAX5974C.TheMAX5969AiscompliantwiththeIEEE802.3atstandardinapower-over-Ethernet(PoE)system.Thedevicecanalsogetpowerfromawalladapter(WAD).TheMAX5900hot-swapcontrollersmoothlyenablesanonstandardhighpowerfromanonstandardPowerSourcingEquipment(PSE).TheMAX5974Cprovidescontrolforwide-input-voltage,ZVSactive-clamped,current-modePWMconvertersandfrequencyfoldbackforPoEandhigh-powerapplications.Usingthesedevices,thisreferencedesignisIEEE802.3atcompliant.Itisalsoahigh-performance,compact,andcost-effectivesolutionforanonstandardhighpowerPD.PoE应用参考设计指标：The3.3V/3.35Aand8.8V/4.6APDisdesignedtomeetthefollowingspecifications:Inputvoltage:42Vto57VNonstandardPSE:37Vto57VWADinputvoltage:30Vupto57VVOUT1:3.3V/3.35AVOUT2:8.8V/4.6AOutputripples:±1%Lineandloadregulation:3.3V=±0.2%,8.8V=±2%Totalefficiencywithloadsof3Aat3.3V,4.2Aat8.8V,and48Vinput:90.4%(includinginputLANtransformerandMOSFETbridge)图6。PoE应用参考设计外形图AO-Electronics傲壹电子官网：www.aoelectronics.com中文网：www.aoelectronics.cn图7。PoE应用参考设计电路图（1）图8。PoE应用参考设计电路图（2）
