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LTC3541-1 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3541-1 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3541-1 0 35411fa tions.TheLTC3541-1willberesetuponthedetectionof eitherevent.TheN-channelMOSFETincorporatedinthe VLDOhasitsdrainconnectedtotheLVINpinasshown inFigure1.Toensurereliableoperation,theLVINvoltage mustbestablebeforetheVLDOisenabled.Forthecase wherethevoltageontheLVINpinissuppliedbythebuck regulator,theinternalpowersupplysequencinglogicas- suresvoltagesareappliedintheappropriatemanner.For thecasewhereanexternalsupplyisusedtopowerthe LVINpin,thevoltageontheLVINpinmustbestablebefore theENVLDOpinisbroughtfromalowtoahigh.Further, theexternalLVINvoltagemustbereducedinconjunction withVINwheneverVINispulledloworremoved. Thelinearregulatorisdesignedtoprovidealoweroutput current(30mA)thanthatavailablefromtheVLDO.The linearregulator’soutputpasstransistorhasitsdraintied totheVINrail.Thisallowsthelinearregulatortobeturned onpriorto,andindependentof,thebuckregulatorwhich wouldordinarilydrivetheVLDOinatypicalapplication. Thelinearregulatorisprovidedwiththermalprotectionthat isdesignedtodisablethelinearregulatorfunctionwhen theoutputpasstransistor’sjunctiontemperaturereaches approximately160°C.Inadditiontothermalprotection, short-circuitdetectionisprovidedtodisablethelinear regulatorfunctionwhenashort-circuitconditionissensed. Thiscircuitisdesignedsuchthatanoutputcurrentof approximately120mAcanbeprovidedbeforethiscircuit willtrigger.AsdetailedintheElectricalCharacteristics, thelinearregulatorwillbeoutofregulationwhenthis eventoccurs.Boththethermalandshort-circuitfaultsare treatedascatastrophicfaultconditions.TheLTC3541-1 willberesetuponthedetectionofeitherevent. TheN-channelMOSFETincorporatedinthelinearregulator hasitsdrainconnectedtotheVINpinasshowninFigure1. ThesizeofthisMOSFETanditsassociatedpowerbussing isdesignedtoaccommodate30mAofDCcurrent.Currents abovethiscanbesupportedforshortperiodsasstipulated intheAbsoluteMaximumRatingssection. TransitioningfromlinearregulatormodetoVLDOmode, accomplishedbybringingENBUCKfromalogiclowtoa logichighwhileENVLDOisalogichigh,isdesignedtobe asseamlessandtransientfreeaspossible.Theprecise transientresponseofLVOUTduetothistransitionisa functionofCOUTandtheloadcurrent.Waveformsgiven intheTypicalPerformanceCharaceristicsshowtypical transientresponsesusingtheminimumCOUTof2.2µFand loadcurrentsof1mAand30mArespectively.Generally,the amplitudeofanytransientspresentwilldecreaseasCOUT isincreased.Toensurereliableoperationandadherence totheloadregulationlimitspresentedintheElectrical Characteristicstable,theloadcurrentmustnotexceed thelinearregulatorIOUTlimitof30mAwithin20msafter ENBUCKhastransitionedtoalogichigh.The300mAIOUT limitofVLDOappliesthereafter.Further,forconfigurations thatdonotusetheLTC3541-1’sbuckregulatortoprovide theVLDOinputvoltage(LVIN),theusermustensurea stableLVINvoltageispresentnolessthan1mspriorto ENBUCKtransitioningtoalogichigh. Inasimilarmanner,transitioningfromVLDOmodeto linearregulatormode,accomplishedbybringingENBUCK fromalogichightoalogiclowwhileENVLDOisalogic high,isdesignedtobeasseamlessandtransientfreeas possible.Again,theprecisetransientresponseofLVOUT duetothistransitionisafunctionofCOUTandtheload current. Waveforms given in the Typical Performance Characeristics show typical transient responses using theminimumCOUTof2.2µFandloadcurrentsof1mA and30mArespectively.Generally,theamplitudeofany transientspresentwilldecreaseasCOUTisincreased.To ensurereliableoperationandadherencetotheloadregula- tionlimitspresentedintheElectricalCharactersticstable, theloadcurrentmustnotexceedthelinearregulatorIOUT limitof30mA1mspriortoENBUCKtransitioningtoalogic lowandthereafer.Further,forconfigurationsthatdonot usetheLTC3541-1’sbuckregulatortoprovidetheVLDO inputvoltage(LVIN),theusermustcontinuetoensurea stableLVINvoltagenolessthan1msafterENBUCKhas transitionedtoalogiclow. OPERATIO |
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