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LTC3541-2
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OPERATIO
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WhentheMODEpinisdriventoalogichightheLTC3541-2
operatesinPulse-Skipmodeforlowoutputvoltageripple.
In this mode, the LTC3541-2 continues to switch at a
constantfrequencydowntoverylowcurrents,whereit
willbeginskippingpulsesusedtocontrolthemain(top)
switchtomaintaintheproperaverageinductorcurrent.
Iftheinputsupplyvoltageisdecreasedtoavalueap-
proachingtheoutputvoltage,thedutycycleofthebuck
isincreasedtowardmaximumon-timeand100%duty
cycle.Theoutputvoltagewillthenbedeterminedbythe
input voltage minus the voltage drop across the main
switchandtheinductor.
VLDO/Linear Regulator Loop
IntheLTC3541-2,theVLDOandlinearregulatorloops
consist of an amplifier and N-channel MOSFET output
stagesthatservotheoutputtomaintainaregulatoroutput
voltage,LVOUT.Theinternalreferencevoltageprovidedto
theamplifieris0.4Vallowingforawiderangeofoutput
voltages.LoopconfigurationsenablingtheVLDOorthe
linearregulatorarestablewithanoutputcapacitanceas
lowas2.2
Fandashighas100F.BoththeVLDOand
thelinearregulatorsarecapableofoperatingwithaninput
voltage,VIN,aslowas2.9V.
TheVLDOregulatorisdesignedtoprovideupto300mA
ofoutputcurrentataverylowLVINtoLVOUTvoltage.This
allowsaclean,secondary,analogsupplyvoltagetobe
providedwithaminimumdropinefficiency.TheVLDO
regulatorisprovidedwiththermalprotectionthatisde-
signedtodisabletheVLDOfunctionwhentheoutput,pass
transistor’sjunctiontemperaturereachesapproximately
160°C.Inadditiontothermalprotection,short-circuit
detectionisprovidedtodisabletheVLDOfunctionwhena
short-circuitconditionissensed.Thiscircuitisdesigned
suchthatanoutputcurrentofapproximately1Acanbe
providedbeforethiscircuitwilltrigger.Asdetailedinthe
ElectricalCharacteristics,theVLDOregulatorwillbeout
ofregulationwhenthiseventoccurs.Boththethermal
andshort-circuitfaults,whendetected,aretreatedas
catastrophic fault conditions. The LTC3541-2 will be
resetuponthedetectionofeitherevent.
TheN-channelMOSFET,incorporatedintheVLDOregula-
tor,hasitsdrainconnectedtotheLVINpinasshownin
Figure1.Toensurereliableoperation,theLVINvoltage
mustbestablebeforetheVLDOregulatorisenabled.For
thecasewherethevoltageontheLVINpinissuppliedby
thebuckregulator,theinternalpowersupplysequenc-
inglogicassuresvoltagesareappliedintheappropriate
manner.Forthecasewhereanexternalsupplyisusedto
powertheLVINpin,theexternallysuppliedLVINvoltage
mustbestable1msbeforetheENVLDOisbroughtfrom
alowtoahigh.Further,theexternallysuppliedLVINmust
bereducedinconjunctionwithVINwheneverVINispulled
loworremoved.
Thelinearregulatorisdesignedtoprovidealoweroutput
currentthanthatavailablefromtheVLDOregulator.The
linearregulator’soutput,passtransistorhasitsdraintied
totheVINrail.Thisallowsthelinearregulatortobeturned
onpriorto,andindependentof,thebuckregulatorwhich
ordinarilydrivestheVLDOregulator.Thelinearregulator
isprovidedwiththermalprotectionthatisdesignedto
disablethelinearregulatorfunctionwhentheoutputpass
transistor’sjunctiontemperaturereachesapproximately
160°C. In addition to thermal protection, short-circuit
detectionisprovidedtodisablethelinearregulatorfunc-
tionwhenashort-circuitconditionissensed.Thiscircuit
isdesignedsuchthatanoutputcurrentofapproximately
120mAcanbeprovidedbeforethiscircuitwilltrigger.As
detailedintheElectricalCharacteristics,thelinearregulator
willbeoutofregulationwhenthiseventoccurs.Boththe
thermalandshort-circuitfaultsaretreatedascatastrophic
faultconditions.TheLTC3541-2willberesetuponthe
detectionofeitherevent.
TheN-channelMOSFET,incorporatedinthelinearregu-
lator,hasitsdrainconnectedtotheVINpinasshownin
Figure1.ThesizeoftheseMOSFETsandtheirassociated
powerbussingisdesignedtoaccomodate30mAofDC
current.Currentsabovethisvaluecanbesupportedfor
short periods as stipulated in the Absolute Maximum
Ratings.