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LTC3541EDD Datasheet(PDF) 9 Page - Linear Technology |
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LTC3541EDD Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3541 3541fa OPERATIO needforexternalpincontrol.Adetaileddiscussionofthe transitionsbetweentheVLDOandlinearregulatorcanbe foundintheVLDO/LinearRegulatorLoopsection. Buck Regulator Control Loop TheLTC3541internalbuckregulatorusesaconstantfre- quency,currentmode,step-downarchitecture.Boththe main(top,P-channelMOSFET)andsynchronous(bottom, N-channelMOSFET)switchesareinternal.Duringnormal operation,theinternalmainswitchisturnedonatthebe- ginningofeachclockcycleprovidedtheinternalfeedback voltagetothebuckislessthanthereferencevoltage.The currentintotheinductorprovidedtotheloadincreases untilthecurrentlimitisreached.Oncethecurrentlimitis reachedthemainswitchturnsoffandtheenergystored intheinductorflowsthroughthebottomsynchronous switchintotheloaduntilthenextclockcycle. Thepeakinductorcurrentisdeterminedbycomparingthe buckfeedbacksignaltoaninternal0.8Vreference.When theloadcurrentincreases,theoutputofthebuckand hencethebuckfeedbacksignaldecrease.Thisdecrease causesthepeakinductorcurrenttoincreaseuntiltheaver- ageinductorcurrentmatchestheloadcurrent.Whilethe mainswitchisoff,thesynchronousswitchisturnedon untileithertheinductorcurrentstartstoreversedirection orthebeginningofanewclockcycle. WhentheMODEpinisdriventoalogiclow,theLTC3541 buckregulatoroperatesinBurstModeoperationforhigh efficiency.Inthismode,themainswitchoperatesbased upon load demand. In Burst Mode operation the peak inductorcurrentissettoafixedvalue,whereeachburst eventcanlastfromafewclockcyclesatlightloadsto nearly continuous cycling at moderate loads. Between bursteventsthemainswitchandanyunneededcircuitry areturnedoff,reducingthequiescentcurrent.Inthissleep state,theloadisbeingsuppliedsolelyfromtheoutput capacitor.Astheoutputvoltagedroops,aninternalerror amplifier’soutputrisesuntilawakethresholdisreached causingthemainswitchtoagainturnon.Thisprocess repeatsataratethatisdependantupontheloadcurrent demand. WhentheMODEpinisdriventoalogichightheLTC3541 operatesinPulse-Skipmodeforlowoutputvoltageripple. Inthismode,theLTC3541continuestoswitchataconstant frequencydowntoverylowcurrents,whereitwillbegin skippingpulsesusedtocontrolthemain(top)switchto maintaintheproperaverageinductorcurrent. Iftheinputsupplyvoltageisdecreasedtoavalueap- proachingtheoutputvoltage,thedutycycleofthebuck isincreasedtowardmaximumon-timeand100%duty cycle.Theoutputvoltagewillthenbedeterminedbythe input voltage minus the voltage drop across the main switchandtheinductor. VLDO/Linear Regulator Loop IntheLTC3541,theVLDOandlinearregulatorloopsconsist ofanamplifierandN-channelMOSFEToutputstagesthat, whenconnectedwiththeproperexternalcomponents, willservotheoutputtomaintainaregulatoroutputvolt- age,LVOUT.Theinternalreferencevoltageprovidedtothe amplifieris0.4Vallowingforawiderangeofoutputvolt- ages.LoopconfigurationsenablingtheVLDOorthelinear regulatorarestablewithanoutputcapacitanceaslowas 2.2 µFandashighas100µF.BoththeVLDOandthelinear regulatorsarecapableofoperatingwithaninputvoltage, VIN,aslowas2.7V,butaresubjecttotheconstraintthat VINmustbegreaterthanLVOUT+1.4V. TheVLDOisdesignedtoprovideupto300mAofoutput currentataverylowLVINtoLVOUTvoltage.Thisallows aclean,secondary,analogsupplyvoltagetobeprovided withaminimumdropinefficiency.TheVLDOisprovided withthermalprotectionthatisdesignedtodisablethe VLDOfunctionwhentheoutput,passtransistor’sjunction temperaturereachesapproximately160°C.Inadditionto thermalprotection,short-circuitdetectionisprovidedto disabletheVLDOfunctionwhenashort-circuitconditionis sensed.Thiscircuitisdesignedsuchthatanoutputcurrent ofapproximately1Acanbeprovidedbeforethiscircuit willtrigger.AsdetailedintheElectricalCharacteristics,the VLDOregulatorwillbeoutofregulationwhenthisevent occurs.Boththethermalandshort-circuitfaultswhen detectedaretreatedascatastrophicfaultconditions.The |
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