LT3071EUFD datasheet(16/28 Pages) LINER

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LT3071

3071f

ApplicAtions inFormAtion

TheREF/BYPpinmustnotbeDCloadedbyanythingexcept

forapplicationsthatparallelotherLT3071regulatorsfor

higheroutputcurrents.ConsulttheApplicationsSection

onParallelingforfurtherdetails.

Output Voltage Margining

TheLT3071’sanalogmarginingpin,MARGA,providesa

continuousoutputvoltageadjustmentrangeof±10%.It

marginsVOUTbyadjustingtheinternal600mVreference

voltage up and down. The MARGA pin’s typical input

impedanceis190kΩbetweenMARGAandtheinternal

VREFnode.DrivingMARGAwith600mVto1.2Vprovides

0%to10%ofadjustment.DrivingMARGAwith600mVto

0Vprovides0%to–10%ofadjustment.Ifunused,allow

MARGAtofloatorbypassthispinwitha1nFcapacitor

toGND.Notethattheanalogmarginingfunctiondoesnot

adjustthePWRGDthreshold.Therefore,negativeanalog

marginingmaytripthePWRGDcomparatorandtoggle

thePWRGDflag.

Enable Function—Turning On and Off

TheENpinenables/disablestheoutputdeviceonly.The

LT3071referenceandallsupportfunctionsremainactive

ifVBIASisaboveitsUVLOthreshold.PullingtheENpin

lowputstheLT3071intonapmode.Innapmode,the

referencecircuitisactive,buttheoutputisdisabledand

quiescentcurrentdecreases.

DrivetheENpinwitheitheradigitallogicportoranopen-

collectorNPNoranopen-drainNMOSterminatedwith

apull-upresistortoVBIAS.Thepull-upresistormustbe

lessthan35ktomeettheVIHconditionoftheENpin.If

unused,connectENtoBIAS.

Input Undervoltage Lockout on BIAS Pin

An internal undervoltage lockout (UVLO) comparator

monitorstheBIASsupplyvoltage.IfVBIASdropsbelow

theUVLOthreshold,allfunctionsshutdown,thepass

transistorisgatedoffandoutputcurrentfallstozero.The

typicalBIASpinUVLOthresholdis1.55Vontherising

edgeofVBIAS.TheUVLOcircuitincorporatesabout150mV

ofhysteresisonthefallingedgeofVBIAS.

High Efficiency Linear Regulator—Input-to-Output

Voltage Control

TheVIOC(voltageinput-to-outputcontrol)pinisafunction

tocontrolaswitchingregulatorandfacilitateadesignsolu-

tionthatmaximizessystemefficiencyathighloadcurrents

andstillprovideslowdropoutvoltageperformance.

The VIOC pin is the output of an integrated transcon-

ductanceamplifierthatsourcesandsinksabout250µA

ofcurrent.IttypicallyregulatestheoutputofmostLTC®

switchingregulatorsorLTM®powermodules,bysinking

currentfromtheITHcompensationnode.TheVIOCfunction

controlsabuckregulatorpoweringtheLT3071’sinputby

maintainingtheLT3071’sinputvoltagetoVOUT+300mV.

This 300mV VIN-VOUT differential voltage is chosen to

providefasttransientresponseandgoodhighfrequency

PSRRwhileminimizingpowerdissipationandmaximizing

efficiency.Forexample,1.5Vto1.2Vconversionand1.3V

to1Vconversionyield1.5Wmaximumpowerdissipation

at5Afulloutputcurrent.

Figure2depictsthattheswitcher’sfeedbackresistornet-

worksetsthemaximumswitchingregulatoroutputvoltage

ifthelinearregulatorisdisabled.However,oncetheLT3071

isenabled,theVIOCfeedbackloopdecreasestheswitching

regulatoroutputvoltagebacktoVOUT+300mV.

UsingtheVIOCfunctioncreatesafeedbackloopbetween

theLT3071andtheswitchingregulator.Assuch,thefeed-

backloopmustbefrequencycompensatedforstability.

Fortunately,theconnectionofVIOCtomanyLTCswitching

regulatorITHpinsrepresentsahighimpedancecharac-

teristicwhichistheoptimumcircuitnodetofrequency

compensatethefeedbackloop.Figure2illustratesthe

typicalfrequencycompensationnetworkusedattheVIOC

nodetoGND.

TheVIOCamplifiercharacteristicsare:

gm=3.2mS,IOUT=±250µA,BW=10MHz.

If the VIOC function is not used, terminate the VIOC

pintoGNDwithasmallcapacitor(1000pF)toprevent

oscillations.

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