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LT3070

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ApplicAtions inFormAtion

Load Regulation

TheLT3070providesaKelvinsensepinforVOUT,allowing

theapplicationtocorrectforparasiticpackageandPCB

I-Rdrops.However,LTCrecommendsthattheSENSEpin

terminateincloseproximitytotheLT3070’sOUTpins.

Thisminimizesparasiticinductanceandoptimizesregula-

tion.TheLT3070handlesmoderatelevelsofoutputline

impedance,butexcessiveimpedancebetweenVOUTand

COUTcausesexcessivephaseshiftinthefeedbackloop

andadverselyaffectsstability.

Figure1inthePinFunctionssectionillustratestheKelvin-

Senseconnectionmethodthateliminatesvoltagedrops

duetoPCBtraceresistance.However,notethatthevoltage

dropacrosstheexternalPCBtracesaddstothedropout

voltageoftheregulator.TheSENSEpininputbiascurrent

dependsontheselectedoutputvoltage.SENSEpininput

currentvariesfrom50µAtypicallyatVOUT=0.8Vto300µA

typicallyatVOUT=1.8V.

Short-Circuit and Overload Recovery

LikemanyICpowerregulators,theLT3070hassafeop-

eratingarea(SOA)protection.Thesafeareaprotection

decreasescurrentlimitasinput-to-outputvoltageincreases

andkeepsthepowertransistorinsideasafeoperating

regionforallvaluesofinput-to-outputvoltageuptothe

absolutemaximumvoltagerating.VBIASmustbeabove

theUVLOthresholdforanyfunction.TheLT3070hasa

precisioncurrentlimitspecifiedat±20%thatisactiveif

VBIASisaboveUVLO.

Under conditions of maximum ILOAD and maximum

VIN-VOUTthedevice’spowerdissipationpeaksatabout

3W.Ifambienttemperatureishighenough,diejunction

temperaturewillexceedthe125°Cmaximumoperating

temperature. If this occurs, the LT3070 relies on two

additionalthermalsafetyfeatures.Atabout145°C,the

PWRGDoutputpulls

lowprovidinganearlywarningofan

impendingthermalshutdowncondition.At165°Ctypically,

theLT3070’sthermalshutdownengagesandtheoutputis

shutdownuntiltheICtemperaturefallsbelowthethermal

hysteresislimit.TheSOAprotectiondecreasescurrentlimit

astheIN-to-OUTvoltageincreasesandkeepsthepower

dissipationatsafelevelsforallvaluesofinput-to-output

voltage.TheLT3070providessomeoutputcurrentatall

valuesofinput-to-outputvoltageuptotheabsolutemaxi-

mumvoltagerating.SeetheCurrentLimitvsVINcurvein

theTypicalPerformanceCharacteristics.

Duringstart-up,aftertheBIASvoltagehascleareditsUVLO

thresholdandVINisincreasing,outputvoltageincreases

attherateofcurrentlimitchargingCOUT.

Withahighinputvoltage,aproblemcanoccurwherethe

removalofanoutputshortwillnotallowtheoutputvolt-

agetorecover.Otherregulatorswithcurrentlimitfoldback

alsoexhibitthisphenomenon,soitisnotuniquetothe

LT3070.Theloadlineforsuchaloadmayintersectthe

outputcurrentcurveattwopoints:normaloperationand

theSOArestrictedloadcurrentsettings.Acommonsitu-

ationisimmediatelyaftertheremovalofashortcircuit,

butwithastaticload≥1A.Inthissituation,removalofthe

loadorreductionofIOUTto<1Awillclearthiscondition

andallowVOUTtoreturntonormalregulation.

Reverse Voltage

TheLT3070incorporatesacircuitthatdetectsifVINde-

creasesbelowVOUT.Thisreverse-voltagedetectorhas

atypicalthresholdofabout(VIN–VOUT)=–6mV.Ifthe

thresholdisexceeded,thisdetectorcircuitturnsoffthe

drivetotheinternalNMOSpasstransistor,therebyturning

offtheoutput.Theoutputpullslowwiththeloadcurrent

dischargingtheoutputcapacitance.Thiscircuit’sintent

istolimitandpreventback-feedcurrentfromOUTtoIN

iftheinputvoltagecollapsesduetoafaultoroverload

condition.

Thermal Considerations

The LT3070’s maximum rated junction temperature of

125°Climitsitspowerhandlingcapabilityandisdomi-

natedbytheoutputcurrentmultipliedbytheinput/output

voltagedifferential:

IOUT•(VIN–VOUT)

TheLT3070’sinternalpowerandthermallimitingcircuitry

protectitunderoverloadconditions.Forcontinuousnor-

malloadconditions,donotexceedthemaximumjunction

temperatureof125°C.Givecarefulconsiderationtoall

sourcesofthermalresistancefromjunctiontoambient.

Thisincludesjunctiontocase,case-to-heatsinkinterface,

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