LT3757 datasheet(13/36 Pages) LINER

13 / 36 page

LT3757

3757fb

applicaTions inForMaTion

FBX Frequency Foldback

WhenVOUTisverylowduringstart-uporashort-circuit

faultontheoutput,theswitchingregulatormustoperate

atlowdutycyclestomaintainthepowerswitchcurrent

withinthecurrentlimitrange,sincetheinductorcurrent

decayrateisverylowduringswitchofftime.Theminimum

on-timelimitationmaypreventtheswitcherfromattaining

asufficientlylowdutycycleattheprogrammedswitch-

ingfrequency.So,theswitchcurrentwillkeepincreasing

througheachswitchcycle,exceedingtheprogrammed

currentlimit.Topreventtheswitchpeakcurrentsfrom

exceedingtheprogrammedvalue,theLT3757contains

a frequency foldback function to reduce the switching

frequencywhentheFBXvoltageislow(seetheNormal-

ized Switching Frequency vs FBX graph in the Typical

PerformanceCharacteristicssection).

Thetypicalfrequencyfoldbackwaveformsareshownin

theTypicalPerformanceCharacteristicssection.Thefre-

quencyfoldbackfunctionpreventsILfromexceedingthe

programmedlimitsbecauseoftheminimumon-time.

Duringfrequencyfoldback,externalclocksynchroniza-

tion is disabled to prevent interference with frequency

reducingoperation.

Thermal Lockout

IfLT3757dietemperaturereaches165°C(typical),the

partwillgointothermallockout.Thepowerswitchwill

beturnedoff.Asoft-startoperationwillbetriggered.The

partwillbeenabledagainwhenthedietemperaturehas

droppedby5°C(nominal).

Loop Compensation

Loopcompensationdeterminesthestabilityandtransient

performance.TheLT3757usescurrentmodecontrolto

regulatetheoutputwhichsimplifiesloopcompensation.

Theoptimumvaluesdependontheconvertertopology,the

componentvaluesandtheoperatingconditions(including

theinputvoltage,loadcurrent,etc.).Tocompensatethe

feedbackloopoftheLT3757,aseriesresistor-capacitor

networkisusuallyconnectedfromtheVCpintoGND.

Figure1showsthetypicalVCcompensationnetwork.For

mostapplications,thecapacitorshouldbeintherangeof

470pFto22nF,andtheresistorshouldbeintherangeof

5kto50k.Asmallcapacitorisoftenconnectedinparal-

lelwiththeRCcompensationnetworktoattenuatethe

VCvoltagerippleinducedfromtheoutputvoltageripple

throughtheinternalerroramplifier.Theparallelcapacitor

usuallyrangesinvaluefrom10pFto100pF.Apractical

approachtodesignthecompensationnetworkistostart

withoneofthecircuitsinthisdatasheetthatissimilar

toyourapplication,andtunethecompensationnetwork

to optimize the performance. Stability should then be

checkedacrossalloperatingconditions,includingload

current,inputvoltageandtemperature.

SENSE Pin Programming

For control and protection, the LT3757 measures the

powerMOSFETcurrentbyusingasenseresistor(RSENSE)

betweenGNDandtheMOSFETsource.Figure4showsa

typicalwaveformofthesensevoltage(VSENSE)acrossthe

senseresistor.ItisimportanttouseKelvintracesbetween

theSENSEpinandRSENSE,andtoplacetheICGNDas

closeaspossibletotheGNDterminaloftheRSENSEfor

properoperation.

Figure 4. The Sense Voltage During a Switching Cycle

3757 F04

VSENSE(PEAK)

VSENSE =

VSENSE(MAX)

VSENSE

DTS

VSENSE(MAX)

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