LT3757 datasheet(14/36 Pages) LINER

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LT3757

3757fb

applicaTions inForMaTion

DuetothecurrentlimitfunctionoftheSENSEpin,RSENSE

shouldbeselectedtoguaranteethatthepeakcurrentsense

voltageVSENSE(PEAK)duringsteadystatenormaloperation

islowerthantheSENSEcurrentlimitthreshold(seethe

Electrical Characteristics table). Given a 20% margin,

VSENSE(PEAK) is set to be 80mV. Then, the maximum

switchripplecurrentpercentagecanbecalculatedusing

thefollowingequation:

c =

−

∆

SENSE

0 5

. •

cisusedinsubsequentdesignexamplestocalculateinduc-

torvalue.∆VSENSEistheripplevoltageacrossRSENSE.

TheLT3757switchingcontrollerincorporates100nstiming

intervaltoblanktheringingonthecurrentsensesignal

immediatelyafterM1isturnedon.Thisringingiscaused

bytheparasiticinductanceandcapacitanceofthePCB

trace,thesenseresistor,thediode,andtheMOSFET.The

100nstimingintervalisadequateformostoftheLT3757

applications.Intheapplicationsthathaveverylargeand

longringingonthecurrentsensesignal,asmallRCfilter

canbeaddedtofilterouttheexcessringing.Figure5

showstheRCfilteronSENSEpin.Itisusuallysufficient

to choose 22Ω for RFLT and 2.2nF to 10nF for CFLT.

KeepRFLT’sresistancelow.Rememberthatthereis65µA

(typical)flowingoutoftheSENSEpin.AddingRFLTwill

affecttheSENSEcurrentlimitthreshold:

VSENSE_ILIM=108mV–65µA•RFLT

APPLICATION CIRCUITS

TheLT3757canbeconfiguredasdifferenttopologies.The

firsttopologytobeanalyzedwillbetheboostconverter,

followedbytheflyback,SEPICandinvertingconverters.

Boost Converter: Switch Duty Cycle and Frequency

TheLT3757canbeconfiguredasaboostconverterfor

the applications where the converter output voltage is

higherthantheinputvoltage.Rememberthatboostcon-

vertersarenotshort-circuitprotected.Underashorted

outputcondition,theinductorcurrentislimitedonlyby

theinputsupplycapability.Forapplicationsrequiringa

step-upconverterthatisshort-circuitprotected,please

refer to the Applications Information section covering

SEPICconverters.

Theconversionratioasafunctionofdutycycleis

OUT

−

incontinuousconductionmode(CCM).

ForaboostconverteroperatinginCCM,thedutycycle

ofthemainswitchcanbecalculatedbasedontheoutput

voltage(VOUT)andtheinputvoltage(VIN).Themaximum

duty cycle (DMAX) occurs when the converter has the

minimuminputvoltage:

MAX

OUT

IN MIN

OUT

−

(

)

Discontinuousconductionmode(DCM)provideshigher

conversionratiosatagivenfrequencyatthecostofreduced

efficienciesandhigherswitchingcurrents.

Figure 5. The RC Filter on SENSE Pin

CFLT

3757 F05

LT3757

RFLT

RSENSE

SENSE

GATE

GND

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