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LT3757

3757fb

applicaTions inForMaTion

Main Control Loop

TheLT3757usesafixedfrequency,currentmodecontrol

schemetoprovideexcellentlineandloadregulation.Op-

erationcanbebestunderstoodbyreferringtotheBlock

DiagraminFigure1.

ThestartofeachoscillatorcyclesetstheSRlatch(SR1)and

turnsontheexternalpowerMOSFETswitchM1through

driverG2.Theswitchcurrentflowsthroughtheexternal

currentsensingresistorRSENSEandgeneratesavoltage

proportional to the switch current. This current sense

voltageVISENSE(amplifiedbyA5)isaddedtoastabilizing

slopecompensationrampandtheresultingsum(SLOPE)

isfedintothepositiveterminalofthePWMcomparatorA7.

WhenSLOPEexceedsthelevelatthenegativeinputofA7

(VCpin),SR1isreset,turningoffthepowerswitch.The

levelatthenegativeinputofA7issetbytheerroramplifier

A1(orA2)andisanamplifiedversionofthedifference

betweenthefeedbackvoltage(FBXpin)andthereference

voltage(1.6Vor–0.8V,dependingontheconfiguration).

Inthismanner,theerroramplifiersetsthecorrectpeak

switchcurrentleveltokeeptheoutputinregulation.

TheLT3757hasaswitchcurrentlimitfunction.Thecurrent

sensevoltageisinputtothecurrentlimitcomparatorA6.

IftheSENSEpinvoltageishigherthanthesensecurrent

limitthresholdVSENSE(MAX)(110mV,typical),A6willreset

SR1andturnoffM1immediately.

The LT3757 is capable of generating either positive or

negativeoutputvoltagewithasingleFBXpin.Itcanbe

configuredasaboost,flybackorSEPICconvertertogen-

eratepositiveoutputvoltage,orasaninvertingconverter

togeneratenegativeoutputvoltage.Whenconfiguredas

aSEPICconverter,asshowninFigure1,theFBXpinis

pulleduptotheinternalbiasvoltageof1.6Vbyavolt-

agedivider(R1andR2)connectedfromVOUTtoGND.

Comparator A2 becomes inactive and comparator A1

performstheinvertingamplificationfromFBXtoVC.When

theLT3757isinaninvertingconfiguration,theFBXpin

ispulleddownto–0.8Vbyavoltagedividerconnected

fromVOUTtoGND.ComparatorA1becomesinactiveand

comparatorA2performsthenoninvertingamplification

fromFBXtoVC.

The LT3757 has overvoltage protection functions to

protect the converter from excessive output voltage

overshootduringstart-uporrecoveryfromashort-circuit

condition.AnovervoltagecomparatorA11(with20mV

hysteresis)senseswhentheFBXpinvoltageexceedsthe

positiveregulatedvoltage(1.6V)by8%andprovidesa

reset pulse. Similarly, an overvoltage comparator A12

(with10mVhysteresis)senseswhentheFBXpinvoltage

exceedsthenegativeregulatedvoltage(–0.8V)by11%

andprovidesaresetpulse.Bothresetpulsesaresentto

themainRSlatch(SR1)throughG6andG5.Thepower

MOSFETswitchM1isactivelyheldoffforthedurationof

anoutputovervoltagecondition.

Programming Turn-On and Turn-Off Thresholds with

the SHDN/UVLO Pin

The SHDN/UVLO pin controls whether the LT3757 is

enabled or is in shutdown state. A micropower 1.22V

reference,acomparatorA10andacontrollablecurrent

sourceIS1allowtheusertoaccuratelyprogramthesup-

plyvoltageatwhichtheICturnsonandoff.Thefalling

valuecanbeaccuratelysetbytheresistordividersR3

andR4.WhenSHDN/UVLOisabove0.7V,andbelowthe

1.22Vthreshold,thesmallpull-downcurrentsourceIS1

(typical2µA)isactive.

Thepurposeofthiscurrentistoallowtheusertoprogram

therisinghysteresis.TheBlockDiagramofthecomparator

andtheexternalresistorsisshowninFigure1.Thetypical

fallingthresholdvoltageandrisingthresholdvoltagecan

becalculatedbythefollowingequations:

R R

µA

VIN FALLING

VIN RISING

. •

(

)

•

1 22

RR VIN FALLING

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