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LT3757HMSE-PBF Datasheet(PDF) 10 Page - Linear Technology

Part # LT3757HMSE-PBF
Description  Boost, Flyback, SEPIC and Inverting Controller
Download  36 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3757HMSE-PBF Datasheet(HTML) 10 Page - Linear Technology

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LT3757
0
3757fb
applicaTions inForMaTion
ForapplicationswheretheSHDN/UVLOpinisonlyused
asalogicinput,theSHDN/UVLOpincanbeconnected
directlytotheinputvoltageVINforalways-onoperation.
INTVCC Regulator Bypassing and Operation
Aninternal,lowdropout(LDO)voltageregulatorproduces
the7.2VINTVCCsupplywhichpowersthegatedriver,as
showninFigure1.Ifalowinputvoltageoperationisex-
pected(e.g.,supplyingpowerfromalithium-ionbattery
ora3.3Vlogicsupply),lowthresholdMOSFETsshould
beused.TheLT3757containsanundervoltagelockout
comparatorA8andanovervoltagelockoutcomparator
A9fortheINTVCCsupply.TheINTVCCundervoltage(UV)
threshold is 2.7V (typical), with 100mV hysteresis, to
ensurethattheMOSFETshavesufficientgatedrivevoltage
beforeturningon.ThelogiccircuitrywithintheLT3757is
alsopoweredfromtheinternalINTVCCsupply.
TheINTVCCovervoltage(OV)thresholdissettobe17.5V
(typical)toprotectthegateofthepowerMOSFET.When
INTVCCisbelowtheUVthreshold,orabovetheOVthresh-
old,theGATEpinwillbeforcedtoGNDandthesoft-start
operationwillbetriggered.
TheINTVCCregulatormustbebypassedtogroundim-
mediatelyadjacenttotheICpinswithaminimumof4.7µF
ceramiccapacitor.Goodbypassingisnecessarytosupply
thehightransientcurrentsrequiredbytheMOSFETgate
driver.
Inanactualapplication,mostoftheICsupplycurrentis
usedtodrivethegatecapacitanceofthepowerMOSFET.
Theon-chippowerdissipationcanbeasignificantconcern
whenalargepowerMOSFETisbeingdrivenatahigh
frequencyandtheVINvoltageishigh.Itisimportantto
limitthepowerdissipationthroughselectionofMOSFET
and/oroperatingfrequencysotheLT3757doesnotexceed
itsmaximumjunctiontemperaturerating.Thejunction
temperature TJ can be estimated using the following
equations:
TJ=TA+PIC•θJA
TA=ambienttemperature
θJA=junction-to-ambientthermalresistance
PIC=ICpowerconsumption
=VIN•(IQ+IDRIVE)
IQ=VINoperationIQ=1.6mA
IDRIVE=averagegatedrivecurrent=f•QG
f=switchingfrequency
QG=powerMOSFETtotalgatecharge
TheLT3757usespackageswithanExposedPadforen-
hancedthermalconduction.Withpropersolderingtothe
ExposedPadontheundersideofthepackageandafull
copperplaneunderneaththedevice,thermalresistance
(
θJA)willbeabout43°C/WfortheDDpackageand40°C/W
fortheMSEpackage.Foranambientboardtemperatureof
TA=70°Candmaximumjunctiontemperatureof125°C,
themaximumIDRIVE(IDRIVE(MAX))oftheDDpackagecan
becalculatedas:
I
T T
V
I
W
V
DRIVE MAX
J
A
JA
IN
Q
IN
(
)
(
)
(
)
.
.
=
− =
θ
1 28
1 66mA
TheLT3757hasaninternalINTVCCIDRIVEcurrentlimit
functiontoprotecttheICfromexcessiveon-chippower
dissipation.TheIDRIVEcurrentlimitdecreasesastheVIN
increases(seetheINTVCCMinimumOutputCurrentvsVIN
graphintheTypicalPerformanceCharacteristicssection).
IfIDRIVEreachesthecurrentlimit,INTVCCvoltagewillfall
andmaytriggerthesoft-start.
BasedontheprecedingequationandtheINTVCCMinimum
OutputCurrentvsVINgraph,theusercancalculatethe
maximumMOSFETgatechargetheLT3757candriveat
agivenVINandswitchfrequency.Aplotofthemaximum
QGvsVINatdifferentfrequenciestoguaranteeaminimum
4.5VINTVCCisshowninFigure2.
AsillustratedinFigure2,atrade-offbetweentheoperating
frequencyandthesizeofthepowerMOSFETmaybeneeded
inordertomaintainareliableICjunctiontemperature.
Priortoloweringtheoperatingfrequency,however,be
suretocheckwithpowerMOSFETmanufacturersfortheir
mostrecentlowQG,lowRDS(ON)devices.PowerMOSFET
manufacturingtechnologiesarecontinuallyimproving,with
newerandbetterperformancedevicesbeingintroduced
almostyearly.


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