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LT3757HMSE-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3757HMSE-PBF Datasheet(HTML) 10 Page - Linear Technology |
10 / 36 page 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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