LT3757 datasheet(11/36 Pages) LINER

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LT3757

3757fb

applicaTions inForMaTion

Figure 2. Recommended Maximum QG vs VIN at Different

Frequencies to Ensure INTVCC Higher Than 4.5V

Aneffectiveapproachtoreducethepowerconsumption

oftheinternalLDOforgatedriveistotietheINTVCCpin

toanexternalvoltagesourcehighenoughtoturnoffthe

internalLDOregulator.

If the input voltage VIN does not exceed the absolute

maximumratingofboththepowerMOSFETgate-source

voltage(VGS)andtheINTVCCovervoltagelockoutthreshold

voltage(17.5V),theINTVCCpincanbeshorteddirectly

totheVINpin.Inthiscondition,theinternalLDOwillbe

turnedoffandthegatedriverwillbepowereddirectly

fromtheinputvoltage,VIN.WiththeINTVCCpinshortedto

VIN,however,asmallcurrent(around16µA)willloadthe

INTVCCinshutdownmode.Forapplicationsthatrequire

thelowestshutdownmodeinputsupplycurrent,donot

connecttheINTVCCpintoVIN.

InSEPICorflybackapplications,theINTVCCpincanbe

connectedtotheoutputvoltageVOUTthroughablocking

diode,asshowninFigure3,ifVOUTmeetsthefollowing

conditions:

1.VOUT<VIN(pinvoltage)

2.7.2<VOUT<17.5V

3.VOUT<maximumVGSratingofpowerMOSFET

AresistorRVCCcanbeconnected,asshowninFigure3,to

limittheinrushcurrentfromVOUT.Regardlessofwhether

Figure 3. Connecting INTVCC to VOUT

CVCC

4.7µF

VOUT

3757 F03

INTVCC

GND

LT3757

RVCC

DVCC

VIN (V)

200

250

150

100

300

3757 F02

300kHz

1MHz

ornottheINTVCCpinisconnectedtoanexternalvoltage

source,itisalwaysnecessarytohavethedrivercircuitry

bypassedwitha4.7µFlowESRceramiccapacitortoground

immediatelyadjacenttotheINTVCCandGNDpins.

Operating Frequency and Synchronization

The choice of operating frequency may be determined

byon-chippowerdissipation,otherwiseitisatrade-off

betweenefficiencyandcomponentsize.Lowfrequency

operationimprovesefficiencybyreducinggatedrivecur-

rentandMOSFETanddiodeswitchinglosses.However,

lower frequency operation requires a physically larger

inductor.Switchingfrequencyalsohasimplicationsfor

loopcompensation.TheLT3757usesaconstant-frequency

architecturethatcanbeprogrammedovera100kHzto

1000kHzrangewithasingleexternalresistorfromthe

RTpintoground,asshowninFigure1.TheRTpinmust

haveanexternalresistortoGNDforproperoperationof

theLT3757.AtableforselectingthevalueofRTforagiven

operatingfrequencyisshowninTable1.

Table 1. Timing Resistor (RT) Value

OSCILLATOR FREQUENCY (kHz)

RT (kΩ)

100

140

200

63.4

300

41.2

400

30.9

500

24.3

600

19.6

700

16.5

800

900

12.1

1000

10.5

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