LT3757 datasheet(16/36 Pages) LINER

16 / 36 page

LT3757

3757fb

applicaTions inForMaTion

Figure 6. The Output Ripple Waveform of a Boost Converter

VOUT

(AC)

tON

VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

VCOUT

3757 F05

tOFF

TJ must not exceed the MOSFET maximum junction

temperaturerating.Itisrecommendedtomeasurethe

MOSFETtemperatureinsteadystatetoensurethatabsolute

maximumratingsarenotexceeded.

Boost Converter: Output Diode Selection

Tomaximizeefficiency,afastswitchingdiodewithlow

forwarddropandlowreverseleakageisdesirable.The

peak reverse voltage that the diode must withstand is

equaltotheregulatoroutputvoltageplusanyadditional

ringingacrossitsanode-to-cathodeduringtheon-time.

Theaverageforwardcurrentinnormaloperationisequal

totheoutputcurrent,andthepeakcurrentisequalto:

D PEAK

L PEAK

L MAX

(

)

(

)

(

)

•

= +









Itisrecommendedthatthepeakrepetitivereversevoltage

ratingVRRMishigherthanVOUTbyasafetymargin(a10V

safetymarginisusuallysufficient).

Thepowerdissipatedbythediodeis:

PD=IO(MAX)•VD

andthediodejunctiontemperatureis:

TJ=TA+PD•RθJA

TheRθJAtobeusedinthisequationnormallyincludes

theRθJCforthedeviceplusthethermalresistancefrom

theboardtotheambienttemperatureintheenclosure.TJ

mustnotexceedthediodemaximumjunctiontemperature

rating.

Thechoiceofcomponent(s)beginswiththemaximum

acceptableripplevoltage(expressedasapercentageof

theoutputvoltage),andhowthisrippleshouldbedivided

betweentheESRstep∆VESRandthecharging/discharg-

ing∆VCOUT.Forthepurposeofsimplicity,wewillchoose

2%forthemaximumoutputripple,tobedividedequally

between∆VESRand∆VCOUT.Thispercentageripplewill

change,dependingontherequirementsoftheapplica-

tion,andthefollowingequationscaneasilybemodified.

Fora1%contributiontothetotalripplevoltage,theESR

oftheoutputcapacitorcanbedeterminedusingthefol-

lowingequation:

ESR

COUT

OUT

D PEAK

≤

0 01

. •

(

)

Boost Converter: Output Capacitor Selection

ContributionsofESR(equivalentseriesresistance),ESL

(equivalentseriesinductance)andthebulkcapacitance

mustbeconsideredwhenchoosingthecorrectoutput

capacitorsforagivenoutputripplevoltage.Theeffectof

thesethreeparameters(ESR,ESLandbulkC)ontheoutput

voltageripplewaveformforatypicalboostconverteris

illustratedinFigure6.

LT3757 Datasheet(PDF) 16 Page - Linear Technology

LT3757 Datasheet(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL