HT7963 datasheet(5/11 Pages) HOLTEK

5 / 11 page

Rev. 1.00

August 14, 2015

HT7963

Thermal Shutdown

Athermalshutdownfunctionisimplementedto

preventdamageduetoexcessiveheatandpower

dissipation.Typicallythethermalshutdownthreshold

is150°C.Whenthethermalshutdownistriggered

thedevicestopsswitchinguntilthetemperaturefalls

belowatypicaltemperatureof125°C,afterwhichthe

devicewillagainbeginoperation.

Component Selection Guide

Inductor

The selected inductor should have a saturation

currentthatmeetsthemaximumpeakcurrentofthe

converter.Anotherimportantinductorparameteris

thedcresistance.Lowerdcresistancevaluesresultsin

higherconverterefficiencies.Formostapplications,

theinductorvaluecanbeobtainedasbelow.

VIN×(VOUT-VIN)

∆IL×fSW×VOUT

Ahighervalueofripplecurrentreducestheinduc-

tancevalue,butincreasestheconductanceloss,core

lossandcurrentstressfortheinductorandswitching

devices.Asuggestedchoiceisfortheinductorripple

currenttobe30%ofthemaximumloadcurrent.This

requiresthattheinductorsaturationcurrentbeabove

IL(PEAK).

IIN(MAX)=

VOUT×IOUT(MAX)

VIN×η

∆IL=30%×IIN(MAX)

IL(PEAK)=IIN(MAX)+12∆IL

IOUT(MAX)isthemaximumloadcurrent,ΔIListhepeak-

to-peakinductorripplecurrent,ηistheconverteref-

ficiency,fSWistheswitchingfrequencyandIL(PEAK)is

thepeakinductorcurrent.

Schottky Diode

ItisrecommendedtouseaSchottkydiodewithlow

forwardvoltagetominimisethepowerdissipation

andthereforetomaximisetheconverterefficiency.

Thebreakdownvoltageratingofthediodeshouldbe

higherthanthemaximumoutputvoltage.Theaverage

andpeakcurrentratingmustbegreaterthanthe

maximumoutputcurrentandpeakinductorcurrentto

ensurethebestreliabilityinmostapplications.

MOSFET

ItisrecommendedtouseaMOSFETwithsmallon

resistancetominimisethepowerdissipationandthere-

sethepowerdissipationandthere-

ethepowerdissipationandthere-

dissipationandthere-

foretomaximisetheconverterefficiency.MOSFETs

withsmallgatecapacitancevaluesneedtobeselected

tohavehigh-speedswitching.Themaximumvoltage

ratingoftheMOSFETshouldbehigherthanthesum

ofVOUTandtherectifyingdiodeVF.Themaximum

currentratingmustbegreaterthantheovercurrent

protectionsettingandpeakinductorcurrenttoensure

thebestreliabilityinmostapplications.

Input Capacitor

A low ESR ceramic capacitor is required to be

connectedbetweentheVINandGNDpins.Use

ceramiccapacitorswithX5RorX7Rdielectricsfor

theirlowESRsandsmalltemperaturecoefficients.

Thiscapacitormustbeconnectedveryclosetothe

VINpinandtheinductorwithshorttracesforgood

noiseperformance.

Output Capacitor

AlowESRceramiccapacitorissuggestedforuse

hereasitwillresultinloweroutputripplevoltages.

Theselectionofoutputcapacitorisdrivenbythe

maximumallowableoutputvoltageripple.Aceramic

capacitorwithalowESRvaluewillprovidethe

lowestvoltagerippleandisthereforerecommended.A

capacitanceintherangeof33μFto47μFissufficient.

Capacitorvoltageratingsneedstobeselectedtohave

anadequatemarginagainstthehighestoutputvoltage.

LED Current Selection

TheLEDcurrentiscontrolledbythecurrentsense

feedbackresistorRFB,Thecurrentsensefeedbackref-

erencevoltage(VFB)is0.3V.Inordertoobtainaccu-

is0.3V.Inordertoobtainaccu-

0.3V.Inordertoobtainaccu-

V.Inordertoobtainaccu-

obtainaccu-

accu-

rateLEDcurrents,precisionresistorsarethepreferred

typewitha1%tolerance.TheLEDcurrent(ILED)can

becalculatedfromthefollowingformula.

ILED=

VFB

RFB

0.3V

RFB

CS Pin and FB Pin RC Filter

Whether an RC low pass filer is employed or

notdependsuponthedesignlayoutandmustbe

consideredonacase-by-casebasis.

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