HT7991 datasheet(10/13 Pages) HOLTEK

Part #	HT7991

Description	Asynchronous Step-up PWM Converter

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Manufacturer	HOLTEK [Holtek Semiconductor Inc]

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10 / 13 page

Rev. 1.20

10

June 01, 2017

HT7991

Thermal Considerations

Forcontinuousoperation,donotexceedtheabsolute

maximum junction temperature.The maximum

powerdissipationdependsonthethermalresistance

oftheICpackage,PCBlayout,rateofsurrounding

airflow,andthealloweddifferencebetweenthe

junctionandambienttemperatures.Themaximum

powerdissipationcanbecalculatedbythefollowing

formula:

PD(MAX)=(TJ(MAX)–TA)/θJA(W)

WhereTJ(MAX)isthemaximumjunctiontemperature,

TAistheambienttemperature,andθJAisthejunction

toambientthermalresistance.

Therecommendedoperatingconditionsspecifyamax-

max-

imumjunctiontemperatureis125°C.Thejunctionto

ambientthermalresistance,θJA,islayoutdependent.On

astandardJEDEC51-7four-layerthermaltestboard,

thethermalresistance,θJA,oftheQFN-10packageis

50°C/W.ThemaximumpowerdissipationatTA=25°C

canbecalculatedbythefollowingformula.

PD(MAX)=(125oC–25oC)/(220oC/W)=455mW

ForafixedTJ(MAX)of125oC,themaximumpowerdis-

dis-

sipationdependsontheoperatingambienttemperature

andthepackage'sthermalresistance,θJA.Thede-rating

curvebelowshowstheeffectofrisingambienttemperature

onthemaximumrecommendedpowerdissipation.

Ambient Temperature (oC)

25

0

50

75

100

125

0.0

125

250

375

500

625

Four-Layer

PCB

85

Layout Consideration Guide

Suggested Layout

Toreduceproblemswithconductednoise,PCBlayoutisveryimportanttostability.Thelayoutrecommendations

arelistedbelow:

(1)TheinputbypasscapacitormustbeplacedclosetotheVINpin.

(2)Theinductor,schottkydiode,andoutputcapacitortraceshouldbeasshortaspossibletoreduceconductedand

radiatednoiseandincreaseoverallefficiency.

(3)Keepthepowergroundandsupplypathsasshortandwideaspossible.

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HT7991 Datasheet(PDF) 10 Page - Holtek Semiconductor Inc

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