HT7718S datasheet(5/29 Pages) HOLTEK

5 / 29 page

Rev. 1.00

February 22, 2012

HT77xxS

Functional Description

TheHT77xxSisaconstantontimesynchronousstep-up

converter,whichusesapulsefrequencymodulation

(PFM)controllerscheme.ThePFMcontrolschemeis

inherentlystable.Therequiredinput/outputcapacitor

andinductorselectionswillnotcreatesituationsof

instability.

Thedeviceincludesafullyintegratedsynchronous

rectifierwhichreducescosts(includesreduceLandC

sizes,eliminatesSchottkydiodecostetc.)andboard

area.Atrueloaddisconnectfunctionensuresthatthe

deviceiscompletelyshutdown.

Low Voltage Start-up

Thedeviceshaveaverylowstartupvoltagedownto

0.7V.Whenpowerisfirstapplied,thesynchronous

switchwillbeinitiallyoffbutenergywillbetrans-

ferredtotheloadthroughitsintrinsicbodydiode.

Shutdown

Duringnormaldeviceoperation,theENpinshouldbe

eitherhighorconnectedtotheVOUTpinortheVIN

powersource.Whenthedeviceisintheshutdown

mode,thatiswhentheENpinispulledlow,theinter-

nalcircuitrywillbeswitchedoff.Duringshutdown,

thePMOSpowertransistorwillbeswitchedoffthus

placingtheoutputintoafloatingcondition.

Synchronous Rectification

AdeadtimeexistsbetweentheNchannelandPchan-

nelMOSFETswitchingoperations.Insynchronous

rectification,thePchannelisreplacedbyaSchottky

diode.HerethePchannelswitchmustbecompletely

offbeforetheNchannelswitchisswitchedon.After

eachcycle,a30nsdelaytimeisinsertedtoensurethe

NchannelswitchiscompletelyoffbeforethePchannel

switchisswitchedontomaintainahighefficiency

overawideinputvoltageandoutputpowerrange.

Application Information

Inductor Selection

Selectingasuitableinductorisanimportantconsider-

ationasitisusuallyacompromisesituationbetween

theoutputcurrentrequirements,theinductorsaturation

limitandtheacceptableoutputvoltageripple.Lower

valuesofinductorvaluescanprovidehigheroutput

currentsbutwillsufferfromhigherripplevoltages

andreducedefficiencies.Higherinductorvaluescan

providereducedoutputripplevoltagesandbetter

efficiencies,butwillbelimitedintheiroutputcurrent

capabilities.Forallinductorsitmustbenotedhowever

thatlowercorelossesandlowerDCresistancevalues

willalwaysprovidehigherefficiencies.

Thepeakinductorcurrentcanbecalculatedusingthe

followingequation:

OUT

PEAK

−

)

(

)

(

Where

VIN=InputVoltage

VOUT=OutputVoltage

IO=OutputCurrent

η=Efficiency

L=Inductor

Capacitor Selection

Astheoutputcapacitorselectedaffectsbothefficien-

cyandoutputripplevoltage,itmustbechosenwith

caretoachievebestresultsfromtheconverter.Output

voltagerippleistheproductofthepeakinductorcurrent

andtheoutputcapacitorequivalentseriesresistance

orESRforshort.ItisimportantthatlowESRvalue

capacitorsareusedtoachieveoptimumperformance.

OnemethodtoachievelowESRvaluesistoconnect

twoormorefiltercapacitorsinparallel.Thecapacitors

valuesandratedvoltagesareonlysuggestedvalues.

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