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HT7718S Datasheet(PDF) 5 Page - Holtek Semiconductor Inc |
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HT7718S Datasheet(HTML) 5 Page - Holtek Semiconductor Inc |
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5 / 29 page  Rev. 1.00 5 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: L V V V V V I V I OUT IN OUT IN IN O OUT PEAK L × × − × + × × = 2 ) ( ) ( η 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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