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HT7727SA Datasheet(PDF) 6 Page - Holtek Semiconductor Inc |
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HT7727SA Datasheet(HTML) 6 Page - Holtek Semiconductor Inc |
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6 / 17 page  Rev. 1.40 6 June 25, 2016 HT77xxSA Functional Description TheHT77xxSAisaconstantontimesynchronousstep- upconverter,whichusesapulsefrequencymodulation (PFM)controllerscheme.ThePFMcontrolschemeis inherentlystable.Therequiredinput/outputcapacitor andinductorselectionswillnotcreatesituationsof instability. Thedeviceincludesafullyintegratedsynchronous rectifierwhichreducescosts(includesreduceLandC sizes,eliminatesSchottkydiodecostetc.)andboard area. Low Voltage Start-up Thedeviceshaveaverylowstartupvoltagedownto 0.7V.Whenpowerisfirstapplied,thesynchronous switchwillbeinitiallyoffbutenergywillbetrans- ferredtotheloadthroughitsintrinsicbodydiode. Shutdown Duringnormaldeviceoperation,theCEpinshouldbe eitherhighorconnectedtotheVOUTpinortheVIN powersource.Whenthedeviceisintheshutdown mode,thatiswhentheCEpinispulledlow,theinter- nalcircuitrywillbeswitchedoff.Duringshutdown, thePMOSpowertransistorwillbeswitchedoff. 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: OSC OUT IN OUT IN IN O OUT PEAK L f L V V V V V I V I × × × − × + × × = 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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