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HT7945 Datasheet(PDF) 7 Page - Holtek Semiconductor Inc |
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HT7945 Datasheet(HTML) 7 Page - Holtek Semiconductor Inc |
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7 / 11 page  Rev. 1.00 7 April 13, 2012 HT7945 Schottky Diode ItisrecommendedtouseaSchottkydiodewitha lowforwardvoltagetominimisepowerdissipation andthereforemaximisetheconverterefficiency. The average and peak current ratings must be greaterthanthemaximumoutputcurrentandpeak inductorcurrent.Therearethreeimportantelectrical parameterstoconsiderwhenchoosingthediode: • Thediodemaximumreversevoltagevaluemustbe greaterthanthemaximumoutputvoltage. • Shortrecoverytimeandlowforwardvoltage–use aSchottkydiodetype. • Diodecurrentratingshouldbegreaterthanthe maximumloadcurrent. Compensation Components TheCOMPpinistheoutputoftheerroramplifierand mustbeproperlyconnectedtoanexternalRCnetwork toensureregulatorloopstability.Recommended valuesare:RComp=24kΩandCComp=1.5nF Oscillator Frequency Setup Therearetwofrequencyoptionsavailable.TheOSC pindefaultswitchingfrequencyis1MHzwhenthepin isunconnectedand500kHzwhenthepinisconnected toground. LED current Setup TheLEDcurrentcanbesetupusinganexternal resistorconnectedfromtheISENTpintoground. Thefollowingequationshowshowthecurrentis calculated: ILED(mA)= RISET(kΩ) 480 ThisshowshowtheLedreferencecurrentcanbe setupatLED1~8andrepresentsthesensedLED currentforeachstring.TheLEDcurrentregulation betweenthestringshasgoodacuracyat±1.5%. Dimming Control ThedeviceincludesanexternalPWMsignaldimming control. PWM dimming control is achieved by applyinganexternalPWMsignalwithafrequencyof 100Hzto20kHz.Thehighlevelofthissignalmust begreaterthan2.0Vandthelowlevelmustbeless than0.8V.A0%dutycyclecorrespondstozeroLED currentwhilea100%dutycyclecorrespondstofull LEDcurrent. Layout Considerations Circuitboardlayoutisaveryimportantconsideration for switching regulators if they are to function properly.Poorcircuitlayoutmayresultinrelated noise problems. In order to minimise EMI and switchingnoise,theguidelinesshouldbenoted: • Alltracksshouldbeaswideaspossible. • Theinputandoutputcapacitors,CINandCOUT, shouldbelocatedclosetotheVIN,VOUTand GNDpins. • TheSchottkydiode,D1,andinductor,L1,mustbe locatedclosetotheLXpin. • TheAGNDanaloggroundpins,andPGNDpower groundpins,musthaveindependentconnections, butmustbeconnectedtogetheratsomefinalpoint ontheusercircuitboard. A recommended PCB layout with component locationsisenclosed. Top Layer Bottom Layer |
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