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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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HT7938A Datenblatt(PDF) 5 Page - Holtek Semiconductor Inc |
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HT7938A Datenblatt(HTML) 5 Page - Holtek Semiconductor Inc |
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5 / 13 page  Rev. 1.00 5 April 19, 2012 HT7938A Schottky Diode Selection The output rectifier diode conducts during the internalMOSFETisturnoff.Theaverageandpeak currentratingmustbegreaterthanthemaximum output current and peak inductor current. The reversebreakdownvoltagemustbegreaterthanthe maximumoutputvoltage.Itisrecommendedtousea schottkydiodewithlowforwardvoltagetominimize thepowerdissipationandthereforetomaximizethe efficiencyoftheconverter.A1N5819typediodeis recommendedforHT7938Aapplications. LED Current Selection TheLEDcurrentiscontrolledbythecurrentsense feedbackresistorRfb,Thecurrentsensefeedback referencevoltageis200mV/300mV.Inordertohave accurateLEDcurrents,precisionresistorsarethe preferredtypewitha1%tolerance.TheLEDcurrent canbecalculatedfromthefollowingformula. FB V LE D I 200m V/ 3 00m V = = FB R FB R Where ILED is the total output LED current, VFB=feedbackvoltage,RFB=currentsenseresistor. Digital and Analog Dimming Control TheLEDilluminationlevelcanbecontrolledusing bothdigitalandanalogmethods. ThedigitaldimmingmethodusesaPWMsignal appliedtotheENpin.Thisisshowninfigure14. WhenthePWMdimmingfrequencyislowerthan 200Hz,LEDcurrentisfollowingbythisPWMsignal. Andnowthedeviceoutputcurrentisunderdigital modedimming.Itisunderanalogmodedimming, whenthePWMdimmingfrequencyishigherthan 200Hz,thePWMsignalisconvertedtoaDCvoltage by internal filter.And the LED current is a DC currentproportionaltoPWMsignal.A0%dutycycle correspondstozeroLEDcurrent.A100%dutycycle correspondstoafullLEDcurrent.Itprovideshigh dimmingaccuracyfromduty5%to100%.Tomake surethisswitchingprocessbetweenonandoffstate isinvisiblebyhumaneyes;theswitchingfrequency mustbegreaterthan100Hz.HT7938Acanbeapplied to the EN pin PWM dimming frequency up to 200kHz.Asshownbelow,toadjusttheanalogmode averageoutputcurrentvaluefollowingtheequation: FB V LE D I = FB R ´ D ut y cycl e 0≤Duty cycle≤1 TherearetwomethodsofanalogLEDbrightness control.ThefirstmethodusesaDCvoltagetocontrol thefeedbackvoltage.IftheDCvoltagerangeisfrom 0Vto3.3V,theselectionofresistorscontroltheLED currentfrom20mAto0mA.Theotherwayistouse afilteredPWMsignal,asshowninfigure16.The filteredPWMsignalapplicationactsinthesameway astheDCvoltagedimmingcontrol. Layout Considerations Circuitboardlayoutisaveryimportantconsideration for switching regulators if they are to function properly.Poorcircuitlayoutmayresultinrelated noiseproblems. InordertominimizeEMIandswitchingnoise,please followtheguidelinesbelow: • Alltracksshouldbeaswideaspossible. • Theinputandoutputcapacitors,C1andC2,should beplacedclosetotheVIN,VOandGNDpins. • TheSchottkydiode,D1,andinductor,L,mustbe placedclosetotheSWpin. • Feedbackresistor,Rfb,mustbeplacedclosetothe FBandGNDpins. • Afullgroundplaneisalwayshelpfulforbetter EMIperformance. A recommended PCB layout with component locationsisshownbelow. Top Layer Bottom Layer |
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