White LED Power Supply Design Techniques

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1、WhiteLEDPowerSupplyDesignTechniquesOliverNachbaurSystemEngineerPortablePowerTexasInstrumentsDeutschlandGmbHHaggertystrasse1,85356Freising,GermanyTel.:+498161803263Fax.:+498161803322Email:o-nachbaur@ti.comAbstractWiththeemergeofcolordisplaysintheportablemarketlikecellularphones,PDAsandPocket

2、PCsawhitebacklightorsidelightisrequiredforahomogeneousilluminationoftheLCDcolors.ThewhiteLEDseemstobeagoodchoiceforbacklightsinceitrequireslesspowerandspacecomparedtothecommonlyusedCCFL(ColdCathodeFluorescentLight)backlight.ThetypicalforwardvoltageofawhiteLEDisintherangeof3Vto5V.Sincethebes

3、tchoicetopowerwhiteLEDsisaconstantcurrentsourceandtheinputvoltagerangeofaLi-IonbatteryisbeloworequaltotheLEDforwardvoltageanewpowersupplysolutionisrequired.Themainpowersupplyrequirementsarehighefficiency,smallsolutionsizeandthepossibilityofadjustingtheLEDbrightness.AcceptableEMIperformanceb

4、ecomesanadditionalfocusforportablesystemswithwirelesscapabilities.Theboostconverterisaveryattractivesolutionwhenhighefficiencyisakeycriteriafortheselectionofthepowersupply.Anotherpopularsolutionistheuseofachargepumpconverter.BothsolutionstodrivewhiteLEDsarediscussedandhowtheyrelatetothemain

5、powersupplyrequirements.AnotherimportantdesignconsiderationisthecontrolmethodtoadjusttheLEDbrightnessthatwillaffecttheoverallconverterefficiencyandanypossiblechromaticityshiftsofthewhiteLED.AsimplesolutionwillbepresentedusingaPWMsignaltocontrolthebrightness.Anadditionaladvantageofthissoluti

6、onisthehigherefficiencycomparedtootherstandardsolutions.TheTaskWhenapowersupplyforawhiteLEDsisselectedthemainrequirementsforaportablesystemisefficiency,overallsolutionsize,solutioncostandlastbutnotleastEMI(electromagneticinterference).Dependingontheportablesystemtheserequirementsarerateddif

7、ferentlyamongstoneanother.Efficiencyusuallycomesoutasthenumberoneortwokeydesignparameterandneedstobeconsideredwhenselectingapowersupply.Infigure1thebasiccircuitofawhiteLEDpowersupplyisshown.WhiteLEDPowerSupplyChargepumporboostconverterLi-IonBatteryI=20mA