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SC635MLTRT Datasheet(PDF) 6 Page - Semtech Corporation |
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SC635MLTRT Datasheet(HTML) 6 Page - Semtech Corporation |
6 / 12 page 6 2005SemtechCorp. www.semtech.com POWER MANAGEMENT SC635 Short-Circuit Current Protection Short-circuitcurrentprotectionisprovidedtolimitthecur- rentthatcanbesourcedwhentheoutputisshortedto ground.WhenashortcircuitforcesVOUTtodropbelow 2V,theSCCPdetectstheconditionandlimitstheoutput current. ResistorSelection TheISETresistorselectioniscriticalingeneratingthecor- rectcurrent.ThevaluecanbechosentosettheSpotlight modecurrentortheFlashmodecurrent,butitmustbe notedthatthetwocurrentsettingsaredependentonthe sameresistance.Itisrecommendedthattheresistorbe selectedtomatchthedesiredLEDcurrentforFlashmode. ThisallowsthedesignertosettheSC635’smaximumcur- rent and select the resistor package size necessary for thepowerdissipationrequiredinFlashmode.Thetypical applicationshownonpage1usesa1.25Ωresistortoset aflashmodecurrentof200mAandspotlightmodecur- rentof80mA.Ahighprecisionresistorshouldbeusedto ensurethespecifiedaccuracyforLEDcurrent. To avoid malfunction of the charge pump, it is recom- mendedthattheresistanceseenattheISETpinremain constant while the device is active (Flash or Spotlight mode).Changing the resistance value or the load cur- rentwhilethedeviceisactivecouldcauseinstabilitythat wouldresultinnon-compliantbehavior. TheloadcurrentreturnpathisfromtheISETsensepoint throughtheresistorandbacktothegroundpins.Resis- tanceinthispathaddstothetotalresistanceandhasthe effectofreducingtheLEDcurrentbyabout4%per0mV ApplicationsInformation(Cont.) ofDCdropacrossthereturncoppertraceinflashmode. Forthisreason,itiscrucialtohavealowresistancereturn path.Placeandgroundtheresistorascloseaspossible tothegroundpinoftheSC635.ThetracefromtheISET pinhasvirtuallynocurrent.TheISETtraceshouldmake contactatthepadofthepowersenseresistortominimize theeffectofvoltagedropbetweentheLEDcathodeand theresistor. CapacitorSelection TheSC635isdesignedtouselow-ESRceramiccapaci- torsfortheinputandoutputbypasscapacitorsaswellas thechargepumpbucketcapacitors.Idealperformance isachievedwhenCisexactlyequaltoC2.Itisrecom- mendedthatX5RorX7Rceramiccapacitorsbeusedfor bestperformance. ThermalResistance TheSC635packageisthermallyefficientwhenthecircuit boardlayoutconnectsthethermalpadthoughmultiple viastothegroundplane.Thethermalresistanceisrated at49°C/W,andthisratingisdependentontheconnec- tionbetweenthethermalpadandthegroundplane.A layoutthatisdonecorrectlyshouldkeepthejunctiontem- perature below the OT limit while operating the SC635 withinthespecifiedelectricalconditionsforIOUTandV ISET. Apoorlayoutmayallowthejunctiontemperaturetoreach theOTlimitwhileinFlashorSpotlightmode.Itiscritical tomaintainadequategroundplanearoundthedeviceto maximizeheattransferandavoidover-temperatureshut- down. |
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