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 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

Apoorlayoutmayallowthejunctiontemperaturetoreach

theOTlimitwhileinFlashorSpotlightmode.Itiscritical

tomaintainadequategroundplanearoundthedeviceto

maximizeheattransferandavoidover-temperatureshut-

down.