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參數(shù)資料
| 型號： | SC635MLTRT |
| 廠商： | Semtech Corporation |
| 英文描述： | Charge Pump Flash LED Driver |
| 中文描述： | 電荷泵閃光LED驅(qū)動器 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大小： | 357K |
| 代理商： | SC635MLTRT |
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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