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APPLICATIONINFORMATION
SelectingtheInputandOutputCapacitors
ThermalConsiderations
JA
T
J
T
一个
P
(9)
P
V
在
V
输出
I
输出
I
BAT
V
输出
V
BAT
I
BAT
(10)
bq24030,bq24031,bq24032,
bq24032a,bq24035,bq24038,bq24039
SLUS618C–AUGUST2004–REVISEDJUNE2005
inmostapplications,allthatisneededisahigh-frequencydecouplingcapacitoroneachinput(acandusb).一个
0.1-µfceramiccapacitor,placedincloseproximitytoacandusbtovsspins,workswell.insomeapplications
dependingonthepowersupplycharacteristicsandcablelength,itmaybenecessarytoaddanadditional10-µf
ceramiccapacitortoeachinput.
thebqtiny-iiionlyrequiresasmalloutputcapacitorforloopstability.a0.1-µfceramiccapacitorplaced
betweentheoutandvsspinistypicallysufficient.
theintegratedldorequiresamaximumof1-µfceramiccapacitoronitsoutput.theoutputdoesnotrequirea
capacitorforasteady-stateloadbuta0.1-µfminimumcapacitanceisrecommended.
itisrecommendedtoinstallaminimumof33-µfcapacitorbetweenthebatpinandvss(inparallelwiththe
电池).thisensuresproperhotplugpowerupwithano-loadcondition(nosystemloadorbatteryattached).
thebqtiny-iiiispackagedinathermallyenhancedmlppackage.thepackageincludesaqfnthermalpadto
provideaneffectivethermalcontactbetweenthedeviceandtheprinted-circuitboard(pcb).fullpcbdesign
guidelinesforthispackageareprovidedintheapplicationnoteentitled
qfn/sonpcbattachment
(SLUA271).
thepowerpadshouldbetiedtothevssplane.themostcommonmeasureofpackagethermalperformanceis
thermalimpedance(
θ
JA
)量过的(ormodeled)fromthechipjunctiontotheairsurroundingthepackage
表面(包围的).
Themathematicalexpressionfor
θ
JA
是:
在哪里
T
J
=chipjunctiontemperature
T
一个
=ambienttemperature
P=devicepowerdissipation
Factorsthatcangreatlyinfluencethemeasurementandcalculationof
θ
JA
包含:
•
whetherornotthedeviceisboardmounted
•
tracesize,composition,厚度,andgeometry
•
orientationofthedevice(horizontalorvertical)
•
volumeoftheambientairsurroundingthedeviceundertestandairflow
•
whetherothersurfacesareincloseproximitytothedevicebeingtested
thedevicepowerdissipation,p,isafunctionofthechargerateandthevoltagedropacrosstheinternalpower
场效应晶体管.itcanbecalculatedfromEquation10:
duetothechargeprofileofli-xxbatteries,themaximumpowerdissipationistypicallyseenatthebeginningof
thechargecyclewhenthebatteryvoltageisatitslowest.看Figure1.typicallytheli-ionbattery'svoltage
quickly(<2vminutes)rampstoapproximately3.5v,whenenteringfastcharge(1-cchargerateandbattery
above3v).因此,itiscustomarytoperformthesteady-statethermaldesignusing3.5vastheminimum
batteryvoltagebecausethesystemboardandchargingdevicedoesnothavetimetoreachamaximum
temperatureduetothethermalmassoftheassemblyduringtheearlystagesoffastcharge.thistheoryiseasily
verifiedbyperformingachargecycleonadischargedbatterywhilemonitoringthebatteryvoltageandchargers
powerpadtemperature.
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