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HT7939A bảng dữ liệu(PDF) 5 Page - Holtek Semiconductor Inc

tên linh kiện HT7939A
Giải thích chi tiết về linh kiện  Performance White LED Driver
PDF  13 Pages
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nhà sản xuất  HOLTEK [Holtek Semiconductor Inc]
Trang chủ  http://www.holtek.com
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HT7939A bảng dữ liệu(HTML) 5 Page - Holtek Semiconductor Inc

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Rev. 1.00
5
April 19, 2012
HT7939A
Input and Output Capacitor Selection
Theoutputcapacitordeterminesthesteadystate
outputvoltageripple.Thevoltagerippleisrelated
tothecapacitor’scapacitanceanditsESRwhichis
theEquivalentSeriesResistance.Ceramiccapacitors
withlowESRvalueswillresultinthelowestripple
voltagevaluesandaretherecommendedtype.Due
totheirlowESRvalues,thecapacitancevaluecanbe
calculatedusingthefollowingequation:
ri ppl e
SW
OU T
OU T
IN
O
ou t
V
F
V
I
V
V
C
´
´
´
-
=
)
(
Intheequationabove,Vripple=peaktopeakoutput
ripple,FSWistheswitchingfrequency.
Theoutputcapacitorisrecommendedtobeinarange
of1μFto10μF.Theinputcapacitorisrequiredtobe
inarangeof2.2μFto10.0μF.Theoutputcapacitor
affectstheloopstabilityoftheboostregulator.Ifthe
outputcapacitorislowerthanthisrange,theboost
regulatorcouldpotentiallybecomeunstable.
Schottky Diode Selection
Theoutputrectifierdiodesuppliescurrenttothe
inductor whentheinternal MOSFETisoff.The
averageandpeakcurrentratingsofthediodemustbe
greaterthanthemaximumoutputcurrentandpeak
inductorcurrent.Thereversebreakdownvoltage
mustbegreaterthanthemaximumoutputvoltage.It
isrecommendedtouseaschottkydiodewithalow
forwardvoltagetominimisepowerdissipationand
thereforetomaximisetheefficiencyoftheconverter.
A1N5819diodetypeisrecommendedforHT7939A
applications.
LED Current Selection
TheLEDcurrentiscontrolledbythecurrentsense
feedbackresistorRfb,Thecurrentsensefeedback
referenceis200mV.InordertoensureaccurateLED
currents,precisionresistorswitha1%tolerancearethe
preferredtypes.TheLEDcurrentcanbecalculated
usingthefollowingformula:
fb
FB
LE D
fb
R
R
V
I
mV
20 0
=
=
WhereILEDistheoutputLEDcurrent,VFB=feedback
voltage,Rfb=currentsenseresistor.
Digital and Analog Dimming Control
TheDigitaldimmingmethodusesaPWMsignal
applied to the EN pin and the EN Pin dimming
frequencyupto200kHz.Thisisshowninfig.19.The
averageLEDcurrentincreasesproportionallywiththe
PWMsignaldutycycle.A0%dutycyclecorresponds
tozeroLEDcurrent.A100%dutycyclecorresponds
toafullLEDcurrent.
TherearetwomethodstocontroltheLEDbrightness
foranalogdimming.ThefirstmethodusesaDC
voltagetocontrolthefeedbackvoltage.IftheDC
voltagerangeisfrom0Vto3.3V,theselectionof
resistorsprovidedinfig.20controlstheLEDcurrent
from20mAto0mA.Otherapplicationsafiltered
PWMsignal,whichisshowninfig.21.Thefiltered
PWMsignalapplicationactsthesamewayastheDC
voltagedimmingcontrol.



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