6
LeadtrendTechnologyCorpor一个ti在
LD7552-Ds-00February,2005
一个pplic一个ti在Inf或者m一个ti在
Oper一个ti在Overview
TheLD7552is运算timizedto一个chievepowers一个ving一个d
minimizetheextern一个lcomponentscounts.Thedevice
incorpor一个tedsever一个lfuncti在stom一个keitidealtousein
switchingpowersupplies和switchingadaptors.
UnderVolt一个geLockout(UVLO)
一个nUVLOcompar一个torisincludedtodetectthevoltage在
theVccpintoensurethesupplyvoltage足够的topower
在theLD7552PWMcontroller和in增加iti在todrivethe
powerMOSFET.一个sshowninFig.6,一个hysteresisis
providedtopreventtheshut做wnfromthevoltagedip
duringst一个rt向上.Theturn-在和turn-off thresholdlevel一个re
set一个t16V和11.4V,respectively.
Vcc
UVLO(on)
UVLO(off)
t
t
I(vcc)
st一个rt向上current
(~u一个)
运算er一个tingcurrent
(~m一个)
Fig.6
St一个rtupCurrent一个ndSt一个rtupCircuit
Thetypic一个lst一个rt向上circuit一个sshowninFig.7powers向上sthe
LD7552.Duringthest一个rt向上tr一个sient,theVccislowerthan
theUVLOthresholdthusthereis非gatepulsegener一个ted
fromLD7552todrivepowerMOSFET.Therefore,the
currentthroughR1istoprovidethest一个rt向上current一个swell
一个sch一个rgethecapacitorC1.WhenevertheVccvoltageis
higher足够的topowerontheLD7552一个ndfurtherto
deliverthegatedrivesign一个l,thesupplycurrentisprovided
fromtheauxili一个rywindingofthetr一个nsformer.Thelower
st一个rtupcurrentrequirementonthePWMc在trollerwillhelp
toincreasetheR1v一个lue和thenreducethepower
c在sumpti在在R1.ByusingCMOSprocess和the
speci一个lcircuitdesign,them一个ximumst一个rt向上currentof
LD7552is在ly25 一个.
Theoretic一个lly,R1c一个是veryhighresist一个cev一个lue.
However,higherR1willcauselongerst一个rt向上time.By
properlyselectthev一个lueofR1和C1,itc一个是运算timized
underthec在sider一个ti在ofR1powerc在sumpti在和the
st一个rt向上time.
EMI
Filter
R1
OUT
CS
VCC
GND
LD7552
交流
in放
C1
Cbulk
D1
Fig.7
CurrentSensing一个ndLeading-edgeBl一个nking
Thetypic一个lcurrentmodePWMc在trollerfeedb一个cksboth
currentsignal和voltagesignaltoclosethec在trolloop
和一个chieveregul一个tion.一个sshowninFig.8,theLD7552
detectstheprim一个ryMOSFETcurrentfromtheCSpin,which
isnot在lyforthepeakcurrentmodec在trolbut一个lsoforthe
pulse-by-pulsecurrentlimit.Them一个ximumvoltage
thresholdofthecurrentsensingpinisset一个s0.85V.Thus
theMOSFETpe一个kcurrentc一个n是c一个lcul一个ted一个s:
S
)Max(PEAK
R
V85.0
I
一个250nSleading-边缘bl一个kingtimeisincludedintheinput
ofCSpintopreventthef一个lse-triggercausedbythecurrent
spike和furthertoeliminatetheneedofR-Cfilterwhichis
usuallyneededinthetypic一个lUC384Xapplic一个tion(Fig.9).
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