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LINEANDLOADREGULATION
OUTPUTCAPACITORSELECTION
V
输出
I
输出
C
输出
1
fs(iout)
–
I
P
L
Vout
Vd–Vin
I
P
等效串联电阻
(7)
TPS61040
TPS61041
SLVS413B–OCTOBER2002–REVISEDMAY2004
thelargerthefeedforwardcapacitortheworsethelineregulationofthedevice.因此,whenconcernforline
regulationisparamount,theselectedfeedforwardcapacitorshouldbeassmallaspossible.seethenextsection
formoreinformationaboutlineandloadregulation.
thelineregulationofthetps61040/41dependsonthevoltagerippleonthefeedbackpin.usuallya50mv
顶峰-至-peakvoltagerippleonthefeedbackpinfbgivesgoodresults.
Someapplicationsrequireaverytightlineregulationandcanonlyallowasmallchangeinoutputvoltageovera
certaininputvoltagerange.ifnofeedforwardcapacitorc
FF
isusedacrosstheupperresistorofthevoltage
feedbackdivider,thedevicehasthebestlineregulation.withoutthefeedforwardcapacitortheoutputvoltage
rippleishigherbecausethetps61040/41showsoutputvoltageburstsinsteadofsinglepulsesontheswitchpin
(sw),increasingtheoutputvoltageripple.increasingtheoutputcapacitorvaluereducestheoutputvoltage
波纹.
ifalargeroutputcapacitorvalueisnotanoption,afeedforwardcapacitorc
FF
canbeusedasdescribedinthe
previoussection.theuseofafeedforwardcapacitorincreasestheamountofvoltageripplepresentonthe
feedbackpin(fb).thegreaterthevoltagerippleonthefeedbackpin(
≥
50mv),theworsethelineregulation.
therearetwowaystoimprovethelineregulationfurther:
1.useasmallerinductorvaluetoincreasetheswitchingfrequencywhichwilllowertheoutputvoltageripple,
aswellasthevoltagerippleonthefeedbackpin.
2.addasmallcapacitorfromthefeedbackpin(fb)togroundtoreducethevoltagerippleonthefeedbackpin
downto50mvagain.asastartingpoint,thesamecapacitorvalueasselectedforthefeedforwardcapacitor
C
FF
canbeused.
forbestoutputvoltagefiltering,alowesroutputcapacitorisrecommended.ceramiccapacitorshavealow
esrvaluebuttantalumcapacitorscanbeusedaswell,dependingontheapplication.
assumingtheconverterdoesnotshowdoublepulsesorpulseburstsontheswitchnode(sw),theoutput
voltageripplecanbecalculatedas:
在哪里:
I
P
=Peakcurrentasdescribedintheprevious
peakcurrentcontrol
部分
L=Selectedinductorvalue
I
输出
=Nominalloadcurrent
fs(i
输出
)=switchingfrequencyatthenominalloadcurrentascalculatedpreviously
vd=rectifierdiodeforwardvoltage(typically0.3v)
C
输出
=Selectedoutputcapacitor
ESR=OutputcapacitorESRvalue
RefertoTable2and
typicalapplications
sectionforchoosingtheoutputcapacitor.
table2.recommendedinputandoutputcapacitors
DEVICECAPACITORVOLTAGERATINGCOMPONENTSUPPLIERCOMMENTS
4.7µf/x5r/08056.3vtayoyudenjmk212by475mgc
在
/c
输出
10µf/x5r/08056.3vtayoyudenjmk212bj106mgc
在
/c
输出
tps61040/411.0µf/x7r/120625vtayoyudentmk316bj105klc
输出
1.0µf/x5r/120635vtayoyudengmk316bj105klc
输出
4.7µf/x5r/121025vtayoyudentmk325bj475mgc
输出
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