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APPLICATIONINFORMATION
inductorselection,maximumloadcurrent
fS
最大值
Vin
最小值
(vout–vin)
I
P
L
Vout
(2)
fS
I
加载
2
I
加载
(vout–vin
vd)
I
2
P
L
(3)
I
加载 最大值
I
2
P
L
fS
最大值
2
(vout
vin)
(4)
TPS61040
TPS61041
SLVS413B–OCTOBER2002–REVISEDMAY2004
sincethepfmpeakcurrentcontrolschemeisinherentlystable,theinductorvaluedoesnotaffectthestabilityof
theregulator.theselectionoftheinductortogetherwiththenominalloadcurrent,inputandoutputvoltageofthe
applicationdeterminestheswitchingfrequencyoftheconverter.dependingontheapplication,inductorvalues
between2.2µhupto47µharerecommended.themaximuminductorvalueisdeterminedbythemaximumon
timeoftheswitch,typically6µs.thepeakcurrentlimitof400ma/250ma(典型地)shouldbereachedwithin
this6-µsperiodforproperoperation.
theinductorvaluedeterminesthemaximumswitchingfrequencyoftheconverter.因此,selecttheinductor
valuethatensuresthemaximumswitchingfrequencyattheconvertermaximumloadcurrentisnotexceeded.
themaximumswitchingfrequencyiscalculatedbythefollowingformula:
在哪里:
I
P
=Peakcurrentasdescribedintheprevious
peakcurrentcontrol
部分
L=Selectedinductorvalue
Vin
最小值
=Thehighestswitchingfrequencyoccursattheminimuminputvoltage
iftheselectedinductorvaluedoesnotexceedthemaximumswitchingfrequencyoftheconverter,thenextstep
istocalculatetheswitchingfrequencyatthenominalloadcurrentusingthefollowingformula:
在哪里:
I
P
=Peakcurrentasdescribedintheprevious
peakcurrentcontrol
部分
L=Selectedinductorvalue
I
加载
=Nominalloadcurrent
vd=rectifierdiodeforwardvoltage(typically0.3v)
asmallerinductorvaluegivesahigherconverterswitchingfrequency,butlowerstheefficiency.
theinductorvaluehaslesseffectonthemaximumavailableloadcurrentandisonlyofsecondaryorder.这
bestwaytocalculatethemaximumavailableloadcurrentundercertainoperatingconditionsistoestimatethe
expectedconverterefficiencyatthemaximumloadcurrent.thisnumbercanbetakenoutoftheefficiency
graphsshowninfigures1,2,3,and4.themaximumloadcurrentcanthenbeestimatedasfollows:
在哪里:
I
P
=Peakcurrentasdescribedintheprevious
peakcurrentcontrol
部分
L=Selectedinductorvalue
fS
最大值
=Maximumswitchingfrequencyascalculatedpreviously
η
=expectedconverterefficiency.typically70%to85%
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