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inputcapacitor(c
在
)
I
c,rms
1.2(
t
在
T
) I
加载
, 在哪里:
t
在
T
V
输出
V
在
{buck regulator}, 和
t
在
T
|
V
输出
|
(|
V
输出
|
V
在
)
{buck−boost regulator}
outputcapacitor(c
输出
)
CatchDiode
Inductor
LM2575
1-asimplestep-downswitchingvoltageregulator
SLVS569D–JANUARY2005–REVISEDJULY2005
applicationinformation(持续)
forstabilityconcerns,aninputbypasscapacitor(electrolytic,c
在
≥
47
µ
f)needstobelocatedascloseas
possibletotheregulator.foroperatingtemperaturesbelow–25
°
c,c
在
mayneedtobelargerinvalue.在
增加,sincemostelectrolyticcapacitorshavedecreasingcapacitancesandincreasingesrastemperature
drops,addingaceramicorsolidtantalumcapacitorinparallelincreasesthestabilityincoldtemperatures.
toextendthecapacitoroperatinglifetime,thecapacitorrmsripplecurrentratingshouldbe:
forbothloopstabilityandfilteringofripplevoltage,anoutputcapacitoralsoisrequired,againincloseproximity
totheregulator.forbestperformance,低-esraluminumelectrolyticsarerecommended,althoughstandard
aluminumelectrolyticsmaybeadequateforsomeapplications.basedonthefollowingequation:
outputripplevoltage=(esrofc
输出
)
×
(inductorripplecurrent)
Outputrippleof50mVto150mVtypicallycanbeachievedwithcapacitorvaluesof220
µ
Fto680
µ
f.大
C
输出
canreducetheripple20mvto50mvpeak-至-顶峰.toimprovefurtheronoutputripple,parallelingof
standardelectrolyticcapacitorsmaybeused.alternatively,高等级的-gradecapacitorssuchas“highfrequency”,
“lowinductance”,or“lowesr”canbeused.
ThefollowingshouldbetakenintoaccountwhenselectingC
输出
:
•
atcoldtemperatures,theesroftheelectrolyticcapacitorscanrisedramatically(typically3
×
nominalvalue
at–25
°
c).becausesolidtantalumcapacitorshavesignificantlybetteresrspecificationsatcold
温度,theyshouldbeusedatoperatingtemperaturelowerthan–25
°
c.asanalternative,tantalums
alsocanbeparalleledtoaluminumelectrolyticsandshouldcontribute10%to20%tothetotalcapacitance.
•
LowESRforC
输出
isdesirableforlowoutputripple.不管怎样,theesrshouldbegreaterthan0.05
Ω
至
avoidthepossibilityofregulatorinstability.hence,asoletantalumcapacitorusedforc
输出
ismost
susceptibletothisoccurrence.
•
thecapacitor’sripplecurrentratingof52khzshouldbeatleast50%higherthanthepeak-至-peakinductor
ripplecurrent.
aswithotherexternalcomponents,thecatchdiodeshouldbeplacedclosetotheoutputtominimizeunwanted
噪音.schottkydiodeshavefastswitchingspeedsandlowforwardvoltagedropsand,因此,offerthebest
效能,especiallyforswitchingregulatorswithlowoutputvoltages(v
输出
<5v).ifahigh-效率,
快-恢复,orultra-快-recoverydiodeisusedinplaceofaschottky,itshouldhaveasoftrecovery(相比
abruptturn-offcharacteristics)toavoidthechanceofcausinginstabilityandemi.standard50-/60-hzdiodes,
suchasthe1n4001or1n5400series,arenotsuitable.
properinductorselectioniskeytotheperformance-switchingpower-supplydesigns.oneimportantfactorto
consideriswhethertheregulatorwillbeusedincontinuous(inductorcurrentflowscontinuouslyandneverdrops
tozero)orindiscontinuousmode(inductorcurrentgoestozeroduringthenormalswitchingcycle).eachmode
hasdistinctivelydifferentoperatingcharacteristicsand,因此,canaffecttheregulatorperformanceand
(所需的)东西.inmanyapplications,thecontinuousmodeisthepreferredmodeofoperation,sinceitoffers
greateroutputpowerwithlowerpeakcurrents,andalsocanresultinloweroutputripplevoltage.theadvantages
ofcontinuousmodeofoperationcomeattheexpenseofalargerinductorrequiredtokeepinductorcurrent
持续的,especiallyatlowoutputcurrentsand/orhighinputvoltages.
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