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OutputVoltageRippleandTransients
FeedbackConnection
在/offinput
Grounding
LM2575
1-asimplestep-downswitchingvoltageregulator
SLVS569D–JANUARY2005–REVISEDJULY2005
applicationinformation(持续)
thelm2575canoperateineithercontinuousordiscontinuousmode.withheavyloadcurrents,theinductor
currentflowscontinuouslyandtheregulatoroperatesincontinuousmode.underlightload,theinductorfully
dischargesandtheregulatorisforcedintothediscontinuousmodeofoperation.forlightloads(大概
200maorless),thisdiscontinuousmodeofoperationisperfectlyacceptableandmaybedesirablesolelyto
keeptheinductorvalueandsizesmall.anybuckregulatoreventuallywilloperateindiscontinuousmodewhen
theloadcurrentislightenough.
thetypeofinductorchosencanhaveadvantagesanddisadvantages.ifhighperformance/qualityisaconcern,
thenmore-expensivetoroidcoreinductorsarethebestchoice,asthemagneticfluxiscontainedcompletely
withinthecore,resultinginlessemiandnoiseinnearbysensitivecircuits.inexpensivebobbincoreinductors,
不管怎样,generatemoreemiastheopencorewillnotconfinethefluxwithinthecore.multipleswitching
regulatorslocatedinproximitytoeachotherareparticularlysusceptibletomutualcouplingofmagneticfluxes
fromeachother’sopencores.inthesesituations,closedmagneticstructures(suchasatoroid,potcore,或者
e-核心)aremoreappropriate.
regardlessofthetypeandvalueofinductorused,theinductornevershouldcarrymorethanitsratedcurrent.
doingsomaycausetheinductortosaturate,inwhichcasetheinductancequicklydrops,andtheinductorlooks
likealow-valueresistor(fromthedcresistanceofthewindings).asaresult,switchingcurrentrisesdramatically
(untillimitedbythecurrent-用-currentlimitingfeatureofthelm2575)andcanresultinoverheatingofthe
inductorandtheicitself.notethatdifferenttypesofinductorshavedifferentsaturationcharacteristics.
aswithanyswitchingpowersupply,theoutputofthelm2575willhaveasawtoothripplevoltageattheswitching
频率.typicallyabout1%oftheoutputvoltage,thisrippleisduemainlytotheinductorsawtoothripple
currentandtheesroftheoutputcapacitor(seenoteonc
输出
).此外,theoutputalsomaycontainsmall
voltagespikesatthepeaksofthesawtoothwaveform.thisisduetothefastswitchingoftheoutputswitchand
theparasiticinductanceofC
输出
.thesevoltagespikescanbeminimizedthroughtheuseoflow-电感
电容.
thereareseveralwaystoreducetheoutputripplevoltage:alargerinductor,alargerc
输出
,orboth.另一
methodistouseasmalllcfilter(20
µ
Hand100
µ
f)attheoutput.thisfiltercanreducetheoutputripple
voltagebyafactorof10(看Figure2).
forfixedvoltageoptions,feedbackmustbewiredtov
输出
.fortheadjustableversion,feedbackmustbe
connectedbetweenthetwoprogrammingresistors.又一次,bothoftheseresistorsshouldbeincloseproximityto
theregulator,andeachshouldbelessthan100k
Ω
tominimizenoisepickup.
在/offshouldbegroundedorbealow-levelttlvoltage(典型地<1.6v)fornormaloperation.toshutdown
thelm2575andputitinstandbymode,ahigh-levelttlorcmosvoltageshouldbesuppliedtothispin.
在/offshouldnotbeleftopenandsafelycanbepulleduptov
在
withorwithoutapullupresistor.
thepowerandgroundconnectionsofthelm2575mustbelowimpedancetohelpmaintainoutputstability.为
the5-pinpackages,bothpin3andtabareground,andeitherconnectioncanbeusedastheyarebothpartof
thesameleadframe.withthe16-pinand24-pinpackages,allthegroundpins(includingsignalandpower
grounds)shouldbesoldereddirectlytowidepcbcoppertracestoensurelow-inductanceconnectionsandgood
thermaldissipation.
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