Order this document by AN920 D Rev

Jade Alberkrack

Description

Niveau: Supérieur, Doctorat, Bac+8
MOTOROLA — SEMICONDUCTOR APPLICATION NOTE Order this document by AN920/D Rev. 2 INTRODUCTION The MC34063 and p.A78S40 are monolithic switching regulator subsystems intended for use as dc to dc con- verters. These devices represent a significant advance- ment in the ease of implementing highly efficient and yet simple switching power supplies. The use of switching regulators is becoming more pronounced over that of linear regulators because the size reductions In new equipment designs require greater conversion efficiency. Another major advantage of the switching regulator is that it has increased application flexibility of output volt- age. The output can be less than, greater than, or of opposite polarity to that of the input voltage. PRINCIPAL OF OPERAllON In order to understand the difference in operation between linear and switching regulators we must com- pare the block diagrams of the two step-down regulators shown in Figure 1. The linear regulator consists of a sta- ble reference, a high gain error amplifier, and a variable resistance series-pass element. The error amplifier mon- itors the outputvoltage level,compares it to the reference and generates a linearcontrol signal that varies between two extremes, saturation and cutoff. This signal is used to vary the resistance of the series-pass element in a corrective fashion in order to maintain a constant output voltage under varying input voltage and output load conditions.

• limit sense


• output switch


• switching regulator


• output voltage


• ramp-up


• current

Sujets

• Switched-mode power supply
• Current

Informations

• Publié par: profil-zyak-2012
• Nombre de lectures: 5
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

029NA9891..CNIALOROTOMO.mV.dedulcnieraatadtsethtiwstiucricsnoitacilppasuoremundnaselpmaxengisedrelfevnoclareveS.smetsysbusrotalugergnihctiwsO4SS7Apdna36043CMehtfonoitarepofoelpicnirpehtliatednisebircsedrepapsihTkcarkreblAedaJybderaperPstiucriClortnoCrotalugeRgnihctiwSO4S87A~dna36043CMehtfosnoitacilppAdnayroehT2.veR029NAALOROTOM)~(.04S87AA1dna36043CMotylppaseodblerugiFninevigelpmaxeeht,gnikaepsyllareneG.lortnocelcycytudybdeveihcasinoitalugerdnadetpurretninusignihctiwsffo-noehterehw,sepytemit-ffoelbairavemit-nOdexifehtdnarotaludomhtdiweslupycneuqerfdexifehteranommoctsomehT.lortnocgnihctiwsfosnoitairavrehtoeraerehT.llamsoslaeranoitapissidrewopehtesiwekildnatnem-eleehthguorhttnerruceht,ffotucninehW.noitapissidehtsisalltuov

aa.LinearRegulator

mb.SwitchingRegulator
FIgure1.Step-DownRegulators

stuov

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a.M~Z34O63

GENERAL
DESCRIPTION
TheMC3406s3eriesisamonolithiccontrolcircuitcon-
tvaeirntienrgsa.lTlthhisedaectviivceefuconcnttiaoinnssraenquiinrteedrfnoraltdectmopdecrcatounr-e
compensatedreference,comparator,controlledduty
dcryicvleero,asncidllaathoirgwhitchurarneantcotiuvtepputesawkictcurhr.eTnhtilsismeitriceirscwuaits,
specificallydesignedtobeincorporatedinstep-up,step-
downandvoltage-invertingconverterapplications.
Thesefunctionsarecontainedinan8pindualin-line
packageshowninFigure2a.
The~A78S4iOsidenticatlotheMC3406w3iththeaddi-
timoitnteodfoapneorna-tiboonaarladpmopwliefirecra.Ttchhisdidoedviec,aeinsdiannau1n6copimn-
dualin-linepackagewhichallowsthereferenceandthe
Tnhoen-sienavdedrittiinoginnaplfuetaotfutrheesgcroematplyaeranthoartnocbetehpeifnlenxeidboiliutty.
toifctahtiesdpaaprtpalincdatailloonwsT.thheeismepmleaymienncltuatdieosnoefrimeosr-epsaosprsheigs--
vuloalttiaogneo,afttrhaecmkianignroeugtuplauttoorrcoofnafigduerriavtieodnsoerceovnednoaustpeuct-
ondswitchingregulator.
FUNCTIONALDESCRIPTION
wThihcehocshcairllgaetosarinsdcdoismcphoasregdoeftshaeceuxrrteernntsaoltiumrcineagncdaspinakc-
itorCTbetweenanupperandlowerpresetthreshold.
Thetypicalchargeanddischargecurrentsare35/LAand
200
J.LA
respectivelyy,ieldingabouta1to6ratio.Thus
theramp-upperiodis6timeslongerthanthatofthe
ramp-downasshowninFigure3.Theupperthreshold
isequaltotheinternalreferencevoltageof1.25voltsand
thelowerisapproximatelyequalto0.75V.Theoscillator
REFINVNOt4INVVCCOUTPUSTWITCHL~IOCE01011
runscontinuouslyataratecontrolledbytheselected
OUTPUTINPUTINPUTOPAMPEIITTERANOCECAThODE
valueofCT.
Duringtheramp-upportionofthecycle,aLogic1is
b.p~A78S4O
vporletsaegnetoafttthheeswAiticnhpiuntgorfetghuelaAtoNriDsgbaetleo.Iwftnhoemoinutapl,uat
Figure2.FunctionaBllockDiagrams
dLiotigoincwi1ll,wsilelttahlseolabtechpraensdecnatuatstethheeBQinoputupt.uTtthoisgcootno-
aLogic1e,nablingthedriverandoutputswitchtocon-
V
dwiullcts.tWarhtteontdhisecohsacrilglaeatonrdreLaocghicesit0swuilplpbeertphrrees~ehnotaltdt,CheT
UPPERTHESNO1.W2V5
TYPICAL
AinputoftheANDgate.Thislogiclevelisalsocon-
nectedtoaninverterwhoseoutputpresentsaLogic1
totheresetinputofthelatch.Thisconditionwillcause
LOYVT!hRRESH0O.7L5DVTYPICAL
Qtogolow,disablingthedriverandoutputswitch.A
logictruthtableofthesefunctionalblocksisshowninI--
itO4ARGEI
t
I
Figure4.
Theoutputofthecomparatorcansetthelatchonly
DISCHARGE
odnu-ricnygctlheoefroaumtpp-uutspowfitCcThacnodncdauncitinoitni.aOtenaceptahreticaolomrfpuallr-
Figure3.CTVoltageWaveform
atorhassetthelatch,itcannotresetit.Thelatchwill
remainsetuntilCTbeginsrampingdown.Thusthecom-cycle,multiplecycles,ormultiplecyclesplusaportion
paratorcaninitiateoutputswitchconduction,butcannot
terminat
i
e
tandthelatchisalwaysresetwhenCTbeginsofCounrerceyntclliem.itingisaccomplishedbymonitoringthe
rampingdown.ThecomparatorosutputwillbeataLogicvoltagedropacrossanexternalsenseresistorplacedin
0whentheoutputvoltageoftheswitchingregulatoris
abovenominal.Undetrheseconditionst,hecomparatorssdeervieelsowpitehdaVcCroCassntdhtihsereosuitsptuotrisswimtcohn.iTtohreevdobltyatgheedrloppk
outputcaninhibitaportionoftheoutputswitchon-cycle,
acompletecycle,acompletecycleplusaportionofone3S3e0nmseVp,tihn.ecWuhrreennttlhiimsitvcoilrtcaugiterybpercoovimdeessagnreaadtdeirttiohnaanl

2
MOTOROLA

~NA

ANDGateLatch
ActiveCondItio~nInputsInputs
TimingCapacitoCrT
ABSRSwitchCommentosnStateofOutputSwitch
BeginsRamp-Up000(SwBit=chi0n).gRegulatorOsutputis~nominal
BeginsRamp-DownRNaocmhpa-nDgosewinnc.eBwas0beforeCT
RampingDown0010output<nominal.Outputswitchcannotbe
RampingDown0
\,~
010Nochangesinceoutputswitchconduction
Switchingregulatorsoutputwent<nominal
RampingUp10duringCTRamp-Up(B~1).Partialon-
cycleforoutputswitch.
SwitchingRegulatorsoutputwent~nominal
RampinUgp
101(8*0)duringCTRamp-Up.Nochange
sinceBcannotresetlatch.
Completeon-cyclesinceBwas1beforeCT
BeginsRamp-Up
J1
\.....
startedRamp-Up.
BeginsRamp-Down
1tOerumtpinutatsewditwc
h
ecnoenvdeurcCtiToinsisRaalwmapiynsgDown.
Figure4.LogicTruthTableofFunctionalBlocks
COMPAROAUTTOPRUT
0
WJLAL
TIMINCGAPACITCOTR
NOOUTPUS~TAtTCH
FFOPJOUNOAULTPVUOTLTA

G
_
E
_____________
iEv~OUWUVTOLTA

GE
H
START-UP
QUIESCEONPTERATION
Figure5.TypicalOperatingWaveforms
currentpathtochargethetimingcapacitorCT.This
butfinitetimeofoutputconductionfollowedbyeithera
causesittorapidlyreachtheupperoscillatorthreshold,normalorextendedoff-timeintervalprovidedbythe
therebyshorteningthetimeofoutputswitchconductionoscillatorramp-downtimeofCT.Theextendedinterval
andthusreducingtheamountofenergystoredintheistheresultofchargingCTbeyondtheupperoscillator
inductor.Thiscanbeobservedasanincreasientheslopethresholdbyoverdrivingthecurrentlimitsenseinput.
ofthechargingportionoftheCTvoltagewaveformasThiscanbecausedbyoperatingtheswitchingregulator
showninFigure5.Operationof
theswitchingregulator
withaseverelyoverloadedorshortedoutputorhaving
inanoverloadorshortedconditionwillcauseaveryshort
theinputvoltagegrosslyabovethe
nominaldesignvalue.
ANS2O
MOTOROLA
3

)0301Ez3II..,
03.01.00.0300.20.40.60.8
v~,
CIJARENT-
S
U
E
MJ
N
TV
S
O
E
LTA(GVEOLTS)
Figure6.TimingCapacitorChargeCurrentversus
Current-UmitSenseVoltage

Underextremeconditions,thevoltageacrossCTwill
approachVCCand
cancausearelativelylongoff-time.
Thisactionmaybeconsideredafeaturesinceitwill
reducethepowerdissipationoftheoutputswitchcon-
siderably.ThisfeaturemaybedisabledonthejLA78540
only,byconnectingasmallsignalPNPtransistorasa
clamp.TheemitterisconnectedtoCT,thebasetothe
referenceoutput,andthecollectortoground.Thiswill
limitthemaximumchargevoltageacrossCTtolessthan
2.0volts.Withtheuseofcurrentlimiting,saturationof
thestorageinductormaybepreventedaswellasachiev-
ingasoftstart-up.
Inpracticethecurrentlimitcircuitwillsomewhatmod-
ifythechargingslopeandpeakamplitudeofCTeach
timethe
outputswitchisrequiredtoconduct.Thisis
becausethethresholdvoltageofthecurrentlimitsense
circuitexhibitsasoftvoltageturn-oncharacteristicand
hasaturn-offtimedelaythatcausessomeovershoot.
The330mVthresholdisdefinedwherethechargeand
dischargecurrentsareofequalvaluewithVCC
=
5.0V,
asshowninFigure6.Thecurrentlimitsensecircuitcan
bedisabledbyconnectingthe