Application of the MC34063 switching regulator

ApplicationReport
SLVA252B–September2006–RevisedNovember2007
ApplicationoftheMC34063SwitchingRegulator
ShafiSekanderandMahmoudHarmouch SLLLinear
ABSTRACT
Thisapplicationreportprovidesthefeaturesthatarenecessarytoimplementdc-to-dc
fixed-frequencyschemeswithaminimumnumberofexternalcomponentsusingthe
MC34063.Thisdevicerepresentssignificantadvancementsineaseofusewithhighly
efficientand,yet,simpleswitchingregulators.Theuseofswitchingregulatoris
becomingmorepronouncedoverthatoflinearregulators,becauseofthesizeand
power-efficiencyrequirementofnewequipmentdesigns.Theuseofswitching
regulatorsincreasesapplicationflexibilityandreducesthecost.
Contents
1MC34063Description 2
2FunctionalDescription 5
3BuckRegulator 6
4BoostSwitchingRegulator 9
5InvertingSwitchingRegulator 11
6SelectingtheRightInductor 13
ListofFigures
1FunctionalBlockDiagram 2
2ReferenceVoltageCircuit 2
3OscillatorVoltageThresholds 3
4TimingCapacitorChargeCurrentvsCurrent-LimitSenseVoltage 3
5TypicalOperationWaveforms 4
6BuckRegulator 6
7BuckSwitchingRegulatorWaveforms 8
8BoostSwitchingRegulator 9
9BoostSwitchingRegulatorWaveforms 11
10SwitchingInverterRegulator 11
11InverterSwitchingRegulatorWaveforms 12

ListofTables
1LogicTruthTableofFunctionalBlocks 5
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1MC34063Description
-
+
QS
1.25-V
Reference
Regulator
R
C
T
I
pk
Oscillator
Q2
Q1
Switch
Collector
4
Switch
Emitter
Timing
Capacitor
GND
3
2

18
7
6
5
Comparator
Inverting Input
V
CC
I
pk
Sense
Drive
Collector
100 W
Comparator
Latch
1.1ReferenceVoltage
Comparator
Inverting Input
Output
R2 R1
V = 1.25(R2/R1 + 1)
out
MC34063Description
TheMC34063isamonolithiccontrolcircuitcontainingalltheactivefunctionsrequiredforswitching
dc-to-dcconverters(seeFigure1).TheMC34063includesthefollowingcomponents:
•Temperature-compensatedreferencevoltage
•Oscillator
•Activepeak-currentlimit
•Outputswitch

•Outputvoltage-sensecomparator
TheMC34063wasdesignedtobeincorporatedinbuck,boost,orvoltage-inverterconverterapplications.
Allthesefunctionsarecontainedinan8-pinDIPorSOICpackage.
Figure1.FunctionalBlockDiagram
Thereferencevoltageissetat1.25Vandisusedtosettheoutputvoltageoftheconverter.
Figure2.ReferenceVoltageCircuit
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1.2Oscillator
Upper Threshold (1.25 V Typical)
Lower Threshold (0.75 V Typical)
Discharge
Time
V
Charge
6t
t
1.3CurrentLimit
V – Current-Limit Sense Voltage – V
CLS
I
– Charging Current – mA
chg
0 0.2 0.4 0.6 0.8
1
0.03
0.1
0.3
1

3
10
30
V = 5V
CC
T = 25°C
A
V = 40V
CC
I = I
chg dischg
MC34063Description
Theoscillatoriscomposedofacurrentsourceandacurrentsinkthatchargeanddischargetheexternal
timingcapacitor(C
T
)betweenanupperandlowerpresetthreshold.Thetypicalchargecurrentis35µA,
andthetypicaldischargecurrentis200µA,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodis
sixtimeslongerthanthatoftheramp-downperiod(seeFigure3).
Theupperthresholdis1.25V,whichissameastheinternalreferencevoltage,andthelowerthresholdis
0.75V.Theoscillatorrunsconstantly,atapacecontrolledbythevalueofC
T
.
Figure3.OscillatorVoltageThresholds
Currentlimitisaccomplishedbymonitoringthevoltagedropacrossanexternalsenseresistorlocatedin
serieswithV
CC
andtheoutputswitch.Thevoltagedropdevelopedacrossthesenseresistorismonitored
bythecurrent-sensepin,I
pk
.Whenthevoltagedropacrossthesenseresistorbecomesgreaterthanthe

presetvalueof330mV,thecurrent-limitcircuitryprovidesanadditionalcurrentpathtochargethetiming
capacitor(C
T
)rapidly,toreachtheupperoscillatorthresholdand,thus,limitingtheamountofenergy
storedintheinductor.Theminimumsenseresistoris0.2Ω.Figure4showsthetimingcapacitorcharge
currentversuscurrent-limitsensevoltage.Tosetthepeakcurrent,I
pk
=330mV/R
sense
.
Figure4.TimingCapacitorChargeCurrentvsCurrent-LimitSenseVoltage
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1.4OutputSwitch
Comparator Output
Timing Capacitor, C
T
Output Switch
Nominal Output Voltage
Output Voltage
1
0
On
Off
Startup Quiescent Operation
MC34063Description
TheoutputswitchisanNPNDarlingtontransistor.Thecollectoroftheoutputtransistoristiedtopin1,
andtheemitteristiedtopin2.ThisallowsthedesignertousetheMC34063inbuck,boost,orinverter
configurations.Themaximumcollector-emittersaturationvoltageat1.5A(peak)is1.3V,andthe

maximumpeakcurrentoftheoutputswitchis1.5A.Forhigherpeakoutputcurrent,anexternaltransistor
canbeused.Figure5showsthetypicaloperationwaveforms.
Figure5.TypicalOperationWaveforms
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2FunctionalDescription
FunctionalDescription
Theoscillatoriscomposedofacurrentsourceandsink,whichchargeanddischargetheexternaltiming
capacitor(C
T
)betweenanupperandlowerpresetthreshold.Thetypicalchargeanddischargecurrents
are35mAand200mA,respectively,yieldingapproximatelya6:1ratio.Thus,theramp-upperiodissix
timeslongerthanthatoftheramp-downperiod(seeFigure3).Theupperthresholdisequaltointernal
referencevoltageof1.25V,andthelowerthresholdisapproximatelyequalto0.75V.Theoscillatorruns
continuouslyataratecontrolledbythevalueofC
T
.
Duringtheramp-upportionofthecycle,alogic1ispresentattheAinputoftheANDgate.Iftheoutput
voltageoftheswitchingregulatorisbelownominal,alogic1isalsopresentattheBinput.Thiscondition
setsthelatchandcausestheQoutputtobealogic1,enablingthedriverandoutputswitchtoconduct.
Whentheoscillatorreachesitsupperthreshold,C
T
startstodischarge,andalogic0ispresentattheA
inputoftheANDgate.Thislogiclevelisalsoconnectedtoaninverterwhoseoutputpresentsalogic1to
theresetinputofthelatch.ThisconditioncausesQtogolow,disablingthedriverandoutputswitch.A
logictruthtableofthesefunctionalblocksisshowninTable1.
Table1.LogicTruthTableofFunctionalBlocks
ANDGateInputsLatchInputs
ActiveConditionofOutput

Comments
TimingCapacitor,C
T
Switch
ABSR
Regulatoroutputisgreaterthanor
Beginrampup000
equaltonominal(B=0).
Nochange,becauseBwas0before
Beginrampdown000
C
T
rampdown.
Nochangeeventhoughregulator
outputlessthannominal.Output
Rampingdown0010
switchcannotbeinitiatedduringR
T
rampdown.
Nochange,becauseoutputswitch
Rampingdown0010
conditionwasterminatedwhenA=0.
Regulatoroutputbecamelessthan
nominalduringC
T
rampup(whenB
Rampingup10
changedto1).Partialoncyclefor
outputswitch.
Regulatoroutputbecamegreaterthan

orequaltonominal(Bchangedto0)
Rampingup101
duringrampupofC
T
.Nochange,
becauseBcannotresetthelatch.
Completeoncycle,becauseB=1
Beginrampup1
beforeC
T
rampupstarted.
Outputswitchconductionisalways
Beginrampdown1
terminatedwhenC
T
isrampingdown.
TheoutputofthecomparatorcansetthelatchonlyduringtherampupofC
T
andcaninitiateapartialor
fulloncycleofoutputswitchconduction.Oncethecomparatorhassetthelatch,itcannotresetit.The
latchremainssetuntilC
T
beginsrampingdown.Thus,thecomparatorcaninitiateoutputswitch
conductionbutcannotterminateit,andthelatchisalwaysresetwhenC
T
beginsrampingdown.The
comparator’soutputisatalogic0whentheoutputvoltageoftheswitchingregulatorisabovenominal.
Undertheseconditions,thecomparator’soutputcaninhibitaportionoftheoutputswitchoncycle,a
completecycle,acompletecycleplusaportionofonecycle,multiplecycle,ormultiplecyclesplusa
portionofonecycle.

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3BuckRegulator
R
L
C
out
D1
Q1
C
in
V
in
GND
V
out
L
+
+
3.1BuckConverterOperation
3.2Time-OnandTime-OffCalculation
3.3SwitchPeakCurrentCalculation
BuckRegulator
Figure6showsthebasicbuckswitchingregulator.Q1interruptstheinputvoltageandprovidesavariable
duty-cyclesquarewavetoanLCfilter.Thefilteraveragesthesquarewaveandproducesadcoutput
voltagethatcanbesettoanylevellessthantheinputbycontrollingthepercentconductiontimeofQ1to
thatofthetotalswitchingcycletime.
V
out

=V
in
(%t
on
)
or
V
out
=V
in
(t
on
/(t
on
+t
off
))
Figure6.BuckRegulator
Asanexample,supposethatthetransistorQ1isoff,theinductorcurrent(I
L
)iszero,andtheoutput
voltageisatitsnominalvalue.TheoutputvoltageacrosscapacitorC
out
willultimatelydecaybelowthe
nominaloutputlevel,becauseitistheonlysourceofsupplycurrenttoloadR
L
.Thisvoltagedeficiencyis
sensedbytheswitchingcontrolcircuitandcausesQ1toturnon.Theinductorcurrentstartstoflowfrom
V
in

throughQ1andC
out
inparallelwithR
L
,anditrisesatarateofΔI/Δt=V/L.Thevoltageacrossthe
inductorisequaltoV
in
–V
sat
–V
out
,andtheinductorpeakcurrentatanyinstantiscalculatedasshown
here:
I
L
=((V
in
–V
sat
–V
out
)/L)t
Attheendoftheonperiod,Q1isturnedoff.Asthemagneticfieldintheinductorstartstocollapse,it
generatesareversevoltagethatforwardbiasesD1,andthepeakcurrentdecaysatarateofΔI/Δt=V/L
asenergyissuppliedtoC
out
andR
L
.Thevoltageacrosstheinductorduringthisperiodisequalto
V

out
+V
F
ofD1.Thecurrentasafunctionoftimeiscalculatedasshownhere:
I
L
=I
L(pk)
–((V
out
+V
F
)/L)t
WhereV
F
istheforwardvoltageofD1.
Asanexample,supposethatduringquiescentoperation,theaverageoutputvoltageisconstant,andthe
systemisoperatinginthediscontinuousmode.ThenI
L(pk)
attainedduringt
on
mustdecaytozeroduring
t
off
,andaratiooft
on
tot
off
canbedetermined.
((V

in
–V
sat
–V
out
)/L)t
on
=((V
out
+V
F
)/L)t
off
∴t
on
/t
off
=(V
out
+V
F
)/(V
in
–V
sat
–V
out
)
Thevolt-timeproductoft
on

mustbeequaltothatoft
off
,andtheinductancevalueisnotafactorwhen
determiningtheirratio.Iftheoutputvoltageinsideaswitchingperiodistoremainconstant,theaverage
currentintotheinductormustbeequaltotheoutputcurrentforacompletecycle.Thepeakinductor
currentwithrespecttooutputcurrentis:
(I
L(pk)
/2)t
on
+(I
L(pk)
/2)t
off
=I
out
t
on
+I
out
t
off
∴I
L(pk)
=2I
out
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3.4TimingCapacitorCalculation

3.5InductanceCalculation
3.6OutputVoltageRipple
BuckRegulator
Thepeakinductorcurrentisalsoequaltothepeakswitchcurrent,sincethetwoareinseries.Theontime
(t
on
)isthemaximumpossibleswitchconductiontime.ItisequaltothetimerequiredforC
T
torampup
fromitslowertoupperthreshold.TherequiredvalueforC
T
canbedeterminedbyusingtheminimum
oscillatorchargingcurrentandthetypicalvalueforthepeak-to-peakoscillatorvoltageswing,bothtaken
fromthedatasheet.
C
T
=I
chg(min)
(Δt/ΔV)
C
T
=20×10
-6
(t
on
/0.5)
C
T
=4.0×10
-5

(t
on
)
TheofftimeisthetimethatdiodeD1isinconductionanditisdeterminedbythetimerequiredforthe
inductorcurrenttoreturntozero.Theofftimeisnotrelatedtotheramp-downtimeofCT.Thecycletime
oftheLCnetworkisequaltot
on(max)
+t
off
,andtheminimumoperationfrequencyiscalculatedasshown
here:
f
min
=1/(t
on(max)
+t
off
)
Theminimumvalueofinductance(L)cannowbecalculated.TheV-knownquantitiesarethevoltage
acrosstheinductorandtherequiredpeakcurrentfortheselectedswitchconductiontime:
L
min
=((V
in
–V
sat
–V
out
)/I
pk(switch)

)t
on
Theminimumvalueofinductanceiscalculatedassumingtheonsetofcontinuousconductionoperation
withafixedinputvoltage,maximumoutputcurrent,andaminimumcharge-currentoscillator.
Thenetchargepercycledeliveredtooutputfiltercapacitor(C
out
)mustbezero(Q+=Q–)iftheoutput
voltageistoremainconstant.
Theripplevoltagecanbecalculatedfromtheknownvaluesofontime,offtime,peakinductorcurrent,and
outputcapacitorvalue:
Duringt
on
ic(t)=I
pk
/t
on
×t,positiveslope
V(t)=1/C
out
∫I
pk
/t
on
×tdt
=I
pk
/(C
out
×t
on

)×t
2
/2+constant
Theaxisoftheparabolapasswaschosenbyitsminimum,soconstant=0.
=I
pk
/(C
out
×t
on
)×t
2
/2
V(t
on
/2)=I
pk
/(C
out
×t
on
)×(t
on
/2)
2
/2
=I
pk
/C
out

×t
on
/8
Duringt
off
ic(t)=–I
pk
/t
off
×t,negativeslope
V(t)=–1/C
out
∫I
pk
/t
off
×tdt
=–I
pk
/(C
out
×t
off
)×t
2
/2+constant
Theaxisoftheparabolapasswaschosenbyitsminimum,soconstant=0.
=–I
pk
/(C

out
×t
off
)×t
2
/2
V(t
off
/2)=–I
pk
/(C
out
×t
off
)×(t
off
/2)
2
/2
=–I
pk
/C
out
×t
off
/8
V
ripple(C)
=|V(t
on

/2)|+|V(t
off
/2)|
=(I
pk
/C
out
)×(t
on
/8)+(I
pk
/C
out
)×(t
off
/8)
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Voltage Across
Switch Q1
V
CE
Diode D1
Voltage
V
KA
Switch Q1
Current
Diode D1

Current
Inductor
Current
Capacitor C
Current
out
Capacitor C
Ripple Voltage
out
V + V
Fin
V
in
V
sat
0
V – V
satin
0
V
F
I
pk
0
0
0
0
I
pk
I

D(AVG)
I
pk
V + V
pkout
V
out
I
out pk C(AVG) D(AVG)
= I /2 = I + I
+I /2
pk
I = I
in C(AVG)
V
in
–I /2
pk
V – V
pkout
t
off/2
t
on/2
V
ripple(p-p)
½I
p/2
Q+
Q–

t
0
t
1
t
2
BuckRegulator
V
ripple(C)
=(I
pk
/C
out
)×(t
on
+t
off
)/8
V
ripple(ESR)
=I
pk
×ESR
V
ripple(p-p)
=I
pk
/C
out
×(t

on
+t
off
)+I
pk
×ESR
V
ripple(p-p)
=I
pk
×[(1/8C)×(t
on
+t
off
)+ESR]
Figure7showsagraphicalderivationofthepeak-to-peakripplevoltagethatwasobtainedfromthe
capacitorcurrentandvoltagewaveforms.
Thecalculationsshownaboveaccountfortheripplevoltagecontributedbytheripplecurrentintoanideal
capacitor.
Inpractice,thecalculatedvalueshouldbeincreasedduetotheinternalequivalentseriesresistance
(ESR)ofthecapacitor.TheadditionalripplevoltageisequaltoI
pk(ESR)
.Increasingthevalueofthefilter
capacitorreducestheoutputripplevoltage.However,apointofdiminishingreturnisreached,because
thecomparatorrequiresafinitevoltagedifferenceacrossitsinputstocontrolthelatch.Thevoltage
differencerequiredtocompletelychangethelatchstatesisabout1.5mV,andtheminimumachievable
rippleattheoutputisthefeedbackdividerratiomultipliedby1.5mV:
V
ripple(p-p)
(min)=(V

out
/V
ref
)(1.5×10
-3
)
Figure7.BuckSwitchingRegulatorWaveforms
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4BoostSwitchingRegulator
R
L
C
out
D1
Q1
C
in
V
in
GND
V
out
L
+
+
4.1OperationofMC34063asBoostConverter
4.2Time-OnandTime-OffCalculation
4.3PeakCurrentCalculation

BoostSwitchingRegulator
Figure8showsabasicswitchingregulator.Energyisstoredintheinductorduringthetimethattransistor
Q1isintheONstate.WhentransistorQ1isturnedoff,theenergyistransferredinserieswithV
in
tothe
outputfiltercapacitor(C
out
)andload(R
L
).Thisconfigurationallowstheoutputvoltagetobesettoany
valuegreaterthanthatofinput.Thefollowingequationscanbeusedtocalculatetheoutputvoltage:
V
out
=V
in
(t
on
/t
off
)+V
in
or
V
out
=V
in
((t
on
/t
off

)+1)
Figure8.BoostSwitchingRegulator
Asanexample,supposethattransistorQ1isoff,theinductorcurrentiszero,andoutputvoltageisatits
nominalvalue.Atthistime,loadcurrentisbeingsuppliedonlybyC
out
,anditwilleventuallyfallbelow
nominalvalue.Whentheoutputvoltagefallsbelowthenominalvalue,itissensedbythecontrolcircuit,
whichinitiatesanoncycle,drivingtransistorQ1intosaturation.Currentstartstoflowfrominputthrough
theinductorandQ1,anditrisesatarateofΔI/Δt=V/L.Thevoltageacrosstheinductorisequalto
V
in
–V
sat
,andthepeakcurrentisroughlyalinearfunctionoft,asshownhere:
I
L
=((V
in
–V
sat
)/L)t
Whentheon-timeiscompleted,Q1turnsoff,andthemagneticfieldintheinductorstartstocollapse,
generatingareversevoltagethatforwardbiasesD1,supplyingenergytoC
out
andR
L
.Theinductorcurrent
decaysatrateofΔI/Δt=V/L,andthevoltageacrossitisequaltoV
out
+V

F
–V
in
.Thecurrentatany
instantiscalculatedasshownhere:
I
L
=I
L(pk)
–((V
out
+V
F
–V
in
)/L)t
Assumingthatthesystemisoperatinginthediscontinuousmode,thecurrentthroughtheinductor
reacheszeroafterthet
off
periodiscompleted.ThentheI
L(pk)
attainedduringt
on
mustdecaytozeroduring
t
off
,andaratiooft
on
tot
off

canbewrittenasshownhere:
((V
in
–V
sat
)/L)t
on
=((V
out
+V
F
–V
in
)/L)t
off
∴t
on
/t
off
=(V
out
+V
F
–V
in
)/(V
in
–V
sat
)

Thevolt-timeproductoft
on
mustbeequaltothatoft
off
,andtheinductancevaluedoesnotaffectthis
relationship.
TheinductorcurrentchargestheoutputfiltercapacitorthroughD1duringt
off
.Iftheoutputvoltageisto
remainconstant,thenetchargepercycledeliveredtooutputfiltercapacitormustbezero(Q+=Q–).
I
chg
t
off
=I
dischg
t
on
Figure9showstheboostswitchingregulatorwaveforms.Byobservingthecapacitorcurrentandmaking
somesubstitutioninthepreviousequation,aformulaforpeakinductorcurrentcanbeobtained.
(I
L(pk)
/2)t
off
=I
out
(t
on
+t
off

)
∴I
L(pk)
=2I
out
(t
on
/t
off
+1)
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4.4InductanceCalculation
4.5OutputVoltageRipple
BoostSwitchingRegulator
Thepeakinductorcurrentisalsoequaltothepeakswitchcurrent,sincethetwoareinseries.Byknowing
thevoltageacrosstheinductorduringt
on
andtherequiredpeakcurrentfortheselectedswitchconduction
time,aminimuminductancevaluecanbedetermined:
L
min
=((V
in
–V
sat
)/I
pk(switch)
)t

on(max)
Calculatetheoutputripplevoltagefromtheknownvaluesoft
on
,t
off
,peakinductorcurrent,outputcurrent,
andoutputcapacitorvalue.ThecapacitorcurrentwaveformsisdepictedinFigure9,t1beingthe
discharginginterval.Solvingfort1inknowntermsyields:
Duringt
off
,thecurrentislinearwithnegativeslope,–ΔI
L
/t
off
ic(t)=–(I
pk
/t
off
)×t
V(t)=–1/C
out
∫(I
pk
/t
off
)×tdt
=–I
pk
/(C
out

×t
off
)xt
2
/2+constant
Theaxisoftheparabolpasswaschosenbythemaximumsoconstant=0.
=–I
pk
/(C
out
×t
off
)×t
2
/2
V(-τ)=–I
pk
/(C
out
×t
off
)×τ
2
/2,τistimefromic(t)=maxtoic(t)=0
(t
off
–τ)/
off
=I
out

/I
pk
,trianglegeometry
τ=t
off
×(I
pk
–I0)/I
pk
(1)
V(-τ)=–I
pk
/2(C
out
×t
off
)×(t
off
)
2
×(I
pk
–I0)
2
/ΔI
L
2
V(-τ)=–t
off
×(I

pk
–I0)
2
/(2C
out
×I
pk
)(2)
Energyconservationintheoutputcapacitor:Q+=Q–
(I
pk
–I0)×τ/2=(t
off
–τ)×I0/2+I0×t
on
(3)
Equation1andEquation2give:
t
off
×(I
pk
–I0)
2
/2∆IL=I0/2×t
off
×(1–(ΔI
L
–I0)/ΔI
L
)+I0×t

on
=t
off
×I0
2
/2ΔI
L
+I0xt
on
t
off
×((I
pk
–I0)
2
–I0
2
)/2I
pk
=I0×t
on
(I
pk
–2I0)×t
off
/2=I0×t
on
Theinductorripplecurrent:
I
pk

=2I
out
×(1+t
on
/t
off
)(4)
Fromoutputcapacitorrippleperiodicityandcontinuity:
V(–τ)=V
ripple(pp)
BysubstitutingEquation4inEquation3:
V
ripple
(C
out
)=I
out
(t
off
+2t
on
)
2
/2C(t
off
+t
on
)
Ift
on

=6.5t
off
,then:
V
ripple
(ESR)=2I
out
×(1+t
on
/t
off
)×ESR
10ApplicationoftheMC34063SwitchingRegulatorSLVA252B–September2006–RevisedNovember2007
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Voltage Across
Switch Q1
V
CE
Diode D1
Voltage
V
KA
Switch Q1
Current
Diode D1
Current
Inductor
Current
Capacitor C

Current
out
Capacitor C
Ripple Voltage
out
V + V
Fout
V
in
V
sat
0
V – V
satout
0
V
F
I
pk
0
0
0
0
I
pk
I
out
I
pk
V + V

pkout
V
out
–I
out
I – I
pk out
t
off
t
on
t
1
V
ripple(p-p)
Q–
I = I
in L(AVG)
½(I – I )
pk out
Q+
5InvertingSwitchingRegulator
R
L
C
out
D1
Q1
C
in

V
in
GND
V
out
L
+
+
InvertingSwitchingRegulator
Figure9.BoostSwitchingRegulatorWaveforms
Abasicvoltage-invertingswitchingregulatorisshowninFigure10.Theenergyisstoredintheinductor
duringtheconductiontimeofQ1.UpontheQ1turnoff,theenergyistransferredtotheoutputfilter
capacitorandload.Inthisconfiguration,theoutputvoltageisderivedonlyfromtheinductor.Thisallows
themagnitudeoftheoutputtobesettoanyvalue.Itmaybelessthan,equalto,orgreaterthanthatofthe
inputandissetbythefollowing:
V
out
=V
in
(t
on
/t
off
)
Figure10.SwitchingInverterRegulator
Theinverterconverteroperatesidenticallytothatoftheboostconverter.Thevoltageacrosstheinductor
duringt
on
isV
in

–V
sat
but,duringt
off
,thevoltageisequaltothenegativemagnitudeofV
out
+V
F
.TheVLT
time-productoft
on
mustbeequaltothatoft
off
,aratiooft
on
tot
off
canbedetermined:
(V
in
–V
sat
)t
on
=(|V
out
|+V
F
)t
off

∴t
on
/t
off
=(|V
out
|+V
F
)/(V
in
–V
sat
)
ThederivationsandtheformulasforI
pk(switch)
,L
(min)
,andC
out
arethesameasthatoftheboostconverter.
Figure11showsthevoltage-inverterswitchingregulatorwaveforms.
SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator11
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Voltage Across
Switch Q1
V
CE
Diode D1
Voltage

V
KA
Switch Q1
Current
Diode D1
Current
Inductor
Current
Capacitor C
Current
out
Capacitor C
Ripple Voltage
out
V – (–V + V )
in Fout
V
in
0
V – V
satin
0
V
F
I
pk
0
0
0
0

I
pk
I
out
I
pk
–V + V
pkout
V
out
–I
out
I – I
pk out
t
off
t
on
t
1
V
ripple(p-p)
Q–
I = I
in C(AVG)
½(I – I )
pk out
Q+
V – V
satin

–V – V
pkout
InvertingSwitchingRegulator
Figure11.InverterSwitchingRegulatorWaveforms
ApplicationoftheMC34063SwitchingRegulator 12SLVA252B–September2006–RevisedNovember2007
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6SelectingtheRightInductor
SelectingtheRightInductor
Properinductorselectioniscrucialtotheperformanceoftheswitchingregulator'sdesign.Theswitching
regulatorhastwomodeofoperation:
•Continuousmode
•Discontinuousmode
Eachmodehascharacteristicallydifferentoperatingcharactersand,therefore,canaffecttheregulator
performanceandrequirements.Inmanyapplications,thecontinuousmodeisthepreferredmodeof
operation,sinceitoffersgreateroutputpowerwithlowerpeakcurrents,widerinputrange,andlower
outputripple.Theseadvantagesofcontinuous-modeoperationcomeattheexpenseofalargerinductor.
Oncetheminimuminductorandpeakcurrentvaluearedetermined,theinductorcanbeselected.Most
manufacturersprovidethefollowingdataintheirdatabook:
•Inductancevalue
•DCR(dcresistance)ofthewinding
•DCsaturationcurrent
•RMScurrent
•Packagetype,size,andpattern
Thegeometryandtheshapeoftheinductorchosencanhaveadvantagesanddisadvantages.Ifhigh
performanceisaconcern,thenthetoroidinductorsarethebestchoices,asthemagneticfluxiscontained
completelywithinthemagneticcore,resultinginlessEMIandnoise.TheEMIandnoisecanaffectnearby
sensitivecircuits.Inthesesituations,closedmagneticstructures,suchastoroid,potcore,orE-core,are
moreappropriate.
Incost-sensitiveapplications,theinexpensivebobbincoreinductorscanbeused.However,thebobbin

coreinductorscangeneratemoreEMI,astheopencoredoesnotconfinethefluxwithinthecoreandcan
affectnearbysensitivecircuits.
SLVA252B–September2006–RevisedNovember2007ApplicationoftheMC34063SwitchingRegulator13
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