Datasheet

www.ti.com
I
L(RMS)
+ I
2
OUT(MAX)
)
1
12
ǒ
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
V
IN(MAX)
L
OUT
F
SW
0.8
Ǔ
2
Ǹ
(12)
I
COUT(RMS)
+
1
12
Ǹ
ȧ
ȡ
Ȣ
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
V
IN(MAX)
L
OUT
F
SW
N
C
ȧ
ȣ
Ȥ
(15)
I
L(PK)
+ I
OUT(MAX)
)
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
1.6 V
IN(MAX)
L
OUT
F
SW
(13)
ESR
MAX
+ N
C
ǒ
V
IN(MAX)
L
OUT
F
SW
0.8
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
Ǔ
DV
p*p(MAX)
(16)
CapacitorSelection
COMPENSATIONCOMPONENTS
C
OUT
+
1
L
OUT
(
K
2pƒ
CO
)
2
(14)
TPS54550
SLVS623A–MARCH2006–REVISEDAPRIL2006
Fortheoutputfilterinductor,itisimportantthatthe39μF.Inthisdesignamoreconsevativefrequency
RMScurrentandsaturationcurrentratingsnotbemultiplierof3isused,resultinginadesiredoutput
exceeded.TheRMSinductorcurrentcanbefoundcapacitanceof200μF.Theselectedoutputcapacitor
fromEquation12:mustberatedforavoltagegreaterthanthedesired
outputvoltageplus1/2theripplevoltage.Any
deratingamountmustalsobeincluded.The
maximumRMSripplecurrentintheoutputcapacitor
isgivenbyEquation15:
andthepeakinductorcurrentcanbedetermined
withEquation13:
WhereN
C
isthenumberofoutputcapacitorsin
parallel.
Forthisdesign,theRMSinductorcurrentis5.04A
andthepeakinductorcurrentis5.35A.Thechosen
ThemaximumESRoftheoutputcapacitoris
inductorisaSumidaCDRH105-6R86.8μH.Ithasa
determinedbytheamountofallowableoutputripple
saturationcurrentratingof5.4AandanRMScurrent
asspecifiedintheinitialdesignparameters.The
ratingof5.4A,meetingtheserequirements.A
outputripplevoltageistheinductorripplecurrent
smallervalueinductorcouldbeused;however,this
timestheESRoftheoutputfilter,sothemaximum
valuewaschosenbecauseithasthelargestvaluein
specifiedESRaslistedinthecapacitordatasheetis
thisstylethatmetthecurrentratingrequirements.
givenbyEquation16:
Largervalueinductorswillhaveloweraccurrentand
resultinloweroutputvoltageripple.Ingeneral,
inductorvaluesforusewiththeTPS54550areinthe
rangeof6.8μHto47μH.
WhereΔV
p-p
isthedesiredpeak-to-peakoutput
ripple.Forthisdesignexample,two100-μFceramic
outputcapacitorsarechosenforC2andC10.These
Theimportantdesignfactorsfortheoutputcapacitor
areTDKC3225X5R0J107M,ratedat6.3Vwitha
aredcvoltagerating,ripplecurrentrating,and
maximumESRof2mΩandaripplecurrentratingin
equivalentseriesresistance(ESR).Thedcvoltage
excessof3A.ThecalculatedtotalRMSripple
andripplecurrentratingscannotbeexceeded.The
currentis161mA(80.6mAeach)andthemaximum
ESRisimportantbecausealongwiththeinductor
totalESRrequiredis43mΩ.Theseoutput
currentitdeterminestheamountofoutputripple
capacitorsexceedtherequirementsbyawide
voltage.Theactualvalueoftheoutputcapacitoris
marginandwillresultinareliable,high-performance
notcritical,butsomepracticallimitsdoexist.
design.itisimportanttonotethattheactual
Considertherelationshipbetweenthedesiredclosed
capacitanceincircuitmaybelessthanthecatalog
loopcrossoverfrequencyofthedesignandLC
valuewhentheoutputisoperatingatthedesired
cornerfrequencyoftheoutputfilter.Ingeneral,itis
outputof3.3V.
desirabletokeeptheclosedloopcrossover
frequencyatlessthan1/5oftheswitching
OthercapacitortypesworkwellwiththeTPS54550,
frequency.Withhighswitchingfrequenciessuchas
dependingontheneedsoftheapplication.
the700-kHzfrequencyofthisdesign,internalcircuit
limitationsoftheTPS54550limitthepractical
maximumcrossoverfrequencytoabout50kHz.
TheexternalcompensationusedwiththeTPS54550
Additionally,toallowforadequatephasegaininthe
allowsforawiderangeofoutputfilterconfigurations.
compensationnetwork,thetheclosedloopcrossover
Alargerangeofcapacitorvaluesandtypesof
frequencyshouldbeatleast30%higherthantheLC
dielectricaresupported.Thedesignexampleuses
cornerfrequency.Thislimitstheminimumcapacitor
Type3compensationconsistingofR1,R3,R5,C6,
valuefortheoutputfilterto:
C7,andC8.Additionally,R2alongwithR1formsa
voltagedividernetworkthatsetstheoutputvoltage.
Thesecomponentreferencedesignatorsarethe
sameasthoseusedintheSWIFTDesigner
WhereKisthefrequencymultiplierforthespread
Software.Thereareanumberofdifferentwaysto
betweenf
LC
andf
CO
.Kshouldbebetween1.3and
designacompensationnetwork.Thisprocedure
15,typically10foronedecadedifference.Fora
outlinesarelativelysimpleprocedurethatproduces
desiredcrossoverof13kHzanda6.8-μ Hinductor,
goodresultswithmostoutputfiltercombinations.
theminimumvaluefortheoutputcapacitorisaround
20
SubmitDocumentationFeedback