Datasheet

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DV
IN
+
I
OUT(MAX)
0.25
C
BULK
ƒ
sw
)
ǒ
I
OUT(MAX)
ESR
MAX
Ǔ
(9)
I
CIN
+
I
OUT(MAX)
2
(10)
SWITCHINGFREQUENCY
OUTPUTFILTERCOMPONENTS
RT(kW) +
46000
ƒ
s
(kHz) * 35.9
(8)
INPUTCAPACITORS
InductorSelection
L
MIN
+
V
OUT(MAX)
ǒ
V
IN(MAX)
* V
OUT
Ǔ
V
IN(max)
K
IND
I
OUT
F
SW
(11)
TPS54550
SLVS623A–MARCH2006–REVISEDAPRIL2006
•Inputvoltagerange
•Outputvoltage
•Inputripplevoltage
WhereI
OUT(MAX)
isthemaximumloadcurrent,f
SW
is
•Outputripplevoltage
theswitchingfrequency,C
BULK
isthebulkcapacitor
•Outputcurrentrating
valueandESR
MAX
isthemaximumseriesresistance
ofthebulkcapacitor.
•Operatingfrequency
ThemaximumRMSripplecurrentalsoneedstobe
Forthisdesignexample,usethefollowingasthe
checked.Forworstcaseconditions,thiscanbe
inputparameters:
approximatedbyEquation10:
DESIGNPARAMETEREXAMPLEVALUE
Inputvoltagerange6Vto17V
Outputvoltage3.3V
Inputripplevoltage300mV
Inthiscase,theinputripplevoltagewouldbe140
Outputripplevoltage30mVmVandtheRMSripplecurrentwouldbe2.5A.Itis
alsoimportanttonotethattheactualinputvoltage
Outputcurrentrating5A
ripplewillbegreatlyaffectedbyparasiticsassociated
Operatingfrequency700kHz
withthelayoutandtheoutputimpedanceofthe
NOTE:Asanadditionalconstraint,thedesignissetuptobe
voltagesource.Theactualinputvoltageripplefor
smallsizeandlowcomponentheight.
thiscircuitisshowninFigure34andislargerthan
thecalculatedvalue.Thismeasuredvalueisstill
belowthespecifiedinputlimitof300mV.The
maximumvoltageacrosstheinputcapacitorswould
TheswitchingfrequencyissetusingtheRTpin.
beVINmaxplusΔVIN/2.Thechosenbulkand
GroundingtheRTpinsetsthePWMswitching
bypasscapacitorsareeachratedfor25Vandthe
frequencytoadefaultfrequencyof250kHz.Floating
combinedripplecurrentcapacityisgreaterthan3A,
theRTpinsetsthePWMswitchingfrequencyto500
bothprovidingamplemargin.Itisveryimportantthat
kHz.ByconnectingaresistorfromRTtoAGND,any
themaximumratingsforvoltageandcurrentarenot
frequencyintherangeof250to700kHzcanbeset.
exceededunderanycircumstance.
UseEquation8todeterminethepropervalueofRT.
Twocomponentsneedtobeselectedfortheoutput
Inthisexamplecircuit,thedesiredswitching
filter,L1andC2.SincetheTPS54550isan
frequencyis700kHzandRTis69.8kΩ.
externallycompensateddevice,awiderangeoffilter
componenttypesandvaluescanbesupported.
TheTPS54550requiresaninputdecoupling
capacitorand,dependingontheapplication,abulk
Tocalculatetheminimumvalueoftheoutput
inputcapacitor.Theminimumrecommendedvalue
inductor,useEquation11:
forthedecouplingcapacitor,C9,is10μF.A
high-qualityceramictypeX5RorX7Ris
recommended.Thevoltageratingshouldbegreater
thanthemaximuminputvoltage.Asmallervalue
maybeusedaslongasallotherrequirementsare
K
IND
isacoefficientthatrepresentstheamountof
met;however10μFhasbeenshowntoworkwellin
inductorripplecurrentrelativetothemaximum
awidevarietyofcircuits.Additionally,somebulk
outputcurrent.Ingeneral,thisvalueisatthe
capacitancemaybeneeded,especiallyifthe
discretionofthedesigner;however,thefollowing
TPS54550circuitisnotlocatedwithinabout2inches
guidelinesmaybeused.FordesignsusinglowESR
fromtheinputvoltagesource.Thevalueforthis
outputcapacitorssuchasceramics,avalueashigh
capacitorisnotcriticalbutshouldberatedtohandle
asK
IND
=0.3maybeused.WhenusinghigherESR
themaximuminputvoltageincludingripplevoltage,
outputcapacitors,K
IND
=0.2yieldsbetterresults.
andshouldfiltertheoutputsothatinputripple
voltageisacceptable.
Forthisdesignexample,useK
IND
=0.3andthe
minimuminductorvalueiscalculatedtobe3μH.For
Thisinputripplevoltagecanbeapproximatedby
thisdesign,alargevaluewaschosen:6.8μH.
Equation9:
19
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