參數(shù)資料
型號： PC8260
廠商： Atmel Corp.
英文描述： PowerPC-based Communications Processors
中文描述： 基于PowerPC的通信處理器
文件頁數(shù)： 35/53頁
文件大?。?/td> 364K
代理商： PC8260
35
PC8260PowerQUICCII
2131B–HIREL–02/03
PowerConsiderations
Theaveragechip-junctiontemperature,T
J
,in°Ccanbeobtainedfromthefollowing:
(1)
T
J
T
A
P
D
+
=
where
T
A
=ambienttemperature°C
Q
JA
=packagethermalresistance,junctiontoambient,°C/W
P
D
=P
INT
+P
I/O
P
INT
=I
DD
xV
DD
Watts–chipinternalpower
P
I/O
=powerdissipationoninputandoutputpins–userdetermined
P
I O
P
INT
Canbeneglectedformostapplications.IfP
I/O
isneglected,anapproximaterelationship
betweenP
D
andT
J
isthefollowing:
P
D
=K/(T
J
+273°C)
(2)
Solvingequations(1)and(2)forKgives:
(3)
K
P
D
T
A
+
P
D
+
=
WhereKisaconstantpertainingtotheparticularpart.Kcanbedeterminedfromequa-
tion(3)bymeasuringP
D
(atequilibrium)foraknownT
A
.UsingthisvalueofK,the
valuesofP
D
andT
J
canbeobtainedbysolvingequations(1)and(2)iterativelyforany
valueofT
A
.
LayoutPractices
EachV
CC
pinonthePC8260shouldbeprovidedwithalow-impedancepathtothe
board’spowersupply.Eachgroundpinshouldlikewisebeprovidedwithalow-imped-
ancepathtoground.Thepowersupplypinsdrivedistinctgroupsoflogiconchip.The
V
CC
powersupplyshouldbebypassedtogroundusingatleastfour0.1μFby-pass
capacitorslocatedascloseaspossibletothefoursidesofthepackage.Thecapacitor
leadsandassociatedprintedcircuittracesconnectingtochipV
CC
andgroundshouldbe
kepttolessthanhalfaninchpercapacitorlead.Afour-layerboardisrecommended,
employingtwoinnerlayersasV
CC
andGNDplanes.
AlloutputpinsonthePC8260havefastriseandfalltimes.Printedcircuit(PC)trace
interconnectionlengthshouldbeminimizedinordertominimizeundershootandreflec-
tionscausedbythesefastoutputswitchingtimes.Thisrecommendationparticularly
appliestotheaddressanddatabusses.MaximumPCtracelengthsofsixinchesare
recommended.Capacitancecalculationsshouldconsideralldeviceloadsaswellas
parasiticcapacitancesduetothePCtraces.AttentiontoproperPCBlayoutandbypass-
ingbecomesespeciallycriticalinsystemswithhighercapacitiveloadsbecausethese
loadscreatehighertransientcurrentsintheV
CC
andGNDcircuits.Pullupallunused
inputsorsignalsthatwillbeinputsduringreset.Specialcareshouldbetakentomini-
mizethenoiselevelsonthePLLsupplypins.
Θ
JA
(
)
0,3
<
273
°
C
(
)
Θ
2
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