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MPC8272 PowerQUICC II Family Hardware Specifications
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11
Thermal Characteristics
4
Thermal Characteristics
Table 6 describes thermal characteristics. Refer to Table 2 for information on a given device’s package.
Discussions of each characteristic are provided in sections 4.1 through 4.7. For the these discussions, P
D
=
(V
DD
×
I
DD
) + PI/O, where PI/O is the power dissipation of the I/O drivers.
Table 6. Thermal Characteristics
4.1
Estimation with Junction-to-Ambient Thermal
Resistance
An estimation of the chip junction temperature, T
J
, in °C can be obtained from the equation:
T
J
= T
A
+ (R
θ
JA
×
P
D
)
where:
T
A
= ambient temperature (oC)
R
θ
JA
= package junction-to-ambient thermal resistance (oC/W)
P
D
= power dissipation in package
The junction-to-ambient thermal resistance is an industry standard value which provides a quick and easy
estimation of thermal performance. However, the answer is only an estimate; test cases have demonstrated
that errors of a factor of two (in the quantity T
J
–
T
A
) are possible.
Characteristic
Symbol
Value
Unit
Air Flow
Junction-to-ambient—
single-layer board
1
1
Assumes no thermal vias
2
Thermal resistance between the die and the printed circuit board per JEDEC JESD51-8. Board temperature is
measured on the top surface of the board near the package.
3
Thermal resistance between the die and the case top surface as measured by the cold plate method (MIL SPEC-883
Method 1012.1).
4
Thermal characterization parameter indicating the temperature difference between package top and the junction
temperature per JEDEC JESD51-2. When Greek letters are not available, the thermal characterization parameter is
written as Psi-JT.
R
θ
JA
27
°
C/W
Natural convection
21
1 m/s
Junction-to-ambient—
four-layer board
R
θ
JA
19
°
C/W
Natural convection
16
1 m/s
Junction-to-board
2
R
θ
JB
11
°
C/W
—
Junction-to-case
3
R
θ
JC
8
°
C/W
—
Junction-to-package top
4
R
θ
JT
2
°
C/W
—
F
Freescale Semiconductor, Inc.
n
.