Electrical Characteristics
MPC561/MPC563 Reference Manual, Rev. 1.2
F-4
Freescale Semiconductor
An estimation of the chip junction temperature, T
J
, in °C
can be obtained from the equation:
T
J
= T
A
+ (R
θ
JA
x P
D
)
where:
T
A
= ambient temperature (°C)
R
θ
JA
= package junction to ambient resistance (°C/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. Unfortunately, the answer is only an estimate; test cases have
demonstrated that errors of a factor of two are possible. As a result, more detailed thermal characterization
is supplied.
Historically, the thermal resistance has frequently been expressed as the sum of a junction to case thermal
resistance and a case to ambient thermal resistance:
R
θ
JA
= R
θ
JC
+ R
θ
CA
where:
R
θ
JA
= junction to ambient thermal resistance (°C/W)
R
θ
JC
= junction to case thermal resistance (°C/W)
R
θ
JA
= case to ambient thermal resistance (°C/W)
R
θ
JC
is device related and cannot be influenced. The user controls the thermal environment to change the
case to ambient thermal resistance, R
θ
CA
. For instance, the air flow can be changed around the device, add
a heat sink, change the mounting arrangement on printed circuit board, or change the thermal dissipation
on the printed circuit board surrounding the device. This description is most useful for ceramic packages
with heat sinks where about 90% of the heat flow is through the case to the heat sink to ambient. For most
packages, a better model is required.
The simplest thermal model of a package which has demonstrated reasonable accuracy (about 20 percent)
is a two resistor model consisting of a junction to board and a junction to case thermal resistance. The
junction to case covers the situation where a heat sink will be used or where a substantial amount of heat
is dissipated from the top of the package. The junction to board thermal resistance describes the thermal
performance when most of the heat is conducted to the printed circuit board. It has been observed that the
thermal performance of most plastic packages and especially PBGA packages is strongly dependent on the
board. temperature.
If the board temperature is known, an estimate of the junction temperature in the environment can be made
using the following equation:
T
J
= T
B
+ (R
θ
JB
x P
D
)
where:
T
B
= board temperature (°C)
R
θ
JB
= package junction to board resistance (°C/W)
P
D
= power dissipation in package (
Ω
)
Summary of Contents for MPC561
Page 84: ...MPC561 MPC563 Reference Manual Rev 1 2 lxxxiv Freescale Semiconductor...
Page 144: ...Signal Descriptions MPC561 MPC563 Reference Manual Rev 1 2 2 46 Freescale Semiconductor...
Page 206: ...Central Processing Unit MPC561 MPC563 Reference Manual Rev 1 2 3 62 Freescale Semiconductor...
Page 302: ...Reset MPC561 MPC563 Reference Manual Rev 1 2 7 14 Freescale Semiconductor...
Page 854: ...Time Processor Unit 3 MPC561 MPC563 Reference Manual Rev 1 2 19 24 Freescale Semiconductor...
Page 968: ...Development Support MPC561 MPC563 Reference Manual Rev 1 2 23 54 Freescale Semiconductor...
Page 1144: ...Internal Memory Map MPC561 MPC563 Reference Manual Rev 1 2 B 34 Freescale Semiconductor...
Page 1212: ...TPU3 ROM Functions MPC561 MPC563 Reference Manual Rev 1 2 D 60 Freescale Semiconductor...
Page 1216: ...Memory Access Timing MPC561 MPC563 Reference Manual Rev 1 2 E 4 Freescale Semiconductor...
