Datasheet

MPC8241 Integrated Processor Hardware Specifications, Rev. 10
Freescale Semiconductor 51
System Design Information
Shin-Etsu MicroSi, Inc. 888-642-7674
10028 S. 51st St.
Phoenix, AZ 85044
Internet: www.microsi.com
Thermagon Inc. 888-246-9050
4707 Detroit Ave.
Cleveland, OH 44102
Internet: www.thermagon.com
7.7.3 Heat Sink Usage
An estimation of the chip junction temperature, T
J
, can be obtained from the equation:
T
J
= T
A
+ (R
θJA
× P
D
)
where:
T
A
= ambient temperature for the package (°C)
R
θJA
= junction-to-ambient thermal resistance (°C/W)
P
D
= power dissipation in the package (W)
The junction-to-ambient thermal resistance is an industry-standard value that provides a quick and easy
estimation of thermal performance. Unfortunately, two values are in common usage: the value determined
on a single-layer board and the value obtained on a board with two planes. For packages such as the PBGA,
these values can be different by a factor of two. Which value is closer to the application depends on the
power dissipated by other components on the board. The value obtained on a single-layer board is
appropriate for the tightly packed printed-circuit board. The value obtained on the board with the internal
planes is usually appropriate if the board has low power dissipation and the components are well separated.
When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance:
R
θJA
= R
θJC
+ R
θCA
where:
R
θJA
= junction-to-ambient thermal resistance (°C/W)
R
θJC
= junction-to-case thermal resistance (°C/W)
R
θCA
= case-to-ambient thermal resistance (°C/W)
R
θJC
is device-related and cannot be influenced by the user. The user controls the thermal environment to
change the case-to-ambient thermal resistance, R
θCA
. For instance, the user can change the size of the heat
sink, the airflow around the device, the interface material, the mounting arrangement on the printed-circuit
board, or the thermal dissipation on the printed-circuit board surrounding the device.
To determine the junction temperature of the device in the application when heat sinks are not used, the
thermal characterization parameter (ψ
JT
) measures the temperature at the top center of the package case
using the following equation:
T
J
= T
T
+ (ψ
JT
× P
D
)