MPC866 Freescale Semiconductor, MPC866 Datasheet - Page 12

• Manufacturer Part Number: MPC866
• Description: (MPC859 / MPC866) Hardware Specifications
• Manufacturer: Freescale Semiconductor

Thermal Calculation and Measurement
Thermal Calculation and Measurement
7.1
An estimation of the chip junction temperature, T
where:
The junction-to-ambient thermal resistance is an industry standard value that 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
7.2
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:
where:
R
affect the case-to-ambient thermal resistance, R
the device, add a heat sink, change the mounting arrangement on the printed-circuit board, or change the
thermal dissipation on the printed-circuit board surrounding the device. This thermal model is most useful
for ceramic packages with heat sinks where some 90% of the heat flows through the case and the heat sink
to the ambient environment. For most packages, a better model is required.
7.3
A simple package thermal model that has demonstrated reasonable accuracy (about 20%) 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 is 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; see
Figure 3.
12
θJC
is device related and cannot be influenced by the user. The user adjusts the thermal environment to
T
R
J
θJA
= T
Estimation with Junction-to-Ambient Thermal
Resistance
T
R
P
Estimation with Junction-to-Case Thermal Resistance
R
R
R
Estimation with Junction-to-Board Thermal Resistance
A
D
= R
θJA
θJA
θJC
θCA
A
= ambient temperature (ºC)
= power dissipation in package
+(R
θJC
= package junction-to-ambient thermal resistance (ºC/W)
= junction-to-ambient thermal resistance (ºC/W)
= junction-to-case thermal resistance (ºC/W)
= case-to-ambient thermal resistance (ºC/W)
θJA
+ R
x P
θCA
D
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)
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J
-T
θCA
A
J
, in °C can be obtained from the equation:
) are possible.
. For instance, the user can change the airflow around
MOTOROLA