Datasheet

MPC852T PowerQUICC™ Hardware Specifications, Rev. 4
10 Freescale Semiconductor
Thermal Calculation and Measurement
where:
T
A
= ambient temperature (ºC)
R
θJA
= 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 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
J
– T
A
) are possible.
7.2 Estimation with Junction-to-Case Thermal Resistance
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
θJA
= R
θJC
+ R
θCA
where:
R
θJA
= junction-to-ambient thermal resistance (ºC/W)
R
θJC
= junction-to-case thermal resistance (ºC/W)
R
θCA
= case-to-ambient thermal resistance (ºC/W)
R
θJC
is device-related and cannot be influenced by the user. The user adjusts the thermal environment to
affect the case-to-ambient thermal resistance, R
θCA
. For instance, the user can change the airflow around
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 Estimation with Junction-to-Board Thermal Resistance
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. 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
θJB
× P
D
)
where:
R
θJB
= junction-to-board thermal resistance (ºC/W)
T
B
= board temperature (ºC)
P
D
= power dissipation in package