E-4
MPC565/MPC566 Reference Manual
MOTOROLA
Thermal Characteristics
An estimation of the chip junction temperature, TJ, in °C can be obtained from the equation:
TJ =TA +(RθJA xPD)
where:
TA = ambient temperature (°C)
RθJA = package junction to ambient resistance (°C/W)
PD = 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:
RθJA =RθJC +RθCA
where:
RθJA = junction to ambient thermal resistance (°C/W)
RθJC = junction to case thermal resistance (°C/W)
RθJA = case to ambient thermal resistance (°C/W)
RθJC is device related and cannot be influenced. The user controls the thermal environment
to change the case to ambient thermal resistance, RθCA. 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
4Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting
site (board) temperature, ambient temperature, air flow, power dissipation of other components on
the board, and the board thermal resistance.
5Per JESD51-6 with the board horizontal.
6Thermal resistance between the die and the printed circuit board (Four layer (2s2p) board, natural con-
vection).
7Indicates the thermal resistance between the die and the case top surface as measured by the cold
plate method (MIL SPEC-883 Method 1012.1) with the cold plate temperature used for the case
temperature.
8Thermal characterization parameter indicating the temperature difference between package top and
the junction temperature per EIA/JESD51-2.