MC68181 Datasheet, PDF (23/119 Pages) Freescale Semiconductor, Inc – Advance Information ROAMING FLEX chip SIGNAL PROCESSOR

Freescale Semiconductor, Inc.
SECTION 4
DESIGN CONSIDERATIONS
THERMAL DESIGN CONSIDERATIONS
An estimation of the chip junction temperature, T_J, in °C can be obtained from the
equation:
Equation 1: T_J = T_A + (P_D × R_θJA)
Where:
T_A = ambient temperature ˚C
R_θJA = package junction-to-ambient thermal resistance ˚C/W
P_D = power dissipation in package
Historically, thermal resistance has been expressed as the sum of a junction-to-case
thermal resistance and a case-to-ambient thermal resistance:
Equation 2: R_θJA = R_θJC + R_θCA
Where:
R_θJA = package junction-to-ambient thermal resistance ˚C/W
R_θJC = package junction-to-case thermal resistance ˚C/W
R_θCA = package 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
example, the user can change the air flow around the device, add a heat sink, change
the mounting arrangement on the Printed Circuit Board, or otherwise change the
thermal dissipation capability of the area surrounding the device on a Printed Circuit
Board. This model is most useful for ceramic packages with heat sinks; some 90% of
the heat flow is dissipated through the case to the heat sink and out to the ambient
environment. For ceramic packages, in situations where the heat flow is split between
a path to the case and an alternate path through the Printed Circuit Board, analysis of
the device thermal performance may need the additional modeling capability of a
system level thermal simulation tool.
The thermal performance of plastic packages is more dependent on the temperature
of the Printed Circuit Board to which the package is mounted. Again, if the
MOTOROLA
MC68181 Technical Data Sheet
4-1
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