Section 4 Design Considerations - Motorola DSP56367 User Manual

SECTION
DESIGN CONSIDERATIONS
4.1 THERMAL DESIGN CONSIDERATIONS
An estimation of the chip junction temperature, T
equation:
(
T = T +
J A
Where:
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance.
R = R
θJA θJC
Where:
R is device-related and cannot be influenced by the user. The user controls the thermal
θJC
environment to change the case-to-ambient thermal resistance, R
can change the air flow around the device, add a heat sink, change the mounting arrangement
on the printed circuit board (PCB), or otherwise change the thermal dissipation capability of
the area surrounding the device on a PCB. 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 PCB, 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
PCB to which the package is mounted. Again, if the estimations obtained from R
satisfactorily answer whether the thermal performance is adequate, a system level model may
be appropriate.
MOTOROLA
4
× )
P R
θJA
D
= ambient temperature °C
T
A
= package junction-to-ambient thermal resistance °C/W
R
qJA
P = power dissipation in package W
D
+ R
θCA
R = package junction-to-ambient thermal resistance °C/W
θJA
R = package junction-to-case thermal resistance °C/W
θJC
R = package case-to-ambient thermal resistance °C/W
θCA
DSP56367
, in °C can be obtained from the following
J
. For example, the user
θCA
do not
θJA
4-1