Intel Embedded Intel486 Hardware Reference Manual page 318

EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
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The worst case junction temperature (T
two reasons. First, the junction-to-case thermal coefficient (θ
to-ambient thermal coefficient (θ
less with power dissipation (P
affected by the airflow in the system, whereas the junction-to-ambient coefficient (θ
vary.
Given the case temperature specification, a designer can either set the ambient temperature or use
fans to control the case temperature. Finned heatsinks or conductive cooling may also be used in
an environment which prohibits the use of fans.
A designer has considerable freedom in designing the heatsink and faces only practical and eco-
nomic limits. Multiple parallel devices may be helpful in reducing θ
to the heat sink is dispersed rather than concentrated, the effective thermal impedance is lower.
To approximate the case temperature for varying environments, the two equations discussed ear-
lier should be combined by making the junction temperature the same for both, resulting in the
following equation:
θ θ
T = T – [( ) P ]
a c ja jc d
Refer to the Intel486 processor datasheets to determine the values of θ
requirement) and the ambient temperature that will yield the desired case temperature. The proper
calculations are important in achieving an efficient and reliable Intel486 processor system.
One packaging option for the Intel486 processors is a 168-pin ceramic PGA. The recommended
heatsinks for the device are offered in the pin fin design that utilizes air cooling. T
proved by adding a heat sink. The heat sink is mounted on the PGA package with a frame and
spring. A typical heat sink is shown in
10-34
) is lower when calculated with case temperature for
j
). Therefore, the calculated junction temperature varies
ja
). Second, the junction-to-case coefficient (θ
d
Figure 10-27. The dimensions are shown in
) is lower than the junction-
jc
) is not
jc
ja
because if the heat input
sa
(per the system's airflow
ja
is greatly im-
a
Figure
) does
10-28.