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MOTOROLA M68040 USER’S MANUAL 11-19
Table 11-4. Thermal Parameters with Forced Airflow and No Heat Sink
for the MC68LC040 and MC68EC040
Thermal Mgmt.
Technique Defined Parameters Measured Calculated
MHz Airflow Velocity PDTJθJC θJA θCA TCTA
20
25
33
100 LFM
4 W
5 W
6.3W
110 °C3 °C/W 12.7 °C/W 9.7 °C/W
98 °C
95 °C
91.1 °C
59.2 °C
46.5 °C
29.9 °C
20
25
33
250 LFM
4 W
5 W
6.3W
110 °C3 °C/W 11 °C/W 8 °C/W
98 °C
95 °C
91.1 °C
66 °C
55 °C
40.70 °C
20
25
33
500 LFM
4 W
5 W
6.3W
110 °C3 °C/W 9.9 °C/W 6.9 °C/W
98 °C
95 °C
91.1 °C
70.4 °C
60.5 °C
47.63 °C
20
25
33
750 LFM
4 W
5 W
6.3W
110 °C3 °C/W 9.5 °C/W 6.5 °C/W
98 °C
95 °C
91.1 °C
72 °C
62.5 °C
50.15 °C
20
25
33
1000 LFM
4 W
5 W
6.3W
110 °C3 °C/W 9.3 °C/W 6.3 °C/W
98 °C
95 °C
91.1 °C
72.8 °C
63.5 °C
51.41 °C
Reviewing the maximum ambient operating temperatures illustrates that using an all small
buffer configuration of the MC68040 with a relatively small amount of airflow (100 LFM)
achieves a 0–70 °C ambient operating temperature. However, depending on the output
buffer configuration and available forced-air cooling, additional thermal management
techniques may be required.
11.9.3 With Heat Sink
The designer must consider many factors in choosing a heat sink: heat-sink size and
composition, method of attachment, and choice of a dry or wet (i.e., thermal grease)
connection. The following paragraphs discuss the relationship of these decisions to the
thermal performance of the design noticed during experimentation.
The heat-sink size is one of the most significant parameters to consider in the selection of
a heat sink. Obviously a larger heat sink provides better cooling. Under forced-air
conditions as low as 100 LFM, the difference between the θCA is very small (0.4 °C/W or
less). This difference continues to decrease as the forced airflow increases.
The area of this example heat-sink base perimeter is 1.8" × 1.8", with a height of 0.65".
The heat-sink used a pin-fin (i.e., bed-of-nails) design composed of aluminum alloy. Figure
11-32 illustrates the heat sink, which can be obtained through Thermalloy, Inc.
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