MC68LC040RC25A Datasheet, PDF (347/442 Pages) Freescale Semiconductor, Inc – M68040 Users Manual

Freescale Semiconductor, Inc.
Table 11-4. Thermal Parameters with Forced Airflow and No Heat Sink
for the MC68LC040 and MC68EC040
MHz
20
25
33
20
25
33
20
25
33
20
25
33
20
25
33
Thermal Mgmt.
Technique
Airflow Velocity
100 LFM
250 LFM
500 LFM
750 LFM
1000 LFM
Defined Parameters
P_D
4W
5W
6.3W
4W
5W
6.3W
4W
5W
6.3W
4W
5W
6.3W
4W
5W
6.3W
T_J
110 °C
110 °C
110 °C
110 °C
110 °C
θ_JC
3 °C/W
3 °C/W
3 °C/W
3 °C/W
3 °C/W
Measured
Calculated
θ_JA
12.7 °C/W
11 °C/W
9.9 °C/W
9.5 °C/W
9.3 °C/W
θ_CA
9.7 °C/W
8 °C/W
6.9 °C/W
6.5 °C/W
6.3 °C/W
T_C
98 °C
95 °C
91.1 °C
98 °C
95 °C
91.1 °C
98 °C
95 °C
91.1 °C
98 °C
95 °C
91.1 °C
98 °C
95 °C
91.1 °C
T_A
59.2 °C
46.5 °C
29.9 °C
66 °C
55 °C
40.70 °C
70.4 °C
60.5 °C
47.63 °C
72 °C
62.5 °C
50.15 °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 θ_CA 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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M68040 USER’S MANUAL
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11-19