
11-18
M68040 USER’S MANUAL
MOTOROLA
11.9.2 Forced Air
In this study, a small sample of MC68040 packages was tested in forced-air cooling in a
wind tunnel with no heat sink. The test was performed with approximately 6 W of power
being dissipated from within the package. As previously mentioned, since the variance in
θ
JA
within the possible power range is negligible, it can be assumed for calculation
purposes that
θ
JA
is constant at all power levels. Using the previous equations, Table 11-3
lists the results of the maximum power dissipation at maximum
θ
JC
with airflow and no
heat sink for the MC68040, and Table 11-4 lists the results for the MC68LC040 and
MC68EC040. Refer to Table 11-1 for calculating other power dissipation values.
Table 11-3. Thermal Parameters with Forced Airflow
and No Heat Sink for the MC68040
Thermal Mgmt.
Technique
Defined Parameters
Measured
Calculated
MHz
Airflow Velocity
P
D
T
J
θ
JC
θ
JA
θ
CA
T
C
T
A
25
25
25
33
33
33
100 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°
C
3
°
C/W
12.7
°
C/W
9.7
°
C/W
91.1
°
C
90.2
°
C
84.9
°
C
86.9
°
C
86.0
°
C
80.0
°
C
91.1
°
C
90.2
°
C
84.2
°
C
86.9
°
C
86.0
°
C
80.0
°
C
91.1
°
C
90.2
°
C
84.2
°
C
86.9
°
C
86.0
°
C
80.0
°
C
91.1
°
C
90.2
°
C
84.2
°
C
86.9
°
C
86.0
°
C
80.0
°
C
91.1
°
C
90.2
°
C
84.2
°
C
86.9
°
C
80.0
°
C
81.8
°
C
29.90
°
C
26.18
°
C
00.76
°
C
12.21
°
C
08.40
°
C
00.00
°
C
40.70
°
C
37.40
°
C
15.40
°
C
25.30
°
C
22.00
°
C
00.00
°
C
47.63
°
C
44.66
°
C
24.86
°
C
33.77
°
C
30.80
°
C
11.00
°
C
50.15
°
C
47.30
°
C
28.30
°
C
36.85
°
C
34.00
°
C
15.00
°
C
51.41
°
C
48.62
°
C
30.02
°
C
38.39
°
C
17.00
°
C
22.58
°
C
25
25
25
33
33
33
250 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°
C
3
°
C/W
11.0
°
C/W
8.0
°
C/W
25
25
25
33
33
33
500 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°
C
3
°
C/W
9.9
°
C/W
6.9
°
C/W
25
25
25
33
33
33
750 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°
C
3
°
C/W
9.5
°
C/W
6.5
°
C/W
25
25
25
33
33
33
1000 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°
C
3
°
C/W
9.3
°
C/W
6.3
°
C/W