MOTOROLA
M68040 USER’S MANUAL
11-17
d. For the example c system running the data bus in small buffer mode with other
outputs in large buffer mode terminated:
P = (Number of Pins Large Buffer High)
×
(Phlb) + (Number of Pins Large Buffer
Low)
×
(Pllb) + (Number of Pins Small Buffer High)
×
(Phsb) + (Number of Pins
Small Buffer Low)
×
(Plsb)
= 19.1 Pins
×
30.1 mW per Pin + 23.8 Pins
×
14.8 mW per Pin + 10.7 Pins
×
0.625
mW per Pin + 10.7 Pins
×
0.625 mW per Pin
= 0.94 W
×
(1 – 0.85)
= 0.14 W
11.9.1 Still Air
In this study, a small sample of MC68040 packages was tested in free-air cooling with no
heat sink. Measurements showed that the average
θ
JA
was 22.8
°
C/W with a standard
deviation of 0.44
°
C/W. The test was performed with approximately 6 W of power being
dissipated from within the package. The test determined that
θ
JA
decreases slightly for the
increasing power dissipation range possible. Therefore, since the variance in
θ
JA
within
the possible power dissipation range is negligible, it can be assumed for calculation
purposes that
θ
JA
is valid at all power levels. Using the previous equations, Table 11-2
lists the results of a maximum power dissipation at maximum
θ
JC
with no heat sink or
airflow (see Table 11-1 to calculate other power dissipation values). The ambient
temperature results illustrate the need to implement some type of thermal management to
obtain a reasonable maximum ambient temperature.
Table 11-2. Thermal Parameters with No Heat Sink or Airflow
Defined Parameters
Measured
Calculated
MHz
P
D
T
J
θ
JC
θ
JA
θ
CA
(
θ
JA
–
θ
JC
)
T
C
(T
J
– P
D
× θ
JC
)
T
A
(T
J
– P
D
× θ
JA
)
MC68040
25
6.3
110
°
C
110
°
C
110
°
C
110
°
C
110
°
C
110
°
C
3
22.8
19.8
91.1
–33.64
25
6.6
3
22.8
19.8
90.2
–40.48
25
8.6
3
22.8
19.8
84.2
–86.08
33
7.7
3
22.8
19.8
86.9
–65.56
33
8.0
3
22.8
19.8
86.0
–72.40
33
10.0
3
22.8
19.8
80.0
–118.00
MC68LC040 and MC68EC040
20
4
110
°
C
110
°
C
110
°
C
3
22.8
19.8
98
18.8
25
5
3
22.8
19.8
95
–4
33
6.3
3
22.8
19.8
91.1
–33.64