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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) × (P hlb) + (Number of Pins Large Buffer
Low) × (P llb ) + (Number of Pins Small Buffer High) × (P hsb) + (Number of Pins
Small Buffer Low) × (P lsb)
= 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 θJA 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 θJA decreases slightly for the
increasing power dissipation range possible. Therefore, since the variance in θJA within
the possible power dissipation range is negligible, it can be assumed for calculation
purposes that θJA is valid at all power levels. Using the previous equations, Table 11-2
lists the results of a maximum power dissipation at maximum θJC 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 PDTJθJC θJA θCA
(θJA θJC )
TC
(TJ – P D × θJC )
TA
(TJ – P D × θJA )
MC68040
25 6.3 110 °C 3 22.8 19.8 91.1 –33.64
25 6.6 110 °C 3 22.8 19.8 90.2 –40.48
25 8.6 110 °C 3 22.8 19.8 84.2 –86.08
33 7.7 110 °C 3 22.8 19.8 86.9 –65.56
33 8.0 110 °C 3 22.8 19.8 86.0 –72.40
33 10.0 110 °C 3 22.8 19.8 80.0 –118.00
MC68LC040 and MC68EC040
20 4 110 °C 3 22.8 19.8 98 18.8
25 5 110 °C 3 22.8 19.8 95 –4
33 6.3 110 °C 3 22.8 19.8 91.1 –33.64
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