MC68LC040RC25A Datasheet, PDF (345/442 Pages) Freescale Semiconductor, Inc – M68040 Users Manual

Freescale Semiconductor, Inc.
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
MHz
25
25
25
33
33
33
20
25
33
Defined Parameters
Measured
P_D
T_J
θ_JC
θ_JA
θ_CA
(θ_JA – θ_JC )
MC68040
6.3
110 °C
3
22.8
19.8
6.6
110 °C
3
22.8
19.8
8.6
110 °C
3
22.8
19.8
7.7
110 °C
3
22.8
19.8
8.0
110 °C
3
22.8
19.8
10.0
110 °C
3
22.8
19.8
MC68LC040 and MC68EC040
4
110 °C
3
22.8
19.8
5
110 °C
3
22.8
19.8
6.3
110 °C
3
22.8
19.8
Calculated
T_C
(T_J – P _D × θ_JC )
T_A
(T_J – P _D × θ _JA )
91.1
–33.64
90.2
–40.48
84.2
–86.08
86.9
–65.56
86.0
–72.40
80.0
–118.00
98
18.8
95
–4
91.1
–33.64
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M68040 USER’S MANUAL
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11-17