In general, the ambient temperature (TA ) in °C is a function of the following equation:
TA = TJ – PD × θJC – PD × θCA
The thermal resistance from case to outside ambient (θCA ) is the only user-dependent
parameter once a buffer output configuration has been determined. Reducing the case to
ambient thermal resistance increases the maximum operating ambient temperature.
Therefore, by utilizing methods such as heat sinks and ambient air cooling to minimize
θCA , a higher ambient operating temperature and/or a lower junction temperature can be
achieved. However, an easier approach to thermal evaluation uses the following
= TJ – PD × θJA or TJ = TA + PD × θJA
θJA = Thermal Resistance from the Junction to the Ambient (θJC + θCA )
The total thermal resistance for a package (θJA ) is a combination of its two components,
θJC and θCA . These components represent the barrier to heat flow from the semiconductor
junction to the package case surface (θJC) and θCA. Although θJC is package related and
the user cannot influence it, θCA is user dependent. Good thermal management by the
user, such as heat sink and airflow, can significantly reduce θCA achieving either a lower
semiconductor junction temperature or a higher ambient operating temperature.
To attain a reasonable maximum ambient operating temperature, the user must reduce
the barrier to heat flow from θJA . The only way to accomplish this is to significantly reduce
θCA by applying thermal management techniques such as heat sinks and forced air
cooling. The following paragraphs discuss thermal study results for the MC68040 that did
not use thermal management techniques, airflow cooling, heat sink, and heat sink
combined with airflow cooling.
The MC68040 power dissipation values given in this section represent the sum of the
power dissipated by the internal circuitry and the output buffers of the MC68040. The
termination network chosen by the system designer strongly influences this last
component of power. Values listed in this section for large buffer terminated entries reflect
a termination network as illustrated in Figure 11-8 and are consistent with specifications
for the MC68040. For additional termination schemes, refer to AN1051,
Transmission Line
Effects in PCB Applications
, or AN1061,
Reflecting on Transmission Line Effects
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