MC68LC040RC25A Datasheet, PDF (342/442 Pages) Freescale Semiconductor, Inc – M68040 Users Manual

Freescale Semiconductor, Inc.
In general, the ambient temperature (T_A ) in °C is a function of the following equation:
T_A = T_J – P_D × θ_JC – P_D × θ_CA
The thermal resistance from case to outside ambient (θ_CA) 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
θ_CA, a higher ambient operating temperature and/or a lower junction temperature can be
achieved. However, an easier approach to thermal evaluation uses the following
equations:
T_A = T_J – P_{D ×} θ_JA or T_J = T_A + P_{D ×} θ_JA
where:
θ_JA = Thermal Resistance from the Junction to the Ambient (θ_JC + θ_CA)
The total thermal resistance for a package (θ_JA) is a combination of its two components,
θ_JC and θ_CA. These components represent the barrier to heat flow from the semiconductor
junction to the package case surface (θ_JC) and θ_CA. Although θ_JC is package related and
the user cannot influence it, θ_CA is user dependent. Good thermal management by the
user, such as heat sink and airflow, can significantly reduce θ_CA achieving either a lower
semiconductor junction temperature or a higher ambient operating temperature.
11.9 MC68040 THERMAL MANAGEMENT TECHNIQUES
To attain a reasonable maximum ambient operating temperature, the user must reduce
the barrier to heat flow from θ_JA. The only way to accomplish this is to significantly reduce
θ_CA 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.
11-14
M68040 USER’S MANUAL
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