Applications Information
The maximum case temperature (Tc) in °C can be obtained from the following equation:
Tc = Tj – Pd × θjc
Tc= Maximum Case Temperature
Tj= Maximum Junction Temperature
Pd= Maximum Power Dissipation of the Device
θjc = Thermal Resistance between the Junction of the Die and the Case
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. There-
fore, 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 equations:
Ta = Tj – Pd × θja or
Tj = Ta + Pd × θja
where: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 junc-
tion 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. The following tables can
be used to aid in deciding how much of air flow and heat sink for proper thermal manage-
Data for the “no heat sink” cases are derived from MC68040 PGA package characteristics.
The MC68060 PGA package has similar thermal characteristics as the MC68060 PGA
Package. The heat sink used for the “with heat sink” cases are based on the Thermalloy
2333B heat sink. Since exact power dissipation figures for the MC68060 are unavailable at
the time of printing, linear interpolation of these tables can be used to provide rough esti-
mates. Table 11-1, Table 11-2, and Table 11-3 list the thermal data.
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