Thermal Resistances and
To calculate the specific power dissipation of a specific design, the termination method
of each signal must be considered. For example, a signal output that is not connected
would not dissipate any additional power if it were configured in the large buffer rather
than the small buffer mode.
Since the maximum operating junction temperature has been specified to be 125°C.
The maximum case temperature, TC, in °C can be obtained from:
TC = TJ - PD · ΦJC
TC = Maximum case temperature
TJ = Maximum junction temperature
PD = Maximum power dissipation of the device
ΦJC = 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 formula:
TA = TJ - PD · ΦJC - PD · ΦCA
Where the thermal resistance from case to ambient, ΦCA, is the only user-dependent
parameter once a buffer output configuration has been determined. As seen from equa-
tion (3), reducing the case to ambient thermal resistance increases the maximum
operating ambient temperature. Therefore, by utilizing such methods as heat sinks and
ambient air cooling to minimize the ΦCA, a higher ambient operating temperature and/or
a lower junction temperature can be achieved.
However, an easier approach to thermal evaluation uses the following formulas:
TA = TJ - PD · ΦJA
TJ = TA - PD · ΦJA
ΦJA = thermal resistance from the junction to the ambient (ΦJC + ΦCA).
This total thermal resistance of a package, ΦJA, is a combination of its two components,
ΦJC and ΦCA. These components represent the barrier to heat flow from the semicon-
ductor junction to the package (case) surface (ΦJC) and from the case to the outside
ambient (ΦJC). Although ΦJC is device related and cannot be influenced by the user, ΦCA
is user dependent. Thus, good thermal management by the user can significantly
reduce ΦCA achieving either a lower semiconductor junction temperature or a higher
ambient operating temperature.
To attain a reasonable maximum ambient operating temperature, a user must reduce
the barrier to heat flow from the semiconductor junction to the outside ambient (ΦJA).
The only way to accomplish this is to significantly reduce ΦCA by applying such thermal
management techniques as heat sinks and ambient air cooling.
The following paragraphs discuss some results of a thermal study of the TS68040
device without using any thermal management techniques; using only air-flow cooling,
using only a heat sink, and using heat sink combined with air-flow cooling.