Package Specifications (Continued)
Table 8-2. Case-to-Ambient Thermal Resistance Examples @ 85°C
θ
for Different Ambient Temperatures (°C/W)
CA
Core Voltage
(VCC2
Core
Frequency
Maximum
Power
)
20°C
25°C
30°C
35°C
40°C
2.9V
266 MHz
233 MHz
200 MHz
180 MHz
7.7W
7.1W
6.4W
6.0W
8.44
9.15
7.79
8.45
7.14
7.75
8.59
9.17
6.49
7.04
7.81
8.33
5.84
6.34
7.03
7.50
(Nominal)
10.16
10.83
9.38
10.00
8.1.1 Heatsink Considerations
While θCA is a useful parameter to calculate, heatsinks are
not typically specified in terms of a single θCA. This is
because the thermal resistivity of a heatsink is not con-
stant across power or temperature. In fact, heatsinks
become slightly less efficient as the amount of heat they
are trying to dissipate increases. For this reason, heatsinks
are typically specified by graphs that plot heat dissipation
(in watts) vs. mounting surface (case) temperature rise
above ambient (in °C). This method is necessary because
ambient and case temperatures fluctuate constantly dur-
ing normal operation of the system. The system designer
must be careful to choose the proper heatsink by match-
ing the required θCA with the thermal dissipation curve of
the device under the entire range of operating conditions in
order to make sure that a case temperature of 85°C is
never surpassed.
As described previously, Table 8-2 shows the maximum
allowed thermal resistance of a heatsink for particular
operating environments. The calculated values, defined
as θCA, represent the required ability of a particular heat-
sink to transfer heat generated by the processor from its
case into the air, thereby maintaining the case tempera-
ture at or below 85°C. Because θCA is a measure of ther-
mal resistivity, it is inversely proportional to the heatsink’s
ability to dissipate heat or it’s thermal conductivity.
Note: A "perfect" heatsink would be able to maintain a
case temperature equal to that of the ambient air
inside the system chassis.
Looking at Table 8-2, it can be seen that as ambient tem-
perature (TA) increases, θCA decreases, and that as power
consumption of the processor (P) increases, θCA
decreases. Thus, the ability of the heatsink to dissipate
thermal energy must increase as the processor power
increases and as the temperature inside the enclosure
increases.
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