Package Specifications (Continued)
Table 7-2. 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
7.7W
8.44
7.79
7.14
6.49
5.84
(Nominal)
2.5V
233 MHz
5.4W
12.04
11.11
10.19
9.26
8.33
(Nominal)
2.2V
(Nominal)
200 MHz
180 MHz
166 MHz
3.8W
3.6W
3.4W
17.11
18.06
19.12
15.08
16.67
17.65
14.47
15.28
16.18
13.18
13.89
14.71
11.84
12.50
13.24
7.1.1 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 the maximum case temperature
from Table 6-4 on page 189 is never exceeded. To choose
the proper heatsink, the system designer must make sure
that the calculated θCA falls above the curve (shaded
area). The curve itself defines the minimum temperature
rise above ambient that the heatsink can maintain.
Table 7-2 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 heatsink to transfer heat generated
by the processor from its case into the air, thereby main-
taining the case temperature at or below 85°C. Because
θCA is a measure of thermal resistivity, it is inversely pro-
portional 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 7-2, 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.
See Figure 7-1 as an example of a particular heatsink
under consideration.
θ
CA = 45/5 = 9
50
40
30
20
10
0
θ
CA = 45/9 = 5
2
4
6
8
10
Heat Dissipated - Watts
Figure 7-1. Heatsink Example
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