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196

Revision 3.1

Package Specifications (Continued)

8.1.1

Heatsink Considerations

As described previously, Table 8-2 shows the maximum

allowed thermal resistance of a heatsink for particular

operating environments. The calculated values, defined

CA, 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

CA 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 wouldbeabletomaintaina

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

the

processor

(P)

increases,

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.

While

CA is a useful parameter to calculate, heatsinks are

not typically specified in terms of a single

CA.Thisis

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

CA 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.

Table 8-2. Case-to-Ambient Thermal Resistance Examples @ 85°C

Core Voltage

(VCC2)

Core

Frequency

Maximum

Power

CA for Different Ambient Temperatures (°C/W)

20°C

25°C

30°C

35°C

40°C

2.9V

(Nominal)

266 MHz

7.7W

8.44

7.79

7.14

6.49

5.84

233 MHz

7.1W

9.15

8.45

7.75

7.04

6.34

200 MHz

6.4W

10.16

9.38

8.59

7.81

7.03

180 MHz

6.0W

10.83

10.00

9.17

8.33

7.50

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