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Revision 1.0

Package Specifications (Continued)

7.1.1

Heatsink Considerations

Table 7-2 shows the maximum allowed thermal resistance

of a heatsink for particular operating environments. The

calculated values, defined as

CA, represent the required

ability of a particular heatsink to transfer heat generated by

the processor from its case into the air, thereby maintaining

the case temperature at or below 85°C. Because

CA is a

measure of thermal resistivity, it is inversely proportional to

the heatsink’s ability to dissipate heat or it’s thermal conduc-

tivity.

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

the

processor

(P)

increases,

decreases. Thus, the ability of the heatsink to dissipate ther-

mal 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 constant

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 typi-

cally specified by graphs that plot heat dissipation (in watts)

vs. mounting surface (case) temperature rise above ambi-

ent (in °C). This method is necessary because ambient and

case temperatures fluctuate constantly during normal oper-

ation of the system. The system designer must be careful

to choose the proper heatsink by matching the required

CA 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 Table

6-4 on page 190) is never exceeded. To choose the proper

heatsink, the system designer must make sure that the cal-

culated

CA falls above the curve (shaded area). The curve

itself defines the minimum temperature rise above ambient

that the heatsink can maintain.

See Figure 7-1 as an example of a particular heatsink

under consideration.

Figure 7-1. Heatsink Example

Table 7-2. Case-to-Ambient Thermal Resistance Examples @ 85°C

Core Voltage

(VCC2)

Core

Frequency

Maximum

Power (W)

CA for Different Ambient Temperatures (°C/W)

20°C

25°C

30°C

35°C

40°C

2.0V

(Nominal)

300 MHz

3.7

1.8V

(Nominal)

266 MHz

3.0

233 MHz

2.8

1.6V

(Nominal)

200 MHz

2.3

CA = 45/9 = 5

Heat Dissipated - Watts

CA = 45/5 = 9

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