AMD SC3200 Heatsink Considerations, Example, Assume P max = 5W and TA max = 40C Therefore, 422

32581C Package Specifications

10.1.1Heatsink Considerations

Table 10-2 on page 421 shows the maximum allowed ther- mal resistance of a heatsink for particular operating envi- ronments. The calculated values, defined as θCA, represent the required ability of a particular heatsink to transfer heat generated by the SC3200 processor from its

Example 1

Assume P (max) = 5W and TA (max) = 40°C.

Therefore:

θCA = TC − TA

case into the air, thereby maintaining the case temperature at or below 85°C. Because θCA is a measure of thermal resistivity, it is inversely proportional to the heatsinks ability to dissipate heat or its thermal conductivity.

θCA =

P

85 − 40

5

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 10-2,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.

While θCA is a useful parameter to calculate, heatsinks are

θ

θCA = 9

The heatsink must provide a thermal resistance below 9°C/ W. In this case, the heatsink under consideration is more than adequate since at 5W worst case, it can limit the case temperature rise above ambient to 40°C (θCA =8).

Example 2

Assume P (max) = 9W and TA (max) = 40°C.

Therefore:

θCA = TC − TA

not typically specified in terms of a single CA.This is 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-

θCA =

P

85 − 40

9

cally specified by graphs that plot heat dissipation (in watts) vs. mounting surface (case) temperature rise above ambient (in °C). This method is necessary because ambi- ent and case temperatures fluctuate constantly during nor- mal operation of the system. The system designer must be careful to choose the proper heatsink by matching 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 9-3 on page 352) 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.

Figure 10-1is an example of a particular heatsink under consideration

TemperatureSurfaceMounting	°C–AmbientAboveRise	θCA = 45/5 = 9
		50
		40
		30			θCA = 45/9 = 5
		20
		10
		0
		2	4	6	8	10
			Heat Dissipated - Watts

Figure 10-1. Heatsink Example

θCA = 5

In this case, the heatsink under consideration is NOT ade- quate to limit the case temperature rise above ambient to 45°C for a 9W processor.

For more information on thermal design considerations or heatsink properties, refer to the Product Selection Guide of any leading vendor of thermal engineering solutions.

Note: The power dissipations P used in these examples are not representative of the power dissipation of the SC3200 processor, which is always less than 4 Watts.

422 AMD Geode™ SC3200 Processor Data Book