AMD SC1200 Heatsink Considerations, Example, Assume P max = 5W and TA max = 40C Therefore, 438

32579B Package Specifications

10.1.1Heatsink Considerations

Table 10-2 on page 437 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 SC1200/SC1201 processor from its case

Example 1

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

Therefore:

θCA = TC − TA

into the air, thereby maintaining the case temperature at or below 85°C. Because θCA is a measure of thermal resistiv- ity, it is inversely proportional to the heatsinks ability to dis- sipate 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 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 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 366) is never exceeded. To choose the proper heatsink, the system designer must make sure that the cal- culated 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 SC1200/SC1201 processor, which is always less than 4 Watts.

438 AMD Geode™ SC1200/SC1201 Processor Data Book