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Application Note 126 Cyrix III CPU Thermal Design Considerations REVISION HISTORY Date 5/20/99 3/22/99 3/1/99 1/29/1999 Version 1.0 0.21 0.2 0.1 Revision Updated Cyrix III Information Changed name from MXs to Cyrix III processor. Changed case temperature from 70 to 85 degrees. Initial Version C:\documentation\joshua\appnotes\cIII_thermal.fm Based on App Note 103 APPLICATION NOTE 126 Cyrix III Thermal Design Considerations Introduction This Application Report serves as a guide in the thermal design of a personal computer using the Via Cyrix III CPUTM Microprocessor. A simplified thermal model is presented that utilizes thermal resistances to describe the heat flow from the CPU. Two case studies are included to show how to measure the thermal performance of the microprocessor in a typical computer enclosure. Additional examples illustrate the calculation of expected maximum case and ambient temperatures. The D.C. Specifications and thermal data in the Cyrix III Microprocessor Data Book (when available) are expanded and updated by the Appendix in this Application Report. 3 Cyrix Application Note 126 - Cyrix III Thermal Design Considerations Heat Flow Heat Flow The Cyrix III CPU dissipates as much as 11.4 watts of power depending on the CPU clock frequency. The CPU is mounted up-side-down in a PGA package (Figure 1). Most of the heat is concentrated at the surface of the semiconductor chip and is passed to the package through three main paths: (1) through the bulk of the silicon chip to where the chip is mounted to the package, (2) through the bond wires to the package, (3) through radiation across the void between the chip and the bottom of the package. The package is cooled by radiation, convection and conduction. Some heat is conducted through the pins and the socket, but most of the heat passes from the package into the flowing air stream that carries the heat out of the equipment enclosure. The transfer of heat from the package to the ambient air can be greatly enhanced through the use of a heatsink. Our thermal model will concentrate on the heat flow from the case and heatsink to the surrounding air. COPPER / TUNGSTEN HEAT SPREADER SILICON CHIP CERAMIC BOND WIRES NOT TO SCALE Cyrix III CPU PGA Package Cross-Sectional View 4 Cyrix Application Note 126 - Cyrix III Thermal Design Considerations Thermal Resistance Model Thermal Resistance Model As heat flows from a heat source to a cooler object, there is a temperature drop (T0 -T1) which is similar to the voltage drop (E) across an electrical resistor. Electrical power dissipated in the chip (P) generates heat. The heat flows away from the source analogous to electrical current (I). By dividing the temperature drop (T0 - T1) by the power producing the heat (P), we obtain thermal resistance () expressed in Celsius degrees (°C) per watt (W). T0 ­ T1 = ----------------P °C ----W This equation is similar to (the dual of) ohms law: E R = -I Cyrix Application Note 126 - Cyrix III