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Cyrix III CPU Thermal Design Considerations
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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.
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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
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Cyrix Application Note 126
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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
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Thermal Resistances
Thermal Resistances
Three thermal resistances (Figure 2) can be used to idealize the heat flow from the case of the Cyrix III CPU to ambient: CS = thermal resistance from case to heatsink in °C/W, SA = thermal resistance from heatsink to ambient in °C/W, CA = CS + SA, thermal resistance from case to ambient in °C/W. Additional symbols are used for the temperatures of the, case, heatsink and ambient air: TC = case temperature (top dead center) in °C, TS = heatsink in °C, TA = ambient (free air) temperature in °C. The power applied to the semiconductor is: P = power applied, VC C * IC C in watts (W).
TC Case CS
TS Heatsink SA CA
TA Ambient
Thermal Resistor Model for Semiconductor
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Cyrix III Thermal Design Considerations
Controlling Case Temperature
Controlling Case Temperature
Before power is applied, the case temperature is at ambient.
T C = TA
When power is applied, the case temperature rises as a function of the power applied and of the amount of heat lost to the ambient from the case.
TC = TA + P * CA
The case temperature of the Cyrix III CPU must be controlled in such a way as to maintain a 85°C maximum temperature. The case temperature can be reduced by: · · · · decreasing the ambient temperature of the room improving the air flow geometry in the electronic enclosure to decrease the box ambient temperature (TA ). decreasing the case-to-ambient thermal resistance (C A) through the use of a heatsink or a heatsink/fan reducing the power generated by decreasing the CPU frequency
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Heatsinks and Heatsink/Fans
Heatsinks and Heatsink/Fans
The case-to-air thermal resistance (CA) can be greatly decreased through the use of a heatsink. Heatsinks improve radiation and convection efficiency. Using a heatsink, the thermal resistance (CA ) becomes the sum of the case-to-heatsink thermal resistance CS and heatsink-to-ambient thermal resistances (SA ): CA = CS + SA. Note: Some manufacturers use the symbol R SA instead of SA To take full advantage of the heatsink, it is important to provide a good case-toheatsink fit. Using sufficient clamping force between the heatsink and case, and the application of thermal grease can reduce CS to about 0.1 °C/W. This allows the following approximation to be made: CA SA. The heatsink-to-ambient thermal resistance can be improved by a factor of about five using a heatsink/fan combination. A heatsink/fan reduces C A by increasing the airflow across the heatsink.
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Cyrix Application Note 126
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Cyrix III Thermal Design Considerations
Required Case-to-Ambient Thermal Resistance
Required Case-to-Ambient Thermal Resistance
If the maximum ambient temperature TA(MAX) inside the electronic enclosure is known, the required case-to-ambient thermal resistance can be calculated. The results of this calculation can be used to select which type of heatsink or heatsink /fan is required. The equation below calculates the thermal resistance of the heatsink required for an application. The table and chart below are based on Vcc2 = 2.2 V.
T C ( MA X ) TA ( MA X ) = -------------------------------------------------V CC (M A X ) × I C C(M A X )
°C -----W
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Required Case-to-Ambient Thermal Resistance
CYRIX III P ERFORMANCE
M AX AC TIVE C YRIX III Actual MHz Current (A) 9.15 9.76 10.40 10.85 11.20
M AX A CTIVE P OWER (W) 20.1 21.5 22.9 23.9 24.6
CA FOR DIFFERENT A MBIENT T EMPERAT URES
25°C 2.98 2.79 2.62 2.51 2.41 30°C 2.74 2.55 2.40 2.30 2.24 35°C 2.50 2.33 2.18 2.09 2.03 40°C 2.24 2.09 1.96 1.88 1.83 45°C 1.99 1.86 1.75 1.67 1.63
RA TING PR 433 PR 466 PR 500 PR 533 PR 533 333 MHz 366 MHz 400 MHz 433 MHz 450 MHz
Required
CA to Maintain 85°C Case Temperature
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Cyrix III Thermal Design Considerations
Expected Results for Cyrix III - 500 CPU with Recommended Heatsink/Fan
Expected Results for Cyrix III - 500 CPU with Recommended Heatsink/Fan
The maximum CPU power dissipation is found in the previous table.
PMAX = 22.9 W
Assuming the maximum ambient temperature of 40°C within the electronic enclosure and the case-to-ambient thermal resistance of 1.09°C/W, the maximum case temperature can be calculated using the equation below
TC(MAX) = TA(MAX) + P(MAX) * CA = 40°C + 22.9 W * 1.09°C/W = 64.96°C
Cyrix Application Note 126
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©1999 Copyright Via Technologies. All rights reserved. Printed in the United States of America Trademark Acknowledgments: Cyrix is a registered trademark of Via Technologies. Cyrix III is a internal code name used by Via Technologies. Product names used in this publication are for identification purposes only and may be trademarks of their respective companies. Via-Cyrix 2703 North Central Expressway Richardson, Texas 75080-2010 United States of America Via Technologies and Via-Cyrix reserve the right to make changes in the devices or specifications described herein without notice. Before design-in or order placement, customers are advised to verify that the information is current on which orders or design activities are based. Via-Cyrix warrants its products to conform to current specifications in accordance with Via-Cyrix' standard warranty. Testing is performed to the extent necessary as determined by Via-Cyrix to support this warranty. Unless explicitly specified by customer order requirements, and agreed to in writing by Via-Cyrix, not all device characteristics are necessarily tested. Via-Cyrix assumes no liability, unless specifically agreed to in writing, for customers' product design or infringement of patents or copyrights of third parties arising from use of Via-Cyrix devices. No license, either express or implied, to Via-Cyrix patents, copyrights, or other intellectual property rights pertaining to any machine or combination of Via-Cyrix devices is hereby granted. Via-Cyrix products are not intended for use in any medical, life saving, or life sustaining system. Information in this document is subject to change without notice.