Geode GXLV Processor Series Low Power Integrated x86 Solutions

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Geode GXLV Processor Series8.0 Package SpecificationsThe thermal characteristics and mechanical dimensionsfor the Geode GXLV processor are provided on thefollowing pages.8.1 THERMAL CHARACTERISTICSTable 8-1 shows the junction-to-case thermal resistanceof the SPGA and BGA package and can be used to calculatethe junction (die) temperature under any given circumstance.Table 8-1. Junction-to-Case Thermal Resistancefor SPGA and BGA PackagesNote that there is no specification for maximum junctiontemperature given since the operation of both SPGA andBGA devices are guaranteed to a case temperature rangeof 0°C to 85°C (see T C in Table 6-4 on page 189). As longas the case temperature of the device is maintained withinthis range, the junction temperature of the die will also bemaintained within its allowable operating range. However,the die (junction) temperature under a given operatingcondition can be calculated by using the following equation:T J =T C +(P* θ JC )where:PackageSPGABGAT J = Junction temperature (°C)T C = Case temperature at top center of package (°C)P = Maximum power dissipation (W)θ JC1.7 °C/W1.1 °C/Wθ JC = Junction-to-case thermal resistance (°C/W)These examples are given for reference only. The actualvalue used for maximum power (P) and ambient temperature(T A ) is determined by the system designer based onsystem configuration, extremes of the operating environment,and whether active thermal management (via SuspendModulation) of the processor is employed.A maximum junction temperature is not specified since amaximum case temperature is. Therefore, the followingequation can be used to calculate the maximum thermalresistance required of the thermal solution for a givenmaximum ambient temperature:where:θ CS = Max case-to-heatsink thermal resistance(°C/W) allowed for thermal solutionθ SA = Max heatsink-to-ambient thermal resistance(°C/W) allowed for thermal solutionT A = Max ambient temperature (°C)T C = Max case temperature at top center of package(°C)P = Max power dissipation (W)If thermal grease is used between the case and heatsink,θ CS will reduce to about 0.01 °C/W. Therefore, the aboveequation can be simplified to:where:θ CS+ θ SA=T C– T A---------------------PT C– T Aθ CA= ---------------------Pθ CA = θ CS = Max case-to-ambient thermal resistance(°C/W) allowed for thermal solution.The calculated θ CA value (examples shown in Table 8-2)represents the maximum allowed thermal resistance ofthe selected cooling solution which is required to maintainthe maximum T C (shown in Table 6-4 on page 189) for theapplication in which the device is used.www.national.com 240 Revision 1.3
Package Specifications (Continued)Core Voltage(V CC2 )2.9V(Nominal)2.5V(Nominal)2.2V(Nominal)Table 8-2. Case-to-Ambient Thermal Resistance Examples @ 85°CCoreFrequencyMaximumPowerθ CA for Different Ambient Temperatures (°C/W)20°C 25°C 30°C 35°C 40°C266 MHz 7.7W 8.44 7.79 7.14 6.49 5.84233 MHz 5.4W 12.04 11.11 10.19 9.26 8.33200 MHz 3.8W 17.11 15.08 14.47 13.18 11.84180 MHz 3.6W 18.06 16.67 15.28 13.89 12.50166 MHz 3.4W 19.12 17.65 16.18 14.71 13.24Geode GXLV Processor Series8.1.1 Heatsink ConsiderationsTable 8-2 shows the maximum allowed thermal resistanceof a heatsink for particular operating environments. Thecalculated values, defined as θ CA , represent the requiredability of a particular heatsink to transfer heat generatedby the processor from its case into the air, thereby maintainingthe case temperature at or below 85°C. Becauseθ CA is a measure of thermal resistivity, it is inversely proportionalto the heatsink’s ability to dissipate heat or it’sthermal conductivity.Note:A"perfect"heatsinkwouldbeabletomaintainacase temperature equal to that of the ambient airinside the system chassis.Looking at Table 8-2, it can be seen that as ambient temperature(T A ) increases, θ CA decreases, and that as powerconsumption of the processor (P) increases, θ CAdecreases. Thus, the ability of the heatsink to dissipatethermal energy must increase as the processor powerincreases and as the temperature inside the enclosureincreases.While θ CA is a useful parameter to calculate, heatsinks arenot typically specified in terms of a single θ CA .Thisisbecause the thermal resistivity of a heatsink is not constantacross power or temperature. In fact, heatsinksbecome slightly less efficient as the amount of heat theyare trying to dissipate increases. For this reason, heatsinksare typically specified by graphs that plot heat dissipation(in watts) vs. mounting surface (case) temperature riseabove ambient (in °C). This method is necessary becauseambient and case temperatures fluctuate constantly duringnormal operation of the system. The system designermust be careful to choose the proper heatsink by matchingthe required θ CA with the thermal dissipation curve ofthe device under the entire range of operating conditions inorder to make sure that the maximum case temperaturefrom Table 6-4 on page 189 is never exceeded. To choosethe proper heatsink, the system designer must make surethat the calculated θ CA falls above the curve (shadedarea). The curve itself defines the minimum temperaturerise above ambient that the heatsink can maintain.See Figure 8-1 as an example of a particular heatsinkunder consideration.Mounting Surface TemperatureRise Above Ambient – °C50403020100θ CA = 45/5 = 9θ CA = 45/9 = 52 4 6 8 10Heat Dissipated - WattsFigure 8-1. Heatsink ExampleRevision 1.3 241 www.national.com
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Geode GXLV Processor Series Low Power Integrated x86 Solutions

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