Intel Pentium 4 Processor In the 423-pin Package Thermal Design Guidelines

Pentium® 4 processor in the 423-pin package Thermal Design Guidelines
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7.2 Thermal Test Vehicle to Processor Thermal Performance Correction Factor
Intel releases Thermal Test Vehicles (TTV) for use by system and heat sink solution thermal designers prior to
processor availability. The Thermal Test Vehicles approximate the thermal behavior of the processor; however,
there is typically a difference in power density and power uniformity. Any thermal solution performance measured
on Thermal Test Vehicles requires the application of a TTV-to-CPU correction factor in order to predict that thermal
solution performance on a processor. For the Pentium® 4 processor, a TTV-to-CPU correction factor is not
necessary.
8 THERMAL MANAGEMENT LOGIC AND THERMAL MONITOR FEATURE
8.1 Processor Power Dissipation
An increase in processor operating frequency not only increases system performance, but also increases the
processor power dissipation. The relationship between frequency and power is generalized in the following
equation: P=CV
2
F (where P = power, C = capacitance, V = voltage, F = frequency). From this equation it is evident
that Power increases linearly with frequency and with the square of voltage. In the absence of power saving
technologies, ever increasing frequencies will result in processors with power dissipations in the hundreds of Watts.
Fortunately, there are numerous ways to reduce the power consumption of a processor. Decreasing the voltage and
transistor size are two examples, a third is clock modulation, which is used extensively in laptop designs.
Clock modulation is defined as periodically removing the clock signal from the processor core, which effectively
reduces its power consumption to a few Watts. A zero watt power dissipation level is not achievable due to
transistor leakage current and the need to keep a few areas of the processor active (cache coherency circuitry, phase
lock loops, interrupt recognition, etc.). Therefore, by cycling the clocks on and off at a 50% duty cycle, the average
power dissipation can drop by up to 50%. Note that the processor performance will also drop by about 50% during
this period, since program execution halts while the clocks are removed. Varying the duty cycle will have a
corresponding influence on power dissipation and processor performance.
Laptop systems use clock modulation to control system and processor temperatures. By using various external
measurement devices, laptops monitor the processor case temperature and turn on fans or initiate clock modulation
to reduce processor power dissipation and ensure that all elements of the system operate within their temperature
specifications. Unfortunately, using thermocouples on the processor packages have some inherent disadvantages
when used to control a thermal management mechanism. Thermal conductivity (Θ
JC
) through the processor package
creates a gradient between the processor case and silicon temperatures. This delta may be large with the silicon
temperature always being higher than the case temperature. Since thermocouples measure case temperature, not
silicon temperature, a significant guard band may be necessary to ensure the processor silicon does not exceed its
maximum specification. Or, more clearly, clock modulation may have to be turned on when the case temperature is
significantly below maximum specification to ensure the processor does not overheat. This large guard band will
have a substantial, and unacceptable, impact on system performance.
Thermal ramp rates, or change in die temperature over a specified time period (∆T/∆t), may be extremely high in
high power processors where ramp rates in excess of 50°C/sec are anticipated to be normal. With this type of
thermal characteristic, it would not be possible to control fans or other cooling devices based on processor
temperature. By the time the fans have spun up to speed, the processor may be well beyond a safe operating
temperature, which would render any increase in cooling capability useless. Just as large guardbands would be
necessary due to package thermal gradients, equally large guardbands would be necessary if temperature controlled
fans were implemented.
Clearly, a new thermal management approach is needed to support the continued increases in processor frequency
and performance.