Intel Pentium 4 Processor in the 478-Pin Package Thermal Design Guidelines

Intel
®
Pentium
®
4 Processor in the 478-Pin Package Thermal Design Guidelines
R
Design Guide 29
Table 2. TTV Correction Factors
Thermal
Resistance
Intel
®
Pentium
®
4 Processor
in the 478-Pin Package
Intel
®
Pentium
®
4 Processor with
512KB L2 Cache on .13 Micron Process
Θ
CS
0.948 1.151
Θ
SA
0.999 1.014
Θ
CA
0.985 1.053
Θ
CA
correction factors should be used for the reference thermal design described in Chapter 3, or
when the ratio Θ
CS
/Θ
SA
is similar to this design (~ 0.53 for the Intel
®
Pentium
®
4 processor with
512KB L2 cache on .13 micron process and ~0.6 for the Intel Pentium 4 processor in the 478-pin
package). If this ratio is significantly different then it is recommended to use individual Θ
CS
and
Θ
SA
correction factors and add corrected Θ
CS
and Θ
SA
to get Θ
CA
.
2.4 Thermal Management Logic and Thermal Monitor
Feature
2.4.1 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 for example, the average power
dissipation can drop by up to 50%. Note that the processor performance also drops by about 50%
during this period, since program execution halts while the clocks are removed. Varying the duty
cycle has a corresponding influence on power dissipation and processor performance. The duty
cycle is specific to the processor (typically 30-50%).
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
external thermocouples connected to the processor package to monitor and control a thermal
management solution has some inherent disadvantages. Thermal conductivity through the
processor package creates a temperature gradient between the processor case and silicon. This
temperature difference may be large with the silicon temperature always being higher than the case
temperature. Since thermocouples measure case temperature, not silicon temperature, significant