3. Pentium 4-M Consumption Modes: What's Behind TDP & Co

What really characterizes a mobile CPU is the fact that its maximum power consumption is incredibly low. This Thermal Design Power (TDP) determines the maximum quantity of heat that a cooling system for a notebook processor has to be able to dissipate to ensure error-free processor operation. As such, it is the yardstick of the CPU's maximum heat dissipation ("energy consumption").

In normal operation, a Pentium 4-M will never be run in this grey zone: Intel speaks of Average Power in this context. "This supplies the CPU's average power consumption when running typical office applications," is Intel's official wording. Active Power refers to power consumption when the CPU is in constant operation, for example, when encoding video or audio files. In order to use the time between two keystrokes to save energy, Pentium 4-M processors employ Quickstart technology. With the help of Quickstart, the processor can switch to a low-power mode and revert to an active mode in virtually no time at all (within nanoseconds). In Quickstart Power mode, the mobile CPU consumes less than 0.5 W, which is substantially less than in Average Power mode (< 2 W).

A comparison of Quickstart, Average, Active and Thermal Design Power.

Relation Between TDP, Core Frequency And Operating Voltage

So why are a mobile CPU's maximum power consumption and, by extension, dissipation so much less? The answer lies in the lower supply voltage that can be used to operate these processors.

According to the formula:

P = C*f*V2CC where
C: total capacity of all CPU gates
f: CPU clock speed
VCC: CPU supply voltage

the power consumption P is proportional to the square of the supply voltage VCC and proportional to the CPU clock speed. Therefore, the most effective method of keeping dissipation in a mobile CPU as low as possible is to lower the supply voltage VCC. Because of the technology, though, reducing VCC raises both the switching times of the transistors and the propagation times within the processor. This means that, in order to be able to guarantee stable, error-free operation, starting with a certain reduction of VCC, it also becomes necessary to reduce the core clock speed. This means that the maximum clock speed and the minimum supply voltage of the final processor are already set at the CPU design phase. Yet even clock speeds that are higher than the theoretical max can be realized by optimizing the production process over time or with a redesign.