Energy considerations are no longer confined to battery-operated circuits. Power dissipation of high-performance CPU chips is on the order of 50 to 120 W, see table 9.1, roughly as much as a craftsman's soldering iron. Removing that much thermal power necessitates sophisticated packages, heat sinks, heat pipes, forced ventilation, and other costly options. On the input side, fat supply rails and elaborate multiphase step-down converters built from numerous and bulky power transistors, inductors, and capacitors are required to handle massive supply currents without critical voltage drops. Not only costs but also packing density suffers.

Observation 9.1.The problem in battery-operated circuits is where to get the energy from whereas getting the heat out is a major problem in high-performance circuits.

The first section in this chapter analyzes what CMOS circuits spend energy for at a fairly detailed level. Section 9.2 then gives practical guidelines for how to improve energy efficiency before section 9.3 summarizes the very basics of heat flow and heat removal.

What does energy get dissipated for in CMOS circuits?

Note, to begin with, that power is not an adequate yardstick when it comes to evaluating alternative schemes for the processing of information because it does not relate to performance in any way. A circuit's efficiency is better defined as dissipated energy per computational operation or, which is the same, as energy per processed data item.