Power estimation for CMOS circuits appears to be deceptively
straightforward. Most vendors provide a table or an
equation that basically splits out the various components of
power dissipation for a CMOS part (see Figure 1). Typically,
these include a component for the input receivers,
which must be de-rated if driven from TTL rather than
CMOS external drivers. Next, there is a component for the
internal core of the chip, which usually includes a negligible
d.c. component and an a.c. component that requires
detailed knowledge of the various switching frequencies
encountered. It also requires knowledge of exactly how
much circuitry is used or unused at any time. Finally, there
is a component attributed to the output stages which are
functions of both the switching frequencies as well as the
external load capacitance. The power is found as the sum
of all three components:
Power = PIN + PCORE + POUT