Dynamic supply current is dominant in CMOS circuits because most of the power is consumed in moving charges in the parasitic
capacitor in the CMOS gates. As a result, the simplified model of a CMOS circuit consisting of several gates can be viewed as
one large capacitor that is charged and discharged between the power-supply rails. Therefore, the power–dissipation capacitance
(Cpd) is often specified as a measure of this equivalent capacitance and is used to approximate the dynamic power consumption.
Cpd is defined as the internal equivalent capacitance of a device calculated by measuring operating current without load
capacitance. Depending on the output switching capability, Cpd can be measured with no output switching (output disabled) or
with any of the outputs switching (output enabled). Cpd is discussed in greater detail in the next section.