Next: 3.2.2 Logic Style
Up: 3.2 Power Consumption
Previous: 3.2 Power Consumption
The basic equations for static and dynamic power consumption were
given already in Chapter 2 to derive the UltraLowPower strategies.
This section reviews them and
discusses the impact of circuit and system properties in more detail.
Looking at plain physics there are three major contributions to the
power consumption in digital CMOS circuits:
 dynamic power consumption
 which is due to the diabatic charging and discharging of the
circuit's internal and external capacitances when the circuit
is operating.
The dynamic power consumption per transistor is

(3.13) 
where
is the clock frequency,
is the activity ratio, i.e.,
the probability for a transistor to switch during a clock period.
 static power consumption
 which is caused by the finite offstate current
that
flows through turnedoff transistors from the supply rail to
ground.
The static power consumption power per transistor is

(3.14) 
where
is the leakage ratio, i.e., the percentage of the
supply voltage which is the average
of a turnedoff
transistor.
^{3.2}
 shortcircuit power consumption
 which originates from the socalled crowbar or inverter crow bar current
that flows during a short time through the series connected
transistors from the supply rail to ground
The shortcircuit power consumption power per transistor is

(3.15) 
The inverter crow bar current
depends largely on the design and
operation of the circuit: the longer the rise and fall times
are in comparison to the delay time
,
the larger
will be.
Usually, in a welldesigned system the effect of
is negligible.
Figure 3.3:
A CMOS inverter with a capacitive load

The total power consumption per transistor is then the sum of all
three contributions.

(3.16) 
For a system consisting of N transistors the power consumption
would then be
In a typical digital CMOS VLSI system, like a microprocessor or
a cache memory module, the dynamic power consumption dominates by far. The reason
is that the threshold voltage is usually in the order of
(
)
so that the contribution from the leakage current
is
also negligible.
Footnotes
 ... transistor.^{3.2}
 In most cases
is assumed to be 1 for simplicity.
Next: 3.2.2 Logic Style
Up: 3.2 Power Consumption
Previous: 3.2 Power Consumption
G. Schrom