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1.10 Power Dissipation
Introduction
STD110
1-36
SEC ASIC
1.10
Power
Dissipation
1.10.1 ESTIMATION OF POWER DISSIPATION IN CMOS CIRCUIT
CMOS circuits have been traditionally considered to consume low power since
they draw very small amount of current in a steady state. However, the recent
revolution in a CMOS technology that allows very high gate density has changed
the way the power dissipation should be understood. The power dissipation in a
CMOS circuit is affected by various factors such as the number of gates, the
switching frequency, the loading on the output of a gate, and so on.
Power dissipation is important when designers decide the amount of necessary
power supply current for the device to operate in safety. Propagation delays and
reliability of the device also depend on power dissipation that determines the
temperature at which the die operates. To obtain high speed and reliability,
designers must estimate power dissipation of the device accurately and
determine the appropriate environments including the package and system
cooling methods.
This section describes the concepts of two types of power dissipation (static and
dynamic) in a CMOS circuit, the method of calculating those in the SEC STD110
library.
1.10.2 STATIC (DC) POWER DISSIPATION
There are two types of static or DC current contributing to the total static power
dissipation in CMOS circuits.
One is the leakage current of the gates resulted by a reverse bias between a well
and a substrate region. There is no DC current path from power to ground in a
CMOS because one of the transistor pair is always off, therefore, no static
current except the leakage current flows through the internal gates of the device.
The amount of this leakage current is, however, in the range of tens of nano
amperes, which is negligible.
The other is DC current that flows through the input and output buffers when the
circuit is interfaced with other devices, especially TTL. The current of pull-up/
pull-down transistor in the input buffers is about 33
μ
A (at 3.3V) and 25uA (at
2.5V) typically, which is also negligible. Therefore, only DC current that the
output buffers source or sink has to be counted to estimate the total static power
dissipation.
DC power dissipation of output and bi-directional buffers is determined by the
following formula:
n
∑
where,
n = Number of output and bidirectional buffers
T = Total operation time in output mode
t
H
= The sum of logic high state time
t
L
= The sum of logic low state time
t
L
+ t
H
= T (Supposed that all output and bidirectional buffers have just logic
high or low state)
Sout is the output mode ratio of bidirectional buffers (typically 0.5)
P
DC_OUTPUT
[mW]
V
OL k
( )
I
OL k
( )
t
L k
( )
×
×
(
)
k
1
=
V
DD
V
OH k
( )
–
(
)
I
OH k
( )
t
H k
( )
×
×
(
)
k
1
=
n
∑
+
T
=
P
DC_BI
[mW]
V
OL k
( )
I
OL k
( )
t
L k
( )
×
×
(
)
k
1
=
n
∑
V
DD
V
OH k
( )
–
(
)
I
OH k
( )
t
H k
( )
×
×
(
)
k
1
=
n
∑
+
S
out
T
×
=