Datasheet

7–147
Power Calculation
When calculating the total power consumption of a circuit, both the static and the dynamic currents must be taken into account.
Both bipolar and BiCMOS devices have varying static-current levels, depending on the state of the output (I
CCL
, I
CCH
, or
I
CCZ
), while a CMOS device has a single value for I
CC
. These values are given in the individual data sheets. All inputs or I/Os
(except GTL or BTL I/Os), when driven at TTL levels, consume additional current because they may not be driven all the way
to V
CC
or GND; therefore, the input transistors are not completely turned off. This value is known as I
CC
and is provided
in the data sheet.
Dynamic power consumption results from charging and discharging both internal parasitic capacitances and external load
capacitance. The parameter for CMOS devices that accounts for the parasitic capacitances is known as C
pd
. It is obtained using
equation 2 and is found in the data sheet.
C
pd
+
I
CC
(dynamic)
V
CC
f
i
* C
L
Where:
f
i
= Input frequency (Hz)
V
CC
= Supply voltage (V)
C
L
= Load capacitance (F)
I
CC
= Measured value of current into the device (A)
Although a C
pd
value is not provided for ABT and LVT, the I
CC
-versus-frequency curves display essentially the same
information (see Figures 13 through 15). The slope of the curve provides a value in the form of mA/(MHz × bit) that, when
multiplied by the number of outputs switching and the desired frequency, provides the dynamic power dissipated by the device
without the load current.
Equations 4 through 7 are used to calculate total power for CMOS or BiCMOS devices:
P
T
+ P
S(tatic)
) P
D(ynamic)
CMOS
CMOS-level inputs
P
S
+ V
CC
I
CC
P
D
+
ǒ
C
pd
f
i
) C
L
f
o
Ǔ
V
CC
2
N
sw
TTL-level inputs
P
S
+
[
I
CC
)
(
N
TTL
DI
CC
DC
d
)
]
P
D
+
ǒ
C
pd
f
i
) C
L
f
o
Ǔ
V
CC
2
N
sw
(3)
(4)
(5)
(6)
(7)
(8)