Micrel Inc.
MIC4451/4452
Transition Power Dissipation
Transition power is dissipated in the driver each time its
output changes state, because during the transition, for
a very brief interval, both the N- and P-channel
MOSFETs in the output totem-pole are ON
simultaneously, and a current is conducted through them
from V
S
to ground. The transition power dissipation is
approximately:
P
T
= 2 f V
S
(A
×
s)
where (A
×
s) is a time-current factor derived from the
typical characteristic curve “Crossover Energy vs.
Supply Voltage.” Total power (P
D
) then, as previously
described is:
P
D
= P
L
+ P
Q
+ P
T
Definitions
C
L
= Load Capacitance in Farads.
D = Duty Cycle expressed as the fraction of time the
input to the driver is high.
f = Operating Frequency of the driver in Hertz
I
H
= Power supply current drawn by a driver when both
inputs are high and neither output is loaded.
I
L
= Power supply current drawn by a driver when both
inputs are low and neither output is loaded.
I
D
= Output current from a driver in Amps.
P
D
= Total power dissipated in a driver in Watts.
P
L
= Power dissipated in the driver due to the driver’s
load in Watts.
P
Q
= Power dissipated in a quiescent driver in Watts.
P
T
= Power dissipated in a driver when the output
changes states (“shoot-through current”) in Watts.
NOTE: The “shoot-through” current from a dual
transition (once up, once down) for both drivers is
stated in Figure 7 in ampere-nanoseconds. This
figure must be multiplied by the number of
repetitions per second (frequency) to find Watts.
R
O
= Output resistance of a driver in Ohms.
V
S
= Power supply voltage to the IC in Volts.
January 2011
11
M9999-011811
MICREL [ MICREL SEMICONDUCTOR ]
