Datasheet

Micrel Inc. MIC4451/4452
October 2011 10
M9999-103111-B
Inductive Load Power Dissipation
For inductive loads the situation is more complicated.
For the part of the cycle in which the driver is actively
forcing current into the inductor, the situation is the same
as it is in the resistive case:
P
L1
= I
2
R
O
D
However, in this instance the R
O
required may be either
the on resistance of the driver when its output is in the
high state, or its on resistance when the driver is in the
low state, depending on how the inductor is connected,
and this is still only half the story. For the part of the
cycle when the inductor is forcing current through the
driver, dissipation is best described as:
P
L2
= I V
D
(1 – D)
where V
D
is the forward drop of the clamp diode in the
driver (generally around 0.7V). The two parts of the load
dissipation must be summed in to produce P
L
:
P
L
= P
L1
+ P
L2
Quiescent Power Dissipation
Quiescent power dissipation (P
Q
, as described in the
input section) depends on whether the input is high or
low. A low input will result in a maximum current drain
(per driver) of  0.2mA; a logic high will result in a
current drain of  3.0mA. Quiescent power can therefore
be found from:
P
Q
= V
S
[D I
H
+ (1 – D) I
L
]
where:
I
H
= quiescent current with input high
I
L
= quiescent current with input low
D = fraction of time input is high (duty cycle)
V
S
= power supply voltage
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.
R
O
= Output resistance of a driver in Ωs.
V
S
= Power supply voltage to the IC in volts.