MIC4423/4424/4425
= 12 x [(0.5 x 0.0035) + (0.5 x 0.0003)]
= 0.0228W
Total power dissipation, then, is:
P
D
= 0.2160 + 0.0066 + 0.0228
= 0.2454W
Assuming an SOIC package, with an
胃
JA
of 120掳C/W, this will
result in the junction running at:
0.2454 x 120 = 29.4掳C
above ambient, which, given a maximum ambient temperature
of 60掳C, will result in a maximum junction temperature of
89.4掳C.
EXAMPLE 2: A MIC4424 operating on a 15V input, with one
driver driving a 50鈩?resistive load at 1MHz, with a duty cycle
of 67%, and the other driver quiescent, in a maximum ambient
temperature of 40掳C:
P
L
= I
2
x R
O
x D
First, I
O
must be determined.
I
O
= V
S
/ (R
O
+ R
LOAD
)
Given R
O
from the characteristic curves then,
I
O
= 15 / (3.3 + 50)
I
O
= 0.281A
and:
P
L
= (0.281)
2
x 3.3 x 0.67
= 0.174W
P
T
= F x V
S
x (A鈥)/2
(because only one side is operating)
= (1,000,000 x 15 x 3.3 x 10
鈥?
) / 2
= 0.025 W
and:
P
Q
= 15 x [(0.67 x 0.00125) + (0.33 x 0.000125) +
(1 x 0.000125)]
(this assumes that the unused side of the driver has its input
grounded, which is more efficient)
= 0.015W
then:
P
D
= 0.174 + 0.025 + 0.0150
R
O
= Output resistance of a driver in Ohms.
V
S
= Power supply voltage to the IC in Volts.
= 0.213W
Micrel
In a ceramic package with an
胃
JA
of 100掳C/W, this amount of
power results in a junction temperature given the maximum
40掳C ambient of:
(0.213 x 100) + 40 = 61.4掳C
The actual junction temperature will be lower than calculated
both because duty cycle is less than 100% and because the
graph lists R
DS(on)
at a T
J
of 125掳C and the R
DS(on)
at 61掳C
T
J
will be somewhat lower.
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鈥檚 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 (鈥渟hoot-through current鈥? in Watts. NOTE: The
鈥渟hoot-through鈥?current from a dual transition (once
up, once down) for both drivers is stated in the graph
on the following page in ampere-nanoseconds. This
figure must be multiplied by the number of repetitions
per second (frequency to find Watts).
MIC4423/4424/4425
10
January 1999
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