MIC4423/4424/4425 Micrel, Inc.
MIC4423/4424/4425 10 July 2005
Then quiescent power loss:
P
Q
= V
S
x [D x I
H
+ (1 – D) x I
L
]
= 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 θ
JA
of 120°C/W, this will
result in the junction running at:
0.2454 x 120 = 29.4°C
above ambient, which, given a maximum ambient tempera
-
ture of 60°C, will result in a maximum junction temperature
of 89.4°C.
EXAMPLE 2: A MIC4424 operating on a 15V input, with one
driver driving a 50Ω resistive load at 1MHz, with a duty cycle
of 67%, and the other driver quiescent, in a maximum ambi-
ent temperature of 40°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•s)/2
(because only one side is operating)
= (1,000,000 x 15 x 3.3 x 10
–9
) / 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
= 0.213W
In a ceramic package with an θ
JA
of 100°C/W, this amount of
power results in a junction temperature given the maximum
40°C ambient of:
(0.213 x 100) + 40 = 61.4°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°C and the R
DS(on)
at 61°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’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 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).
R
O
= Output resistance of a driver in Ohms.
V
S
= Power supply voltage to the IC in Volts.
