MIC4123 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Dual 3A-Peak Low-Side MOSFET Driver

MIC4123/4124/4125
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 E-Pad SOIC package, with an θ_JA of 58°C/W,
this will result in the junction running at:
0.2454 x 58 = 14.2°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 MIC4124 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 = I² 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
Micrel, Inc.
In a MLF with an θ_JA of 60°C/W, this amount of power results
in a junction temperature given the maximum 40°C ambient
of:
(0.213 x 100) + 40 = 52.8°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 52.8°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.
M9999-052405
9
May 2005