MIC4421A Datasheet, PDF (10/13 Pages) Micrel Semiconductor – 9A Peak Low-Side MOSFET Driver Bipolar/CMOS/DMOS Process

Micrel, Inc.
Table 1. MIC4421A Maximum Operating Frequency
V_S
Max Frequency
18V
220kHz
15V
300kHz
10V
640kHz
5V
2MHz
Conditions:
1. θ_JA = 150°C/W
2. T_A = 25°C
3. C_L = 10,000pF
Capacitive Load Power Dissipation
Dissipation caused by a capacitive load is simply the
energy placed in, or removed from, the load capacitance
by the driver. The energy stored in a capacitor is
described by the equation:
E = 1/2 C V²
As this energy is lost in the driver each time the load is
charged or discharged, for power dissipation calculations
the 1/2 is removed. This equation also shows that it is
good practice not to place more voltage in the capacitor
than is necessary, as dissipation increases as the
square of the voltage applied to the capacitor. For a
driver with a capacitive load:
PL = f C (VS)2
where:
f = Operating Frequency
C = Load Capacitance
VS = Driver Supply Voltage
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 = I² 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
MIC4421A/4422A
Quiescent Power Dissipation
Quiescent power dissipation (PQ, 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 just:
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.
June 2007
10
M9999-062707