MIC4451 Micrel Semiconductor, MIC4451 Datasheet - Page 9

• Manufacturer Part Number: MIC4451
• Description: 12A-Peak Low-Side MOSFET Driver Bipolar/CMOS/DMOS Process
• Manufacturer: Micrel Semiconductor

MIC4451/4452
driver. The energy stored in a capacitor is described by the
equation:
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 on 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:
where:
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:
However, in this instance the R
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
where V
(generally around 0.7V). The two parts of the load dissipation
must be summed in to produce P
Quiescent Power Dissipation
Quiescent power dissipation (P
section) depends on whether the input is high or low. A low
input will result in a maximum current drain (per driver) of
Quiescent power can therefore be found from:
where:
0.2mA; a logic high will result in a current drain of 3.0mA.
V
I
C = Load Capacitance
I
D = fraction of time input is high (duty cycle)
H
S
L
f = Operating Frequency
= Driver Supply Voltage
= quiescent current with input high
= quiescent current with input low
E = 1/2 C V
P
P
P
P
P
D
L
L1
L2
L
Q
is the forward drop of the clamp diode in the driver
= f C (V
= P
= V
= I
= I V
L1
2
S
R
D
[D I
+ P
O
S
(1 – D)
2
D
)
H
L2
2
+ (1 – D) I
O
Q
L
required may be either the
, as described in the input
L
]
5-78
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
power dissipation is approximately:
where (A•s) is a time-current factor derived from the typical
characteristic curve “Crossover Energy vs. Supply Voltage.”
Total power (P
Definitions
R
P
V
P
V
C
P
P
I
I
D = Duty Cycle expressed as the fraction of time the
I
H
D
D
Q
O
S
S
L
L
L
T
f = Operating Frequency of the driver in Hertz
= power supply voltage
= Load Capacitance in Farads.
= Power supply current drawn by a driver when both
= Power supply current drawn by a driver when both
= Output current from a driver in Amps.
= Total power dissipated in a driver in Watts.
= Power dissipated in the driver due to the driver’s
= Power dissipated in a quiescent driver in Watts.
= Power dissipated in a driver when the output
= Output resistance of a driver in Ohms.
= Power supply voltage to the IC in Volts.
P
P
T
D
input to the driver is high.
inputs are high and neither output is loaded.
inputs are low and neither output is loaded.
load in Watts.
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 Figure 7 in ampere-nanoseconds. This
figure must be multiplied by the number of repeti-
tions per second (frequency) to find Watts.
= 2 f V
= P
D
L
) then, as previously described is:
+ P
S
(A•s)
Q
+ P
T
S
to ground. The transition
April 1998
Micrel