Micrel Inc.
MIC4451/4452
Resistive Load Power Dissipation
Dissipation caused by a resistive load can be calculated
as:
P
L
= I
2
R
O
D
where:
I = the current drawn by the load
R
O
= the output resistance of the driver when the output
is high, at the power supply voltage used. (See data
sheet)
D = fraction of time the load is conducting (duty cycle)
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 V
VS
18V
15V
10V
5V
2
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
= I
2
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
:
Max. Frequency
220kHz
300kHz
640kHz
2MHz
P
L
= P
L1
+ P
L2
Quiescent Power Dissipation
Quiescent power dissipation (P
Q
, 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
Table 1: MIC4451 Maximum Operating Frequency
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:
P
L
= f C (V
S
)
2
where:
f = Operating Frequency
C = Load Capacitance
V
S
= Driver Supply Voltage
January 2011
10
M9999-011811
MICREL [ MICREL SEMICONDUCTOR ]
