MIC4423/4424/4425
in estimating power dissipation in the driver. Operating
frequency, power supply voltage, and load all affect power
dissipation.
Given the power dissipation in the device, and the thermal
resistance of the package, junction operating temperature for
any ambient is easy to calculate. For example, the thermal
resistance of the 8-pin plastic DIP package, from the datasheet,
is 150°C/W. In a 25°C ambient, then, using a maximum
junction temperature of 150°C, this package will dissipate
960mW.
Accurate power dissipation numbers can be obtained by
summing the three sources of power dissipation in the device:
• Load power dissipation (P
L
)
• Quiescent power dissipation (P
Q
)
• Transition power dissipation (P
T
)
Calculation of load power dissipation differs depending on
whether the load is capacitive, resistive or inductive.
Micrel
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
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
≤2.0mA.
Quiescent power can therefore be found from:
P
Q
= V
S
[D I
H
+ (1 – D) I
L
]
where:
I
H
=
I
L
=
D=
V
S
=
quiescent current with input high
quiescent current with input low
fraction of time input is high (duty cycle)
power supply voltage
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 characteristic curves)
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
2
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:
P
L
= f C (V
S
)
2
where:
f = Operating Frequency
C = Load Capacitance
V
S
= Driver 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
= f V
S
(A•s)
where (A•s) is a time-current factor derived from Figure 2.
Total power (P
D
) then, as previously described is just
P
D
= P
L
+ P
Q
+P
T
Examples show the relative magnitude for each term.
EXAMPLE 1: A MIC4423 operating on a 12V supply driving
two capacitive loads of 3000pF each, operating at 250kHz,
with a duty cycle of 50%, in a maximum ambient of 60°C.
First calculate load power loss:
P
L
= f x C x (V
S
)
2
P
L
= 250,000 x (3 x 10
–9
+ 3 x 10
–9
) x 12
2
= 0.2160W
Then transition power loss:
P
T
= f x V
S
x (A•s)
= 250,000 • 12 • 2.2 x 10
–9
= 6.6mW
Then quiescent power loss:
P
Q
= V
S
x [D x I
H
+ (1 – D) x I
L
]
9
MIC4423/4424/4425
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
January 1999
MICREL [ MICREL SEMICONDUCTOR ]
