MIC4120/4129

Micrel, Inc.

M9999-081105

8

August 2005

Transition Power Dissipation

Transition power is dissipated in the driver each time its out-

put 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

power dissipation is approximately:

where (A•s) is a time-current factor derived from the typical

characteristic curves.

Definitions

D = Duty Cycle expressed as the fraction of time the

input to the driver is high.

f = Operating Frequency of the driver in Hertz.

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 shown by the "Typical Characteristic Curve":

Crossover Area vs. Supply Voltage and is in am-

pere-seconds. This figure must be multiplied by

the number of repetitions per second (frequency)

to find Watts.

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:

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:

f = Operating Frequency

C = Load Capacitance

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:

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 induc-

tor is forcing current through the driver, dissipation is best

described as

(generally around 0.7V). The two parts of the load dissipation

Quiescent Power Dissipation

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:

where:

D = fraction of time input is high (duty cycle)