MIC4421/4422

Micrel, Inc.

August 2005

9

M9999-081005

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:

P

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

characteristic curve “Crossover Energy vs. Supply Volt-

age.”

Total power (P

P

Definitions

C

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

Power supply current drawn by a driver when both

inputs are high and neither output is loaded.

I

Power supply current drawn by a driver when both

inputs are low and neither output is loaded.

I

Output current from a driver in Amps.

P

Total power dissipated in a driver in Watts.

P

Power dissipated in the driver due to the driver’s

load in Watts.

P

Power dissipated in a quiescent driver in Watts.

P

Power dissipated in a driver when the output

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.

R

Output resistance of a driver in Ohms.

V

Power supply voltage to the IC in Volts.

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 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

2

where:

f = Operating Frequency

C = Load Capacitance

V

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

2

R

However, in this instance the R

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

P

where V

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

must be summed in to produce P

L

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

≤0.2mA; a logic high will result in a current drain of ≤ 3.0mA.

Quiescent power can therefore be found from:

P

where:

I

quiescent current with input high

I

quiescent current with input low

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

V

power supply voltage