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DS20006092B-page 13

MIC4420/9

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:

EQUATION 4-3:

4.4.3

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:

EQUATION 4-4:

However, in this instance the RO 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 in Equation 4-5 in

which VD is the forward drop of the clamp diode in the

driver (generally around 0.7V).

EQUATION 4-5:

The two parts of the load dissipation must be summed

in to produce PL.

EQUATION 4-6:

4.4.4

QUIESCENT POWER DISSIPATION

Quiescent power dissipation (PQ, as described in the

Input Stage section) depends on whether the input is

high or low. A low input will result in a maximum current

drain (per driver) of ≤0.2 mA; a logic high will result in a

current drain of ≤2.0 mA. Quiescent power can

therefore be found from:

EQUATION 4-7:

4.4.5

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 +VS to ground. The transition power

dissipation is approximately:

EQUATION 4-8:

Total power dissipation (PD), then, as previously

described, is:

EQUATION 4-9:

PL f C



Where:

= Operating frequency.

= Load capacitance.

= Driver supply voltage.

PL1 I

2 R



PL2 I VD



1 D

–







PL PL1 PL2

PQ VS D IH



1 D

–













Where:

= Quiescent current with input high.

= Quiescent current with input low.

= Duty cycle.

= Power supply voltage.

PT 2 f



A s









Where:

A•s = A time-current factor derived from the typical

characteristic curves.

PD PL PQ PT

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