MIC4425 datasheet(13/26 Pages) MICROCHIP

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DS20006638A-page 13

MIC4423/4/5

4.9

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 as:

EQUATION 4-5:

The two parts of the load dissipation must be summed

in to produce PL.

EQUATION 4-6:

4.10 Quiescent Power Dissipation

Quiescent power dissipation (PQ, 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 ≤0.2 mA; a logic high will result in a

current drain of ≤2.0 mA. Quiescent power can

therefore be found from:

EQUATION 4-7:

4.11 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-Channel 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 (PD) then, is described as:

EQUATION 4-9:

Examples show the relative magnitude for each term.

PL1 I

2 R D



PL2 I VD 1 D

–







Where:

VD =

The forward drop of the clamp diode in the

driver (generally around 0.7V).

PL PL1 PL2

PQ VS D IH 1 D

–











Where:

IH =

Quiescent current with input high

IL =

Quiescent current with input low

D =

Fraction of time input is high (duty cycle)

VS =

Power supply voltage

PT f VS A s









Where:

(A x s) = is a time current factor derived from

Figure 4-1

PD PL PQ PT

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