MIC4451 datasheet(12/24 Pages) MICROCHIP

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MIC4451/52

DS20006616A-page 12



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In this instance, however, 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:

4.8

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 ≤3.0 mA. Quiescent power can

therefore be derived from:

EQUATION 4-7:

4.9

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 (PD) then, as previously described is:

EQUATION 4-9:

4.10 Definitions

CL = 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

IH = Power supply current drawn by a driver when both

inputs are high and neither output is loaded.

IL = Power supply current drawn by a driver when both

inputs are low and neither output is loaded.

ID = Output current from a driver in Amps.

PD = Total power dissipated in a driver in Watts.

PL = Power dissipated in the driver due to the driver’s

load in Watts.

PQ = Power dissipated in a quiescent driver in Watts.

PT = Power dissipated in a driver when the output

changes states (shoot-through current) in watts.

RO = Output resistance of a driver in Ohms.

VS = Power supply voltage to the IC in volts.

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 the input is high (duty cycle).

VS = Power supply voltage.

PT 2 f



A s









Where:

(A x s) = A time-current factor derived from Figure 2-6

PD PL PQ PT

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