Micrel, Inc.

MIC4421A/4422A

June 2007

10

M9999-062707

Table 1. MIC4421A Maximum Operating Frequency

Max Frequency

18V

220kHz

15V

300kHz

10V

640kHz

5V

2MHz

Conditions:

1.

2.

3.

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:

E = 1/2 C V

2

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:

PL = f C (VS)2

where:

f =

Operating Frequency

C =

Load Capacitance

VS =

Driver Supply Voltage

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 inductor is forcing current through the

driver, dissipation is best described as:

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

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 ≤0.2mA; a logic high will result in a current

drain of ≤3.0mA.

Quiescent power can therefore be found from:

where:

Quiescent current with input high

Quiescent current with input low

D =

Fraction of time input is high (duty cycle)

Power supply voltage

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

approximately:

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

typical characteristic curve “Crossover Energy vs.

Supply Voltage.”

Definitions

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.

Power supply current drawn by a driver

when both inputs are high and neither output

is loaded.

Power supply current drawn by a driver

when both inputs are low and neither output

is loaded.

Output current from a driver in Amps.

Total power dissipated in a driver in Watts.

Power dissipated in the driver due to the

driver’s load in Watts.

Power dissipated in a quiescent driver in

Watts.