MIC4452YN datasheet(10/14 Pages) MICREL

10 / 14 page

Micrel Inc.

MIC4451/4452

January 2011

M9999-011811

Resistive Load Power Dissipation

Dissipation caused by a resistive load can be calculated

as:

PL = I

2 R

O D

where:

I = the current drawn by the load

RO = the output resistance of the driver when the output

is high, at the power supply voltage used. (See data

sheet)

D = fraction of time the load is conducting (duty cycle)

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

Max. Frequency

18V

220kHz

15V

300kHz

10V

640kHz

2MHz

Table 1: MIC4451 Maximum Operating Frequency

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:

PL = f C (VS)

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:

PL1 = I

2 R

O D

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:

PL2 = I VD (1 – D)

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

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

dissipation must be summed in to produce PL:

PL = PL1 + PL2

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:

PQ = VS [D IH + (1 – D) IL]

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

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