July 2005

9

MIC4423/4424/4425

MIC4423/4424/4425

Micrel, Inc.

frequency, power supply voltage, and load all affect power

dissipation.

Given the power dissipation in the device, and the thermal

resistance of the package, junction operating temperature

for any ambient is easy to calculate. For example, the ther-

mal resistance of the 8-pin plastic DIP package, from the

datasheet, is 150°C/W. In a 25°C ambient, then, using a

maximum junction temperature of 150°C, this package will

dissipate 960mW.

Accurate power dissipation numbers can be obtained by sum-

ming the three sources of power dissipation in the device:

• Load power dissipation (P

• Quiescent power dissipation (P

• Transition power dissipation (P

Calculation of load power dissipation differs depending on

whether the load is capacitive, resistive or inductive.

Resistive Load Power Dissipation

Dissipation caused by a resistive load can be calculated as:

P

2

R

where:

I = the current drawn by the load

R

output is high, at the power supply voltage used

(See characteristic curves)

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:

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:

P

2

where:

f = Operating Frequency

C = Load Capacitance

V

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:

P

2

R

However, in this instance the R

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

P

where V

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

must be summed in to produce P

L

P

Quiescent Power Dissipation

Quiescent power dissipation (P

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 ≤2.0mA.

Quiescent power can therefore be found from:

P

where:

I

I

D = fraction of time input is high (duty cycle)

V

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 V

power dissipation is approximately:

P

where (A•s) is a time-current factor derived from Figure 2.

P

Examples show the relative magnitude for each term.

EXAMPLE 1: A MIC4423 operating on a 12V supply driving

two capacitive loads of 3000pF each, operating at 250kHz,

with a duty cycle of 50%, in a maximum ambient of 60°C.

First calculate load power loss:

P

2

P

–9

= 0.2160W

Then transition power loss:

P