MIC4123/4124/4125

Micrel, Inc.

M9999-052405

9

May 2005

P

Q

= V

= 12 x [(0.5 x 0.0035) + (0.5 x 0.0003)]

= 0.0228W

Total power dissipation, then, is:

P

D

= 0.2160 + 0.0066 + 0.0228

= 0.2454W

Assuming an E-Pad SOIC package, with an θ

this will result in the junction running at:

0.2454 x 58 = 14.2°C

above ambient, which, given a maximum ambient tempera-

ture of 60°C, will result in a maximum junction temperature

of 89.4°C.

EXAMPLE 2: A MIC4124 operating on a 15V input, with one

driver driving a 50Ω resistive load at 1MHz, with a duty cycle

of 67%, and the other driver quiescent, in a maximum ambi-

ent temperature of 40°C:

P

2

x R

First, I

I

Given R

I

I

and:

P

L

= 0.174W

P

T

= F x V

(because only one side is operating)

= 0.025 W

and:

P

(1 x 0.000125)]

(this assumes that the unused side of the driver has its input

grounded, which is more efficient)

= 0.015W

then:

P

D

= 0.174 + 0.025 + 0.0150

= 0.213W

In a MLF with an θ

in a junction temperature given the maximum 40°C ambient

of:

(0.213 x 100) + 40 = 52.8°C

The actual junction temperature will be lower than calculated

both because duty cycle is less than 100% and because the

graph lists R

will be somewhat lower.

Definitions

C

D = Duty Cycle expressed as the fraction of time the

input

to the driver is high.

f = Operating Frequency of the driver in Hertz.

I

inputs are high and neither output is loaded.

I

inputs are low and neither output is loaded.

I

P

P

load

in Watts.

P

P

changes

states (“shoot-through current”) in Watts.

NOTE: The “shoot-through” current from a dual

transition (once up, once down) for both drivers is

stated in the graph on the following page in ampere-

nanoseconds. This figure must be multiplied by the

number of repetitions per second (frequency to find

Watts).

R

V