14

A high CMR LED drive circuit

must keep the LED on during

common mode transients. This

is achieved by overdriving the

LED current beyond the input

threshold so that it is not

pulled below the threshold

during a transient. A minimum

LED current of 10 mA provides

adequate margin over the

achieve 10 kV/

A high CMR LED drive circuit

transients. For example, during

30, the current flowing through

As long as the low state voltage

developed across the logic gate

will remain off and no common

mode failure will occur.

Figure 30. Equivalent Circuit for Figure 25 During Common Mode Transient

Rg

1

3

2

4

8

6

7

5

+

V CM

SHIELD

* THE ARROWS INDICATE THE DIRECTION

OF CURRENT FLOW DURING

−dVCM/dt

+5 V

+

V CC = 18 V

* * *

0.1

µF

+

_

_

* * *

_

The open collector drive circuit,

shown in Figure 31, cannot keep

transient, since all the current

supplied by the LED, and it is

not recommended for

applications requiring ultra high

an alternative drive circuit

which, like the recommended

application circuit (Figure 25),

does achieve ultra high CMR

performance by shunting the

LED in the off state.

Figure 31. Not Recommended Open Collector

Drive Circuit

Figure 32. Recommended LED Drive Circuit for

Ultra-High CMR

1

3

2

4

8

6

7

5

SHIELD

+5 V

Q1

1

3

2

4

8

6

7

5

SHIELD

+5 V