12

A high CMR LED drive circuit must keep the LED on during

common mode transients. This is achieved by overdriv-

ing the LED current beyond the input threshold so that

it is not pulled below the threshold during a transient. A

minimum LED current of 7 mA provides adequate margin

LED Drive Circuit Considerations for Ultra High CMR Per-

formance

Without a detector shield, the dominant cause of opto-

coupler CMR failure is capacitive coupling from the input

side of the optocoupler, through the package, to the

detector IC as shown in Figure 19. The ACPL-P302/W302

improves CMR performance by using a detector IC with

an optically transparent Faraday shield, which diverts the

capacitively coupled current away from the sensitive IC

circuitry. However, this shield does not eliminate the ca-

pacitive coupling between the LED and optocoupler pins

5-8 as shown in Figure 20. This capacitive coupling causes

perturbations in the LED current during common mode

transients and becomes the major source of CMR failures

for a shielded optocoupler. The main design objective of

a high CMR LED drive circuit becomes keeping the LED in

the proper state (on or off ) during common mode tran-

sients. For example, the recommended application circuit

(Figure 17), can achieve 10 kV/μs CMR while minimizing

component complexity.

Techniques to keep the LED in the proper state are dis-

cussed in the next two sections.

Figure 21. Equivalent Circuit for Figure 15 During Common Mode Transient.

The open collector drive circuit, shown in Figure 22, can

by the LED, and it is not recommended for applications

drive circuit which like the recommended application

circuit (Figure 17), does achieve ultra high CMR perfor-

mance by shunting the LED in the off state.

Figure 19. Optocoupler Input to Output Capacitance Model for Unshielded

Optocouplers.

Figure 20. Optocoupler Input to Output Capacitance Model for Shielded

Optocouplers.

Figure 22. Not Recommended Open Collector Drive Circuit.

will remain off and no common mode failure will occur.

Figure 23. Recommended LED Drive Circuit for Ultra-High CMR Dead Time

and Propagation Delay Specifications.

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