LED Drive Circuit Considerations for
Ultra High CMR Performance
Without a detector shield, the
dominant cause of optocoupler
CMR failure is capacitive
coupling from the input side of
the optocoupler, through the
package, to the detector IC as
shown in Figure 29. The HCPL-
3180 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
Figure 29. Optocoupler Input to Output
Capacitance Model for Unshielded
Optocouplers.
Figure 30. Optocoupler Input to Output
Capacitance Model for Shielded
Optocouplers.
circuitry. However, this shield
does not eliminate the capacitive
coupling between the LED and
optocoupler pins 5-8 as shown
in Figure 30. 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 transients. For example,
the recommended application
circuit (Figure 25), can achieve
10 kV/us CMR while minimizing
component complexity.
Vcc= 20V
Figure 31. Equivalent Circuit for Figure 25
During Common Mode Transient.
Techniques to keep the LED in
the proper state are discussed in
the next two sections.
Figure 32. Not Recommended Open Collector
Drive Circuit.
Figure 33. Recommended LED Drive Circuit for
Ultra-High CMR
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