20
LED Drive Circuit
Considerations for Ultra
High CMR Performance.
(Discussion applies to HCPL-
3120, HCPL-J312, and
by using a detector IC with an
optically transparent Faraday
shield, which diverts the capaci-
tively coupled current away from
the sensitive IC circuitry. How-
ever, this shield does not
eliminate the capacitive coupling
between the LED and optocoup-
ler pins 5-8 as shown in
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
15 kV/µs CMR while minimizing
component complexity.
HCNW3120)
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-
3120 improves CMR performance
Figure 30. This capacitive
coupling causes perturbations in
the LED current during common
mode transients and becomes the
major source of CMR failures for
Techniques to keep the LED in
the proper state are discussed in
the next two sections.
C
1
2
3
4
8
7
6
5
1
8
7
6
5
LEDO1
C
C
C
LEDP
LEDP
2
C
LEDO2
3
4
C
LEDN
LEDN
SHIELD
Figure 29. Optocoupler Input to Output
Capacitance Model for Unshielded Optocouplers.
Figure 30. Optocoupler Input to Output
Capacitance Model for Shielded Optocouplers.
ETC [ ETC ]
