Datasheet

18
Figure 36. Not Recommended Open Collector Drive Circuit.
The open collector drive circuit, shown in Figure 36,
cannot keep the LED o during a +dV
cm
/dt transient, since
all the current owing through C
LEDN
must be supplied
by the LED, and it is not recommended for applications
requiring ultra high CMR
L
performance. Figure 37 is an
alternative drive circuit which, like the recommended
application circuit (Figure 29), does achieve ultra high CMR
performance by shunting the LED in the o state.
Figure 35. Equivalent Circuit for Figure 29 During Common Mode Tran-
sient.
CMR with the LED On (CMR
H
)
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
maximum I
FLH
of 5 mA to achieve 40 kV/μs CMR.
CMR with the LED O (CMR
L
)
A high CMR LED drive circuit must keep the LED o (V
F
≤ V
F(OFF)
) during common mode transients. For example,
during a -dV
cm
/dt transient in Figure 35, the current
owing through C
LEDP
also ows through the R
SAT
and
V
SAT
of the logic gate. As long as the low state voltage
developed across the logic gate is less than V
F(OFF)
, the LED
will remain o and no common mode failure will occur.
Figure 34. Optocoupler Input to Output Capacitance Model for Shielded
Optocouplers.
Figure 33. Optocoupler Input to Output Capacitance Model for Unshield-
ed Optocouplers.
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
SHIELD
C
LEDO1
C
LEDO2
Rg
1
3
V
SAT
2
4
8
6
7
5
+
V
CM
I
LEDP
C
LEDP
C
LEDN
SHIELD
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING - dV
CM
/dt.
+5 V
+
-
V
CC
= 18 V
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0.1
µF
+
-
-
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
SHIELD
+5 V
Q1
I
LEDN
LED Drive Circuit Considerations for Ultra High CMR Per-
formance. (Discussion applies to ACPL-3130, ACPL-J313,
and ACNW3130)
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 33. The ACPL-3130 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 capacitive
coupling between the LED and optocoupler pins 5-8
as shown in Figure 34. 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 o) during common mode transients.
For example, the recommended application circuit
(Figure 29), can achieve 40 kV/µs CMR while minimizing
component complexity.
Techniques to keep the LED in the proper state are dis-
cussed in the next two sections.