acpl-w314 Avago Technologies, acpl-w314 Datasheet - Page 12

• Manufacturer Part Number: acpl-w314
• Description: 0.4 Amp Output Current Igbt Gate Driver Optocouplers
• Manufacturer: Avago Technologies

LED Drive Circuit Considerations for Ultra High CMR Per-
formance
Without a detector shield, the dominant cause of optocou-
pler 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. How-
ever, this shield does not eliminate the capacitive 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 transients. For ex-
ample, 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 19. Optocoupler Input to Output Capacitance Model for Unshielded Optocou-
plers.
Figure 20. Optocoupler Input to Output Capacitance Model for Shielded Optocouplers.
CMR with the LED On (CMR
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 7 mA provides adequate margin over the
maximum I
12
1
1
2
2
3
3
1
1
2
2
3
3
C
C
C
C
LEDP
LEDP
LEDN
LEDN
C
C
C
C
LEDP
LEDP
LEDN
LEDN
C
C
FLH
LED01
LED01
SHIELD
SHIELD
of 5 mA to achieve 10 kV/µs CMR.
C
C
LED02
LED02
H
)
6
6
5
5
4
4
6
6
5
5
4
4
CMR with the LED Off (CMR
A high CMR LED drive circuit must keep the LED off (V
V
during a -dV
ing through C
the logic gate. As long as the low state voltage developed
across the logic gate is less than V
off and no common mode failure will occur.
Figure 21. Equivalent Circuit for Figure 15 During Common Mode Transient.
The open collector drive circuit, shown in Figure 22, can
not keep the LED off during a +dV
the current flowing through C
LED, and it is not recommended for applications requiring
ultra high CMR
which like the recommended application circuit (Figure
17), does achieve ultra high CMR performance by shunting
the LED in the off state.
Figure 22. Not Recommended Open Collector Drive Circuit.
Figure 23. Recommended LED Drive Circuit for Ultra-High CMR Dead Time and Propa-
gation Delay Specifications.
+ 5 V
F(OFF)
+5 V
+5 V
+5 V
+5 V
Q1
Q1
Q1
) during common mode transients. For example,
1
1
2
2
3
3
+
+
+
-
-
-
V
V
V
SAT
SAT
SAT
1
1
2
2
3
3
CM
C
C
C
C
LEDP
LEDP
LEDN
LEDN
LEDP
I
I
L
/dt transient in Figure 21, the current flow-
LEDN
LEDN
C
C
1
2
3
•
LEDP
LEDP
performance. The alternative drive circuit
•
THE ARROWS INDICATE THE DIRECTION
C
C
OF CURRENT FLOW DURING - dV
LEDN
LEDN
also flows through the R
C
C
SHIELD
LEDP
LEDN
I
I
LEDP
SHIELD
SHIELD
SHIELD
L
)
V
CM
LEDN
F(OFF)
CM
must be supplied by the
CM
/dt transient, since all
6
6
5
5
4
4
/ dt
the LED will remain
6
6
5
5
4
4
0.1 µF
6
5
4
SAT
and V
+
+
-
-
V
CC
= 18V
R
SAT
g
F
of
≤