HCPL-3180-500E Datasheet, PDF (14/16 Pages) AVAGO TECHNOLOGIES LIMITED – 2.5 Amp Output Current, High Speed, Gate Drive Optocoupler

LED Drive Circuit Considerations for Ultra High CMR Performance
Without a detector shield, the dominant cause of op-
tocoupler CMR failure is capacitive coupling from the
input side of the optocoupler, through the package, to
the detector IC as shown in Figure 28. 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
circuitry. However, this shield does not eliminate the ca-
pacitive coupling between the LED and optocoupler pins
5-8 as shown in Figure 29. 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 25), 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.
1
8
C_LEDP
2
7
3
6
C_LEDN
4
5
Figure 28. Optocoupler input to output capacitance model for
unshielded optocouplers.
1
C_LEDO1
8
C_LEDP
2
3
C_LEDN
7
C_LEDO2
6
4
5
SHIELD
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
over-driving 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
8 mA to achieve 10 kV/µs CMR.
+5 V
+
V_SAT
–
1
C_LEDP
2
I_LEDP
3
C_LEDN
4
SHIELD
8
0.1
7
µF
+
–
V_CC = 20 V
6
•••
R_g
5
•••
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING –dV_CM/dt.
+–
V_CM
Figure 30. Equivalent circuit for Figure 25 during common mode transient.
CMR with the LED Off (CMR_L)
A high CMR LED drive circuit must keep the LED off (V_F
≤ V_F(OFF)) during common mode transients. For example,
during a -dV_CM/dt transient in Figure 30, the current
flowing through C_LEDP also flows 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 off and no common mode failure will
occur.
The open collector drive circuit, shown in Figure 31,
cannot keep the LED off during a +dV_CM/dt transient,
since all the current flowing through C_LEDN must be
supplied by the LED, and it is not recommended for ap-
plications requiring ultra high CMR_L performance. Figure
32 is an alternative drive circuit, which like the recom-
mended application circuit (Figure 25), does achieve
ultra high CMR performance by shunting the LED in the
off state.
Figure 29. Optocoupler input to output capacitance model for
shielded optocouplers.
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