HCPL-5120 Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – 2.0 Amp Output Current IGBT Gate Drive Optocoupler

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 7 mA to
achieve 10 kV/µs CMR.
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
applications requiring ultra high
CMR_L performance. Figure 32 is
an alternative drive circuit
which, like the recommended
application circuit (Figure 25),
does achieve ultra high CMR
performance by shunting the
LED in the off state.
+5 V
+
V_SAT
1
CLEDP
2
ILEDP
3
C_LEDN
4
SHIELD
8
0.1
µF
+
_
V _CC = 18 V
7
6
***
Rg
5
***
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING ∆dVCM/dt
+_
V _CM
Figure 30. Equivalent Circuit for Figure 25 During Common Mode Transient.
1
8
+5 V
C_LEDP
2
7
Q1
3
C_LEDN
6
ILEDN
4
5
SHIELD
Figure 31. Not Recommended Open Collector
Drive Circuit
1
8
+5 V
C_LEDP
2
7
3
6
C_LEDN
4
5
SHIELD
Figure 32. Recommended LED Drive Circuit for
Ultra-High CMR
14