HCPL3120000E AVAGO TECH, HCPL3120000E Datasheet - Page 22

• Manufacturer Part Number: HCPL3120000E
• Manufacturer: AVAGO TECH

+5 V
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 cur-
rent of 10 mA provides adequate
margin over the maximum I
5 mA to achieve 15 kV/ s CMR.
Figure 31. Equivalent Circuit for Figure 25 During
Common Mode Transient.
Figure 33. Recommended LED Drive
Circuit for Ultra-High CMR.
+5 V
V
+
–
H
SAT
).
1
2
3
4
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING –dV
1
2
3
4
C
C
LEDP
LEDN
SHIELD
C
C
I
LEDP
LEDP
LEDN
SHIELD
V
+
CM
–
FLH
of
8
7
6
5
CM
CMR with the LED Off
(CMR
A high CMR LED drive circuit
must keep the LED off (V
V
transients. For example, during a
-dV
the current flowing through C
also flows through the R
V
the low state voltage developed
across the logic gate is less than
V
and no common mode failure will
occur.
/dt.
SAT
F(OFF)
F(OFF)
8
7
6
5
cm
of the logic gate. As long as
/dt transient in Figure 31,
) during common mode
, the LED will remain off
L
).
0.1
µF
+
–
Rg
V
CC
= 18 V
• • •
• • •
SAT
F
and
LEDP
+5 V
Figure 32. Not Recommended Open
Collector Drive Circuit.
22
The open collector drive circuit,
shown in Figure 32, cannot keep
the LED off during a +dVcm/dt
transient, since all the current
flowing through C
supplied by the LED, and it is not
recommended for applications
requiring ultra high CMR
performance. Figure 33 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.
Q1
1
2
3
4
C
C
I
LEDN
LEDP
LEDN
SHIELD
LEDN
must be
L
8
7
6
5