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AV02-0616EN - July 11, 2014
Likewise for common-mode transients which occur
when the LED is o (i.e. CMR
H
, since the output is “high”),
if an imbalance between I
LP
and I
LN
results in a transient
I
F
equal to or greater than the switching threshold of the
optocoupler, the transient “signal” may cause the output
to spike below 2 V (which consti tutes a CMR
H
failure).
By using the recommended circuit in Figure 13, good
CMR can be achieved. The balanced I
LED
-setting resistors
help equalize I
LP
and I
LN
to reduce the amount by which
I
LED
is modulated from transient coupling through C
LA
and C
LC
.
CMR with Other Drive Circuits
CMR performance with drive circuits other than that
shown in Figure 13 may be enhanced by following these
guidelines:
1. Use of drive circuits where current is shunted from the
LED in the LED “o” state (as shown in Figures 15 and
16). This is benecial for good CMR
H
.
2. Use of I
FH
> 3.5 mA. This is good for high CMR
L
.
Figure 15 shows a circuit which can be used with any
totem-pole-output TTL/LSTTL/HCMOS logic gate. The
buer PNP transistor allows the circuit to be used with
logic devices which have low current-sinking capability.
It also helps maintain the driving-gate power-supply cur-
rent at a constant level to minimize ground shifting for
other devices connected to the input-supply ground.
When using an open-collector TTL or open-drain CMOS
logic gate, the circuit in Figure 16 may be used. When
using a CMOS gate to drive the optocoupler, the circuit
shown in Figure 17 may be used. The diode in parallel
with the R
LED
speeds the turn-o of the optocoupler
LED.
Figure 15. TTL interface circuit
Figure 16. TTL open-collector/open-drain gate drive circuit
Figure 17. CMOS gate drive circuit
420 Ω
(MAX)
1
3
2
4
2N3906
(ANY PNP)
V
CC
74L504
(ANY
TTL/CMOS
GATE)
HCPL-260L
LED
R
1
3
2
4
V
CC
74HC00
(OR ANY
OPEN-COLLECTOR/
OPEN-DRAIN
LOGIC GATE)
HCPL-260L
LED
220 Ω
1
3
2
4
V
CC
74HC04
(OR ANY
TOTEM-POLE
OUTPUT LOGIC
GATE)
HCPL-260L
1N4148
LED
