anode/cathode and output ground
(C
LA
and C
LC
). Also shown in
Figure 21 on the input side is an
AC-equivalent circuit. Table 1
indicates the directions of I
LP
and
I
LN
flow depending on the
direction of the common-mode
transient.
For transients occurring when the
LED is on, common-mode rejec-
tion (CMR
L
, since the output is in
the “low” state) depends upon the
amount of LED current drive (I
F
).
For conditions where I
F
is close
to the switching threshold (I
TH
),
CMR
L
also depends on the extent
which I
LP
and I
LN
balance each
other. In other words, any
condition where common-mode
transients cause a momentary
decrease in I
F
(i.e. when
dV
CM
/dt>0 and |I
FP
| > |I
FN
|,
referring to Table 1) will cause
common-mode failure for
transients which are fast enough.
Likewise for common-mode
transients which occur when the
LED is off (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 20, good CMR
can be achieved. (In the case of
the -261N families, a minimum
CMR of 15 kV/µs is guaranteed
using this circuit.) 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 20 may be enhanced by
following these guidelines:
1. Use of drive circuits where
current is shunted from the
LED in the LED “off” state (as
shown in Figures 22 and 23).
This is beneficial for good
CMR
H
.
2. Use of I
FH
> 3.5 mA. This is
good for high CMR
L
.
Using any one of the drive
circuits in Figures 22-24 with
I
F
= 10 mA will result in a typical
CMR of 8 kV/µs for the HCPL-
261N family, as long as the PC
board layout practices are
followed. Figure 22 shows a
circuit which can be used with
any totem-pole-output TTL/
LSTTL/HCMOS logic gate. The
buffer 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 current 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 23 may
be used. When using a CMOS
gate to drive the optocoupler, the
circuit shown in Figure 24 may
be used. The diode in parallel
with the R
LED
speeds the turn-off
of the optocoupler LED.
1
1/2 R
LED
8
0.01 µF
V
CC
+
350
Ω
2
I
LP
1/2 R
LED
C
LA
7
3
I
LN
6
15 pF
V
O
4
C
LC
5
SHIELD
GND
+
V
CM
–
Figure 21. AC Equivalent Circuit for HCPL-261X.
V
CC
HCPL-261X
420
Ω
(MAX)
2N3906
(ANY PNP)
1
2
LED
74L504
(ANY
TTL/CMOS
GATE)
3
4
Figure 22. TTL Interface Circuit for the HCPL-261A/-
261N Families.
1-179
OPTOCOUPLERS
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
