15
Figure 22. TTL interface circuit for the HCPL-261A/-261N families.
Table 1 indicates the directions of I
LP
and I
LN
ow depend-
ing 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 thresh-
old (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 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 20, good
CMR can be achieved. (In the case of the -261N families,
a minimum CMR of 15 kV/µs is guaranteed using this cir-
cuit.) 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 “o” state (as shown in Figures 22
and 23). This is benecial 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 prac-
tices are followed. Figure 22 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-sink-
ing 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 in-
put-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-o of the optocoupler
LED.
Figure 21. AC equivalent circuit for HCPL-261X.
350 Ω
1/2 R
LED
V
CC
+
15 pF
+
V
CM
8
7
6
1
3
SHIELD
5
2
4
C
LA
V
O
GND
0.01 µF
1/2 R
LED
C
LC
I
LN
I
LP
–
420 Ω
(MAX)
1
3
2
4
2N3906
(ANY PNP)
V
CC
74L504
(ANY
TTL/CMOS
GATE)
HCPL-261X
LED
