HCPL-261A Datasheet, PDF (15/17 Pages) Agilent(Hewlett-Packard) – HCMOS Compatible, High CMR, 10 MBd Optocouplers

Table 1 indicates the directions of I_LP and I_LN flow depend-
ing on the direction of the common-mode transient.
For transients occurring when the LED is on, common-
mode rejec­tion (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 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 consti­tutes 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 “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 prac-
tices 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-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-off of the optocoupler
LED.
1
8
1/2 R_LED
2
7
I_LP
1/2 R_LED
3
I_LN
C_LA
6
4
C_LC
5
SHIELD
+
–
V_CM
Figure 21. AC equivalent circuit for HCPL-261X.
HCPL-�261A fig 20
0.01 µF
350 Ω
V_CC+
15 pF
V_O
GND
74L504
(ANY
TTL/CMOS
GATE)
V_CC
420 Ω
(MAX)
HCPL-261X
1
2N3906
2
(ANY PNP)
3
LED
4
Figure 22. TTL interface circuit for the HCPL-261A/-261N families.
HCPL-�261A fig 21
15