15

Figure 22. TTL interface circuit for the HCPL-261A/-261N families.

420 Ω

(MAX)

HCPL-�261A fig 21

1

3

2

4

2N3906

(ANY PNP)

VCC

74L504

(ANY

TTL/CMOS

GATE)

HCPL-261X

LED

Table 1 indicates the directions of I

ing on the direction of the common-mode transient.

For transients occurring when the LED is on, common-

mode rejection (CMR

state) depends upon the amount of LED current drive (I

For conditions where I

old (I

balance each other. In other words, any condition where

common-mode transients cause a momentary decrease

in I

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

if an imbalance between I

I

optocoupler, the transient “signal”may cause the output

to spike below 2 V (which constitutes a CMR

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

and I

from transient coupling through C

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

2. Use of I

Using any one of the drive circuits in Figures 22-24 with

I

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.

Figure 21. AC equivalent circuit for HCPL-261X.

350 Ω

HCPL-�261A fig 20

1/2 RLED

VCC+

15 pF

+

VCM

8

7

6

1

3

SHIELD

5

2

4

CLA

VO

GND

0.01 µF

1/2 RLED

CLC

ILN

ILP

–