14

Figure 20. Recommended drive circuit for HCPL-261A/-261N families for high-CMR (similar for HCPL-

263A/-263N).

Application Information

Common-Mode Rejection for HCPL-

261A/HCPL-261N Families:

Figure 20 shows the recommended

drive circuit for the HCPL-261N/-

261A for optimal common-mode

rejection performance. Two main

points to note are:

1. The enable pin is tied to V

than floating (this applies to

single-channel parts only).

2. Two LED-current setting resistors

are used instead of one. This is

to balance I

common-mode transients.

If the enable pin is left floating, it is

possible for common-mode tran-

sients to couple to the enable pin,

resulting in common-mode failure.

This failure mechanism only occurs

when the LED is on and the output

is in the Low State. It is identified as

occurring when the transient output

voltage rises above 0.8 V. Therefore,

the enable pin should be connected

to either V

best common-mode performance

with the output low (CMR

failure mechanism is only present

in single-channel parts (HCPL-261N,

-261A, -061N, -061A) which have the

enable function.

Also, common-mode transients can

capacitively couple from the LED an-

ode (or cathode) to the output-side

ground causing current to be shunt-

ed away from the LED (which can be

bad if the LED is on) or conversely

cause current to be injected into the

LED (bad if the LED is meant to be

off). Figure 21 shows the parasitic

capacitances which exists between

LED anode/cathode and output

ground (C

Figure 21 on the input side is an AC-

equivalent circuit.

*Higher CMR may be obtainable by connecting pins 1, 4 to input ground (Gnd1).

SINGLE CHANNEL PRODUCTS

0.01 µF

350 Ω

HCPL-261A fig 19

74LS04

OR ANY TOTEM-POLE

OUTPUT LOGIC GATE

VO

VCC+

8

7

6

1

3

SHIELD

5

2

4

HCPL-261A/261N

GND

GND2

357 Ω

(MAX.)

VCC

357 Ω

(MAX.)

*

*

GND1

Figure 19. Recommended printed circuit board layout.

DUAL CHANNEL PRODUCTS

ENABLE

(IF USED)

GND BUS (BACK)

V

BUS (FRONT)

CC

N.C.

N.C.

N.C.

N.C.

OUTPUT 1

OUTPUT 2

ENABLE

(IF USED)

0.1µF

0.1µF

10 mm MAX. (SEE NOTE 16)

GND BUS (BACK)

V

BUS (FRONT)

CC

OUTPUT 1

OUTPUT 2

0.1µF

10 mm MAX. (SEE NOTE 16)