1-178

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 recom-

mended drive circuit for the

HCPL-261N/-261A for optimal

common-mode rejection

performance. Two main points to

note are:

rather than floating (this

applies to single-channel parts

only).

2. Two LED-current setting

resistors are used instead of

variation during common-

mode transients.

If the enable pin is left floating, it

is possible for common-mode

transients 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

for best common-mode

performance with the output low

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 anode (or cathode) to the

output-side ground causing

current to be shunted 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

*Higher CMR May Be Obtainable by Connecting Pins 1, 4 to Input Ground (Gnd1).

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)

0.1

µF

0.1

µF

10 mm MAX. (SEE NOTE 16)

SINGLE CHANNEL PRODUCTS

0.01 µF

350

Ω

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.

GND BUS (BACK)

V

BUS (FRONT)

CC

OUTPUT 1

OUTPUT 2

0.1

µF

10 mm MAX. (SEE NOTE 16)

DUAL CHANNEL PRODUCTS