SINGLE CHANNEL PRODUCTS
GND BUS (BACK)
V
CC
BUS (FRONT)
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:
1. The enable pin is tied to V
CC
rather 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
LED
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
to either V
CC
or logic-level high
for best common-mode
performance with the output low
(CMR
L
). This 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 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
N.C.
ENABLE
(IF USED)
0.1µF
N.C.
N.C.
OUTPUT 1
0.1µF
ENABLE
(IF USED)
N.C.
OUTPUT 2
10 mm MAX. (SEE NOTE 16)
DUAL CHANNEL PRODUCTS
GND BUS (BACK)
V
CC
BUS (FRONT)
OUTPUT 1
0.1µF
OUTPUT 2
10 mm MAX. (SEE NOTE 16)
Figure 19. Recommended Printed Circuit Board Layout.
*
VCC
357
Ω
(MAX.)
357
Ω
(MAX.)
1
HCPL-261A/261N
8
0.01 µF
350
Ω
VCC+
2
7
3
6
VO
74LS04
OR ANY TOTEM-POLE
OUTPUT LOGIC GATE
GND1
4
SHIELD
5
GND
*
GND2
*
HIGHER CMR MAY BE OBTAINABLE BY CONNECTING PINS 1, 4 TO INPUT GROUND (GND1).
Figure 20. Recommended Drive Circuit for HCPL-261A/-261N Families for High-
CMR (Similar for HCPL-263A/-263N).
*Higher CMR May Be Obtainable by Connecting Pins 1, 4 to Input Ground (Gnd1).
1-178
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