Application Information
Common-Mode Rejection for
HCPL-260L Families:
Figure 13 shows the recom-
mended drive circuit 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 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 14 shows the
parasitic capacitances which
exists between LED
anode/cathode and output ground
(C
LA
and C
LC
). Also shown in
Figure 14 on the input side is an
AC-equivalent circuit.
VCC
220
Ω
*
1
HCPL-260L
8
0.01 F
350
Ω
VCC+
2
7
220
Ω
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 13. Recommended drive circuit for High-CMR.
1
1/2 R
LED
8
0.01 F
V
CC
+
350
Ω
2
I
LP
1/2 R
LED
C
LA
7
3
I
LN
6
15 pF
V
O
4
C
LC
5
SHIELD
GND
.
+
V
CM
–
Figure 14. AC equivalent circuit.
For transients occurring when the
LED is on, common-mode rejec-
tion (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 threshold (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
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 consti-
tutes a CMR
H
failure).
By using the recommended
circuit in Figure 13, good CMR
can be achieved. 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
6 − 116
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
