15
the LED quickly when the LED is
turned off. Upon turn-on of the
LED, the silicon diode capaci-
tance will provide a rapid
charging path (peaking current)
for the LED. In addition, this
silicon diode prevents common-
mode current from entering the
LED anode when the driver IC is
on and no operating LED current
exists.
Data Communication and
Input/Output Interfaces
In data communication, the
this optocoupler. First, use good
high-frequency circuit layout
practices to minimize coupling of
common-mode signals between
input and output circuits. Keep
input traces away from output
traces to minimize capacitive
coupling of interference between
input and output sections. If
HCPL-47XX can be used as a line
receiver on a RS-232-C line or
this optocoupler can be part of a
proprietary data link with low
input current, multi-drop stations
along the data path. Also, this
low-power optocoupler can be
used within equipment that
monitors the presence of high-
voltage. For example, a benefit of
the low input LED current (40
µA) helps the input sections of a
Programmable Logic Controller
(PLC) monitor proximity and limit
switches. The PLC I/O sections
can benefit from low input
current optocouplers because the
total input power dissipation
when monitoring the high voltage
(120 Vac - 220 Vac) inputs is
minimized at the I/O connections.
This is especially important when
many input channels are stacked
together.
possible, parallel, or shunt switch
the LED current as shown in
Figure 13, rather than series
In general, series switching the low
input current of the HCPL-47XX
LED is not recommended. This is
particularly valid when in a high
common-mode interference
switch the LED current as
illustrated in Figure 15. Not only
will CMR be enhanced with these
circuits (Figures 13 and 14), but
the switching speed of the opto-
coupler will be improved as well.
This is because in the parallel
switched case the LED current is
current-steered into or away from
the LED, rather than being fully
turned off as in the series switched
case. Figure 13 illustrates this
type of circuit. The Schottky
diode helps quickly to discharge
and pre-bias the LED in the off
state. If a common-mode voltage
across the optocoupler suddenly
attempts to inject a current into
the off LED anode, the Schottky
diode would divert the interfering
current to ground. The combina-
tion of the Schottky diode forward
voltage and the Vol saturation
voltage of the driver output stage
(on-condition) will keep the LED
voltage at or below 0.8 V. This will
prevent the LED (off-condition)
from conducting any significant
forward current that might cause
the HCPL-47XX to turn on. Also,
if the driver stage is an active
totem-pole output, the Schottky
diode allows the active output
pull-up section to disconnect from
the LED and pull high.
environment. However, if series
switching of the LED current must
be done, use an additional pull-up
resistor from the cathode of the
LED to the input VCC as shown in
Figure 15. This helps minimize any
differential-mode current from
conducting in the LED while the
LED is off, due to a common-mode
signal occurring on the input VCC
(anode) of the LED. The common-
mode signal coupling to the anode
and cathode could be slightly
different. This could potentially
create a LED current to flow that
would rival the normal, low input
current needed to operate the
optocoupler. This additional
parallel resistor can help shunt any
leakage current around the LED
should the drive circuit, in the off
state, have any significant leakage
current on the order of 40 µA.
With the use of this parallel
resistor, the total drive current
conducted when the LED is on is
the sum of the parallel resistor and
LED currents. In the series circuit
of Figure 15 with the LED off, if a
common-mode voltage were to
couple to the LED cathode, there
can be enough imbalance of
Circuit Design Issues
Power Supply Filtering
Since the HCPL-47XX is a high-
gain, split-Darlington amplifier,
any conducted electrical noise on
the VCC power supply to this
optocoupler should be minimized.
A recommended VCC filter circuit
is shown in Figure 12 to improve
the power supply rejection (psr)
of the optocoupler. The filter
should be located near the
combination of pin 8 and pin 5 to
provide best filtering action. This
filter will drastically limit any
sudden rate of change of VCC with
time to a slower rate that cannot
interfere with the optocoupler.
Common-Mode Rejection &
LED Driver Circuits
As shown in Figure 14, most
active output driver integrated
circuits can source directly the
forward current needed to operate
the LED of the HCPL-47XX
optocoupler. The advantage of
using the silicon diode in this
circuit is to conduct charge out of
common-mode voltage across the
LED to cause a LED current to
flow and, inadvertently, turn on the
optocoupler. This series, switching
circuit has no protection against a
negative-transition, input common-
mode signal.
With the combination of a high-
efficiency AlGaAs LED and a
high-gain amplifier in the HCPL-
47XX optocoupler, a few circuit
techniques can enhance the
common-mode rejection (CMR) of
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