
20
LED Drive Circuit Considerations for
Ultra High CMR Performance
Without a detector shield, the
dominant cause of optocoupler
CMR failure is capacitive coupl-
ing from the input side of the
optocoupler, through the package,
to the detector IC as shown in
Figure 16. The HCPL-4506 series
improve CMR performance by
using a detector IC with an
optically transparent Faraday
shield, which diverts the capaci-
tively coupled current away from
the sensitive IC circuitry.
However, this shield does not
eliminate the capacitive coupling
between the LED and the opto-
coupler output pins and output
ground as shown in Figure 17.
This capacitive coupling causes
perturbations in the LED current
during common mode transients
and becomes the major source of
CMR failures for a shielded opto-
coupler. The main design
objective of a high CMR LED drive
circuit becomes keeping the LED
in the proper state (on or off)
during common mode transients.
For example, the recommended
application circuit (Figure 15),
can achieve 15 kV/µs CMR while
minimizing component com-
plexity. Note that a CMOS gate is
recommended in Figure 15 to
keep the LED off when the gate is
in the high state.
Another cause of CMR failure for
a shielded optocoupler is direct
coupling to the optocoupler
output pins through CLEDO1 and
CLEDO2 in Figure 17. Many factors
influence the effect and magni-
tude of the direct coupling
including: the use of an internal
or external output pull-up
resistor, the position of the LED
current setting resistor, the
connection of the unused input
package pins, and the value of the
capacitor at the optocoupler
output (CL).
Techniques to keep the LED in the
proper state and minimize the
effect of the direct coupling are
discussed in the next two
sections.
CMR with the LED On (CMRL)
A high CMR LED drive circuit
must keep the LED on during
common mode transients. This is
achieved by overdriving the LED
current beyond the input
threshold so that it is not pulled
below the threshold during a
transient. The recommended
minimum LED current of 10 mA
provides adequate margin over
the maximum ITH of 5.0 mA (see
Figure 1) to achieve 15 kV/µs
CMR. Capacitive coupling is
higher when the internal load
resistor is used (due to CLEDO2)
and an IF = 16 mA is required to
obtain 10 kV/µs CMR.
The placement of the LED current
setting resistor effects the ability
of the drive circuit to keep the
LED on during transients and
interacts with the direct coupling
to the optocoupler output. For
example, the LED resistor in
Figure 18 is connected to the
anode. Figure 19 shows the AC
equivalent circuit for Figure 18
during common mode transients.
During a +dVcm/dt in Figure 19,
the current available at the LED
anode (Itotal) is limited by the
series resistor. The LED current
(IF) is reduced from its DC value
by an amount equal to the current
that flows through CLEDP and
CLEDO1. The situation is made
worse because the current
through CLEDO1 has the effect of
trying to pull the output high
(toward a CMR failure) at the
same time the LED current is
being reduced. For this reason,
the recommended LED drive
circuit (Figure 15) places the
current setting resistor in series
with the LED cathode. Figure 20
is the AC equivalent circuit for
Figure 15 during common mode
transients. In this case, the LED
current is not reduced during a
+dVcm/dt transient because the
current flowing through the
package capacitance is supplied
by the power supply. During a
-dVcm/dt transient, however, the
LED current is reduced by the
amount of current flowing
through CLEDN. But, better CMR
performance is achieved since the
current flowing in CLEDO1 during a
negative transient acts to keep the
output low.
Coupling to the LED and output
pins is also affected by the con-
nection of pins 1 and 4. If CMR is
limited by perturbations in the
LED on current, as it is for the
recommended drive circuit
(Figure 15), pins 1 and 4 should
be connected to the input circuit
common. However, if CMR
performance is limited by direct
coupling to the output when the
LED is off, pins 1 and 4 should be
left unconnected.
CMR with the LED Off (CMRH)
A high CMR LED drive circuit
must keep the LED off
(VF≤VF(OFF)) during common
mode transients. For example,
during a +dVcm/dt transient in
Figure 20, the current flowing
through CLEDN is supplied by the
parallel combination of the LED
and series resistor. As long as the
voltage developed across the
resistor is less than VF(OFF) the