8
LED Drive Circuit
Considerations For Ultra
High CMR Performance
Without a detector shield, the
dominant cause of optocoupler
CMR failure is capacitive
coupling from the input side of
the optocoupler, through the
package, to the detector IC as
shown in Figure 14. The HCPL-
530X improves CMR
performance by using a detector
IC with an optically transparent
Faraday shield, which diverts the
capacitively 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 15.
This capacitive coupling causes
perturbations in the LED current
during common mode transients
and becomes the major source of
CMR failures for a shielded
optocoupler. 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 13), can achieve 10 kV/µs
CMR while minimizing compo-
nent complexity. Note that a
CMOS gate is recommended in
Figure 13 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 C
LEDO1
and
C
LEDO2
in Figure 15. 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
(CMR
L
)
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 I
TH
of 5.0 mA (see
Figure 1) to achieve 10 kV/µs
CMR. Capacitive coupling is
higher when the internal load
resistor is used (due to C
LEDO2
)
and an I
F
= 16mA is required to
obtain 10 kV/µs CMR.
The placement of the LED
current setting resistor affects 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 16 is connected to the
anode. Figure 17 shows the AC
equivalent circuit for Figure 16
during common mode transients.
During a +dV
CM/dt
in Figure 17,
the current available at the LED
anode (I
TOTAL
) is limited by the
series resistor. The LED current
(I
F
) is reduced from its DC value
by an amount equal to the current
that flows through C
LEDP
and
C
LEDO1
. The situation is made
worse because the current
through C
LEDO1
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 13) places the
current setting resistor in series
with the LED cathode. Figure 18
is the AC equivalent circuit for
Figure 13 during common mode
transients. In this case, the LED
current is not reduced during a
+dV
CM/dt
transient because the
current flowing through the
package capacitance is supplied
by the power supply. During a
-dV
CM/dt
transient, however, the
LED current is reduced by the
amount of current flowing
through C
LEDN
. But better CMR
performance is achieved since the
current flowing in C
LEDO1
during
a negative transient acts to keep
the output low.
Coupling to the LED and output
pins is also affected by the
connection 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 13), 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
(CMR
H
)
A high CMR LED drive circuit
must keep the LED off
(V
F
≤
V
F(OFF)
) during common
mode transients. For example,
during a +dV
CM/dt
transient in
Figure 18, the current flowing
through C
LEDN
is supplied by the
parallel combination of the LED
and series resistor. As long as the
voltage developed across the
resistor is less than V
F(OFF)
the
LED will remain off and no
HP [ AGILENT(HEWLETT-PACKARD) ]
