20
LED Drive Circuit Considerations for Ultra High CMR Performance
Without a detector shield, the dominant cause of op-
tocoupler 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 capacitively coupled current away from the
sensitive IC circuitry. However, this shield does not elimi-
nate the capacitive coupling between the LED and the
optocoupler 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
optocoupler. The main design objective of a high CMR
LED drive circuit becomes keeping the LED in the proper
state (on or o) during common mode transients. For ex-
ample, the recommended application circuit (Figure 15),
can achieve 15 kV/µs CMR while minimizing component
complexity. Note that a CMOS gate is recommended in
Figure 15 to keep the LED o 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 17. Many factors inuence
the eect and magnitude of the direct coupling includ-
ing: the use of an internal or external output pull-up re-
sistor, 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 (C
L
).
Techniques to keep the LED in the proper state and mini-
mize the eect 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 dur-
ing common mode transients. This is achieved by over-
driving the LED current beyond the input threshold so
that it is not pulled below the threshold during a tran-
sient. 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 15 kV/µs CMR. Capacitive
coupling is higher when the internal load resistor is used
(due to C
LEDO2
) and an I
F
= 16 mA is required to obtain
10 kV/µs CMR.
The placement of the LED current setting resistor eects
the ability of the drive circuit to keep the LED on dur-
ing 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 (I
F
) is reduced from its DC value
by an amount equal to the current that ows through
C
LEDP
and C
LEDO1
. The situation is made worse because
the current through C
LEDO1
has the eect 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 owing through the package capacitance is sup-
plied by the power supply. During a -dVcm/dt transient,
however, the LED current is reduced by the amount of
current owing through C
LEDN
. But, better CMR perfor-
mance is achieved since the current owing in C
LEDO1
during a negative transient acts to keep the output low.
Coupling to the LED and output pins is also aected by
the connection of pins 1 and 4. If CMR is limited by per-
turbations in the LED on current, as it is for the recom-
mended 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 o, pins 1 and 4 should be left uncon-
nected.
CMR with the LED O (CMR
H
)
A high CMR LED drive circuit must keep the LED o
(V
F
≤ V
F(OFF)
) during common mode transients. For ex-
ample, during a +dVcm/dt transient in Figure 20, the
current owing 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 o and no common mode failure will
occur. Even if the LED momentarily turns on, the 100 pF
capacitor from pins 6-5 will keep the output from dip-
ping below the threshold. The recommended LED drive
circuit (Figure 15) provides about 10 V of margin between
the lowest optocoupler output voltage and a 3 V IPM
threshold during a 15 kV/µs transient with V
CM
= 1500 V.
Additional margin can be obtained by adding a diode
in parallel with the resistor, as shown by the dashed line
connection in Figure 20, to clamp the voltage across the
LED below V
F(OFF)
.
Since the open collector drive circuit, shown
in Figure 21, cannot keep the LED o during
a +dVcm/dt transient, it is not desirable for applications
requiring ultra high CMR
H
performance. Figure 22 is the
AC equivalent circuit for Figure 21 during common mode
transients. Essentially all the current owing through
C
LEDN
during a +dVcm/dt transient must be supplied by
the LED. CMR
H
failures can occur at dV/dt rates where
the current through the LED and C
LEDN
exceeds the input
threshold. Figure 23 is an alternative drive circuit which
does achieve ultra high CMR performance by shunting
the LED in the o state.
