7
LED Drive Circuit Considerations For Ultra High CMR Per-
formance
Without a detector shield, the dominant cause of op-
tocoupler 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-M456
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 ca-
pacitive coupling between the LED and the optocoupler
output pin and output ground as shown in Figure 15. This
capacitive coupling causes perturbations in the LED cur-
rent 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 example, the rec-
ommended application circuit (Figure 13), can achieve
15 kV/µs CMR while minimizing component complexity.
Note that a CMOS gate is recommended in Figure 13
to keep the LED o when the gate is in the high state.
Another cause of CMR failure for a shielded optocou-
pler is direct coupling to the optocoupler output pins
through CLEDO1 in Figure 15. Many factors inuence the
eect and magnitude of the direct coupling including:
the position of the LED current setting resistor 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 cou-
pling 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 4.0 mA (see Figure 2) to achieve 15 kV/µs CMR.
The placement of the LED current setting resistor eects
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 +dVCM/dt in Figure 17, 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 ows through
CLEDP and CLEDO1. The situation is made worse because
the current through CLEDO1 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 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 +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 CLEDN. But, better CMR perfor-
mance is achieved since the current owing in CLEDO1
during a negative transient acts to keep the output low.
CMR With The LED O (CMRH)
A high CMR LED drive circuit must keep the LED o
(VF ≤ VF(OFF)) during common mode transients. For example,
during a +dVCM/dt transient in Figure 18, the current ow-
ing through CLEDN is supplied by the parallel combination
of the LED and series resistor. As long as the voltage de-
veloped across the resistor is less than VF(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 5-4 will keep the output from dipping below
the threshold. The recommended LED drive circuit (Figure
13) provides about 10 V of margin between the lowest op-
tocoupler output voltage and a 3 V IPM threshold during
a 15kV/µs transient with VCM = 1500 V. Additional margin
can be obtained by adding a diode in parallel with the
resistor, as shown by the dashed line connection in Fig-
ure 18, to clamp the voltage across the LED below VF(OFF).
Since the open collector drive circuit, shown in Figure 19,
cannot keep the LED o during a +dVCM/dt transient, it is
not desirable for applications requiring ultra high CMRH
performance. Figure 20 is the AC equivalent circuit for
Figure 19 during common mode transients. Essentially
all the current owing through CLEDN during a +dVCM/dt
transient must be supplied by the LED. CMRH failures can
occur at dv/dt rates where the current through the LED
and CLEDN exceeds the input threshold . Figure 21 is an
alternative drive circuit which does achieve ultra high
CMR performance by shunting the LED in the o state.