This CMR optimization method fundamentally makes
use of the differential input capability of the LED input.
By ensuring the common mode impedance on both the
cathode and anode of the LED are balanced, it effectively
nullifies the effect of a CMR transient on the LED. This is
most easily achieved by splitting the input bias resistor
into two (as shown in Figure 9).
Table 1 indicates the directions of I and I
flow
LP
LN
depending on the direction of the common-mode
transient. For transients occurring when the LED is on,
common-mode rejection (CML, since the output is in the
“low” state) depends upon the amount of LED current
drive (I ). For conditions where I is close to the switching
F
F
threshold (I ), CML also depends on the extent which I
TH
LP
and I balance each other. In other words, any condition
where common-mode transients cause a momentary
LN
Split resistor configuration for ACPL-061L/ACPL-C61L/
ACNW261L
decrease in IF (i.e. when dV /dt>0 and |I | > |I N|,
CM
FP
F
referring to Table 1) will cause common-mode failure for
transients which are fast enough.
Figure 12 shows the recommended drive circuit for the
ACPL-061L/ACPL-C61L/ACNW261L for optimal common-
mode rejection performance. Two LED-current setting
resistors are used to balance the common mode impe-
dance at LED anode and cathode. Common-mode tran-
sients can capacitively couple from the LED anode (or
cathode) to the output-side ground causing current to be
shunted away from the LED (which can be bad if the LED
is on) or conversely cause current to be injected into the
LED (bad if the LED is meant to be off). Figure12 shows the
parasitic capacitances which exists between LED anode/
Likewise for common-mode transients which occur when
the LED is off (i.e. CM , since the output is “high”), if an
H
imbalance between I and I results in a transient I
LP
LN
F
equal to or greater than the switching threshold of the
optocoupler, the transient “signal” may cause the output
to spike below 2 V (which constitutes a CM failure).
H
The balanced I -setting resistors help equalize the
LED
common mode voltage change at anode and cathode
to reduce the amount by which I
is modulated from
LED
cathode and output ground (C and C ).
LA
LC
transient coupling through C and C
.
LC
LA
RT = R1 + R2, R1/R2 ≈ 1.5
1
2
8
7
VDD
Anode
R1
R2
VI
VE
0.1 µF
VO
6
5
3
4
GND1
Cathode
GND2
Shield
Figure 12. Recommended drive circuit for ACPL-061L/ACPL-C61L/ACNW261L for high-CMR
1
8
VDD
Anode
R1
2
VE
7
0.1 µF
CLA
R2
VO
6
5
3
4
Cathode
CLC
Shield
Figure 13. AC equivalent of ACPL-061L/ACPL-C61L/ACNW261L
16
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
