acpl-w70l Avago Technologies, acpl-w70l Datasheet - Page 12

• Manufacturer Part Number: acpl-w70l
• Description: Single-channel And Dual-channel High Speed 15 Mbd Cmos Optocoupler With Glitch-free Power-up Feature
• Manufacturer: Avago Technologies

Common Mode Rejection for ACPL-W70L AND ACPL-K73L
Figure 13 shows the recommended driving circuit for the
ACPL-W70L and ACPL-K73L for optimal common-mode re-
jection performance. Two LED-current setting resistors are
used instead of one. This is to balance the common mode
impedance at LED anode and cathode. Common-mode
transients 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 ). Figure14 shows the
parasitic capacitances which exists between LED anode/
cathode and output ground (C
Figure 14 on the input side is an AC-equivalent circuit.
Table 1 indicates the directions of I
ing on the direction of the common-mode transient. For
transients occurring when the LED is on, common-mode
rejection (CM
pends upon the amount of LED current drive (I
ditions where I
CM
Figure 13. Recommended drive circuit for ACPL-W70L and ACPL-K73L for high-CMR
12
74LS04 OR ANY TOTEM-
POLE OUTPUT LOGIC GATE
Figure 14. AC equivalent of ACPL-W70L and ACPL-K73L
L
V
½ R
½ R
also depends on the extent which I
DD
total
total
L
, since the output is in the “low” state) de-
V
F
DD1
SHIELD
is close to the switching threshold (I
I
GND
LN
I
C
1
LP
LC
C
LA
LA
1/2R
1/2R
and C
LP
530 Ω
total
total
and I
R
LC
LP
total
). Also shown in
LN
0.1µF
and I
=300Ω for V
= 580Ω for V
SHIELD
1
flow depend-
F
LN
ACPL-W70L
). For con-
balance
15pF
DD
DD
V
GND
V
TH
=3.3V
O
DD2
=5V
),
2
each other. In other words, any condition where common-
mode transients cause a momentary decrease in I
when dV
cause common-mode failure for transients which are fast
enough.
Likewise for common-mode transients which occur when
the LED is off (i.e. CM
balance between I
to or greater than the switching threshold of the optocou-
pler, the transient “signal” may cause the output to spike
below 2V (which constitutes a CM
By using the recommended circuit in Figure 13, good CM
can be achieved. The resistors recommended in Figure 13
include both the output impedence of the logic driver cir-
cuit and the external limiting resistor. The balanced I
setting resistors help equalize the common mode voltage
change at anode and cathode to reduce the amount by
which I
C
0.1µF
LA
and C
TTL/CMOS
LED
74L504
CM
GATE)
LC
(ANY
is modulated from transient coupling through
V
GND
V
V
/dt>0 and |I
.
DD
O
DD2
2
LP
H
and I
, since the output is “high”), if an im-
FP
V
| > |I
LN
DD
results in a transient I
2N3906
530 Ω
FN
(ANY PNP)
|, referring to Table 1) will
H
failure).
530 Ω
1
1
3
ACPL-W70L
F
ACPL-W70L
equal
F
LED
(i.e.
LED
R
-