Datasheet

19
Figure 13. Recommended drive circuit for High-CMR.
Application Information
Common-Mode Rejection for HCPL-260L Families:
Figure 13 shows the recommended drive circuit for op-
timal common-mode rejection performance. Two main
points to note are:
1. The enable pin is tied to V
CC
rather than oating (this
applies to single-channel parts only).
2. Two LED-current setting resistors are used instead of
one. This is to balance I
LED
variation during common-
mode transients.
If the enable pin is left oating, it is possible for common-
mode transients to couple to the enable pin, resulting in
common-mode failure. This failure mechanism only oc-
curs when the LED is on and the output is in the Low
State. It is identied as occurring when the transient out-
put voltage rises above 0.8 V. Therefore, the enable pin
should be connected to either V
CC
or logic-level high for
best common-mode performance with the output low
(CMR
L
). This failure mechanism is only present in single-
channel parts which have the enable function.
Figure 14. AC equivalent circuit.
350 Ω
HCPL-�261A fig 20
1/2 R
LED
V
CC
+
15 pF
+
V
CM
8
7
6
1
3
SHIELD
5
2
4
C
LA
V
O
GND
0.01 µF
1/2 R
LED
C
LC
I
LN
I
LP
–
Also, common-mode transients can capacitively cou-
ple 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 o). Figure 14 shows the parasitic capaci-
tances which exists between LED anode/cathode and
output ground (C
LA
and C
LC
). Also shown in Figure 14 on
the input side is an AC-equivalent circuit.
For transients occurring when the LED is on, common-
mode rejection (CMR
L
, since the output is in the “low”
state) depends upon the amount of LED current drive
(I
F
). For conditions where I
F
is close to the switching
threshold (I
TH
), CMR
L
also depends on the extent which
I
LP
and I
LN
balance each other. In other words, any condi-
tion where common-mode transients cause a momen-
tary decrease in I
F
will cause common-mode failure for
transients which are fast enough.
0.01 µF
350 Ω
HCPL-261A fig 19
74LS04
OR ANY TOTEM-POLE
OUTPUT LOGIC GATE
V
O
V
CC+
8
7
6
1
3
SHIELD
5
2
4
HCPL-260L
GND
GND2
220
Ω
V
CC
220 Ω
*
*
*
HIGHER CMR MAY BE OBTAINABLE BY CONNECTING PINS 1, 4 TO INPUT GROUND (GND1).
GND1