HCPL260L Datasheet, PDF (13/14 Pages) Agilent(Hewlett-Packard) – High Speed LVTTL Compatible 3.3 Volt Optocouplers

Application Information
Common-Mode Rejection for
HCPL-260L Families:
Figure 13 shows the recom-
mended drive circuit for optimal
common-mode rejection
performance. Two main points to
note are:
1. The enable pin is tied to V _CC
rather than floating (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 floating, it
is possible for common-mode
transients to couple to the enable
pin, resulting in common-mode
failure. This failure mechanism
only occurs when the LED is on
and the output is in the Low
State. It is identified as occurring
when the transient output 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.
Also, 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). Figure 14 shows the
parasitic capacitances 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.
VCC
*
HCPL-260L
1
8
VCC+
0.01 F
220 Ω
2
7
350 Ω
220 Ω
3
6
VO
74LS04
OR ANY TOTEM-POLE
OUTPUT LOGIC GATE
GND1
4
5
SHIELD
*
GND
GND2
* HIGHER CMR MAY BE OBTAINABLE BY CONNECTING PINS 1, 4 TO INPUT GROUND (GND1).
Figure 13. Recommended drive circuit for High-CMR.
1
1/2 R_LED
2
1/2 R_LED
3
I_LP
I_LN
C_LA
C_LC
4
SHIELD
.
+
–
V_CM
Figure 14. AC equivalent circuit.
8
V_CC+
0.01 F
7
350 Ω
6
V_O
15 pF
5
GND
For transients occurring when the
LED is on, common-mode rejec-
tion (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
condition where common-mode
transients cause a momentary
decrease in I_F will cause
common-mode failure for
transients which are fast enough.
Likewise for common-mode
transients which occur when the
LED is off (i.e. CMR _H, since the
output is “high”), if an imbalance
between I_LP and I_LN results in a
transient I_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 consti-
tutes a CMR_H failure).
By using the recommended
circuit in Figure 13, good CMR
can be achieved. The balanced
I_LED-setting resistors help equalize
I_LP and I_LN to reduce the amount
by which I_LED is modulated from
transient coupling through C_LA
and C_LC
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