HCPL-260L Datasheet, PDF (18/19 Pages) AVAGO TECHNOLOGIES LIMITED – High Speed LVTTL Compatible 3.3 Volt Optocouplers Low power consumption

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 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 oc-
curs when the LED is on and the output is in the Low
State. It is identified 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.
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 off ). 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.
VCC
*
HCPL-260L
1
8
VCC+
0.01 µF
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
4
I_LP
I_LN
C_LA
C_LC
SHIELD
HCPL-261A fig 19
8
0.01 µF
7
350 Ω
V_CC+
6
V_O
15 pF
5
GND
+
–
V_CM
Figure 14. AC equivalent circuit.
HCPL- 261A fig 20
18