HCPL-261A Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – HCMOS Compatible, High CMR, 10 MBd Optocouplers

anode/cathode and output ground
(C_LA and C_LC). Also shown in
Figure 21 on the input side is an
AC-equivalent circuit. Table 1
indicates the directions of I_LP and
I_LN flow depending on the
direction of the common-mode
transient.
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 (i.e. when
dV_CM/dt>0 and |I_FP| > |I_FN|,
referring to Table 1) 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 20, good CMR
can be achieved. (In the case of
the -261N families, a minimum
CMR of 15 kV/µs is guaranteed
using this circuit.) 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.
CMR with Other Drive
Circuits
CMR performance with drive
circuits other than that shown in
Figure 20 may be enhanced by
following these guidelines:
1. Use of drive circuits where
current is shunted from the
LED in the LED “off” state (as
shown in Figures 22 and 23).
This is beneficial for good
CMR_H.
2. Use of I_FH > 3.5 mA. This is
good for high CMR_L.
Using any one of the drive
circuits in Figures 22-24 with
I_F = 10 mA will result in a typical
CMR of 8 kV/µs for the HCPL-
261N family, as long as the PC
board layout practices are
followed. Figure 22 shows a
circuit which can be used with
any totem-pole-output TTL/
LSTTL/HCMOS logic gate. The
buffer PNP transistor allows the
circuit to be used with logic
devices which have low current-
sinking capability. It also helps
maintain the driving-gate power-
supply current at a constant level
to minimize ground shifting for
other devices connected to the
input-supply ground.
When using an open-collector
TTL or open-drain CMOS logic
gate, the circuit in Figure 23 may
be used. When using a CMOS
gate to drive the optocoupler, the
circuit shown in Figure 24 may
be used. The diode in parallel
with the R_LED speeds the turn-off
of the optocoupler LED.
1
1/2 R_LED
2
1/2 R_LED
3
4
I_LP
I_LN
C_LA
C_LC
SHIELD
8
V_CC+
0.01 µF
7
350 Ω
6
V_O
15 pF
5
GND
+
–
V_CM
Figure 21. AC Equivalent Circuit for HCPL-261X.
74L504
(ANY
TTL/CMOS
GATE)
V_CC
420 Ω
(MAX)
HCPL-261X
1
2N3906
2
(ANY PNP)
3
LED
4
Figure 22. TTL Interface Circuit for the HCPL-261A/-
261N Families.
1-179