209
See also:
for voltage levels of isolated outputs.
NOTE
(1) 0.36 V is obtained considering a worst-case external (pull-up) load of 2
mA ( 180 ohm x 2 mA = 0.36 V), which means that the circuit can support the
presence of an external pull-up resistor up to a (minimum) value of 1K5 ohm
(in 3.3 V) or 2K4 ohm (in 5 V). If needed, an other R value can be chosen
according to the actual pull-up load within the circuit.
NOTE
(2) R
POL
limits the Voh voltage to about 2.7 V in order to match TTL and LVTTL
levels.
2.7 V is obtained considering the R
POL
-R 560 ohm-180 ohm divider and taking
into account that the voltage drop across the opto-coupler pins (V
OUT
+
-
V
OUT
t
-
) is about 0.9 V.
Dynamic limitations
The maximum pulse width of isolated outputs is about 5 µs and the maximum pulse rate is 100
KHz,
Isolated outputs add an extra delay of about 5 µs in the signal propagation.
The resistor value of R = 180 ohm has good dynamic results for a usual capacitive loads as 1 or 2
meters of cable. As example, a 2m cable will add 100 pF of load (50pF/m) which give a rise time
of about 18 µs at 180 ohm ( R x C = 180 ohm x 100 pF = 18 µs ) . If needed, the R value can be
adapted to match special requirements in terms of rise time and/or capacitive load.
If maximizing the opto-coupler switching time is a concern, it is not recommended to not
increase too much the value of the resistor. The opto-coupler circuit behaves better (switching
times) with a load of about 10 mA or higher. R = 180 ohm loads the opto-coupler at 13 mA (3.3
V) and 23 mA (5 V).
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