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Application Note AN-3002
Low Current Input Circuit Ideas
6N138/139 Series
Introduction
Advancements in opto-coupling and LED technology have
given us the 6N139. This unique optocoupler, having an
input LED current specification at 500 microamperes, has
opened some interesting design doors. Besides the obvious
and much written about ability to be directly driven by
CMOS circuits, the 6N139 can be considered for signal
detection, transient detection, matrices and non-loading line
receiving. Following are but a few circuit ideas to stimulate
the designer’s interest.
The LED is provided with a 50 microampere forward current
to charge the LED capacity to the VF level. In this way, the
LED is not causing conduction in its output circuitry but is
prepared to conduct very quickly. Any noise or oscillation on
the “D.C. power source” is coupled through “C” which
develops a signal across the LED. Even small unwanted sig-
nals can cause a large change in the LED forward current.
Once the LED’s forward current equals or exceeds 500
microamperes, the output circuitry will conduct indicating
the presence of the unwanted signal.
Signal Detection
The detection of noise, spikes or oscillations can easily and
directly be detected by the input of the 6N139 as shown in
the circuit of Figure 1.
C
+
R
D.C. POWER SOURCE
LED
-
X
6N139
Transient Detection
The detection of the presence or absence of waveforms can
easily be detected by the circuit in Figure 2.
R
S
CR
6N139
R
L
INPUT
C
LED
X
Figure 2. Pulse or Waveform Detection Circuit
Figure 1. 6N139 Input Circuit For Signal Detection
For the detection of the presence of a desired signal, pulse or
waveform use:
CR=Silicon diode
(
Positive Vpk. of input
)
–
2.5 volts
R
L
= -----------------------------------------------------------------------------------
-
1 milliampere
Pulse interval of 1/f
-
C
min
= ----------------------------------------------
R
L
Pulse width or 1/4f
R
S
max
= ---------------------------------------------
5C
X=Non-Latching output circuitry to follow
LED=Input diode of 6N139
f=frequency
For the detection of undesirable signals on a D.C. power
source use:
Power supply voltage
–
1.5 volts
R
= ------------------------------------------------------------------------------
-
50 microamperes
C=To inject 500 microamperes into LED
X=Latching or non-latching output circuitry to follow
LED=Input diode of 6N139
Examples:
A desired pulse train to be present is shown in Figure 3.
REV. 4.00 4/30/02
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
