Application Note AN-3002
Low Current Input Circuit Ideas
6N138/139 Series
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REV. 4.00 4/30/02
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.
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.
Figure 1. 6N139 Input Circuit For Signal Detection
For the detection of undesirable signals on a D.C. power
source use:
C=To inject 500 microamperes into LED
X=Latching or non-latching output circuitry to follow
LED=Input diode of 6N139
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.
Transient Detection
The detection of the presence or absence of waveforms can
easily be detected by the circuit in Figure 2.
Figure 2. Pulse or Waveform Detection Circuit
For the detection of the presence of a desired signal, pulse or
waveform use:
CR=Silicon diode
X=Non-Latching output circuitry to follow
LED=Input diode of 6N139
f=frequency
Examples:
A desired pulse train to be present is shown in Figure 3.
D.C. POWER SOURCE
R
-
+
C6N139
X
LED
RPower supply voltage 1.5 volts–
50 microamperes
-------------------------------------------------------------------------------=
INPUT
RSRL
CR
CX
6N139
LED
RL
Positive Vpk. of input()2.5 volts–
1 milliampere
------------------------------------------------------------------------------------=
Cmin
Pulse interval of 1/f
RL
-----------------------------------------------=
RSmax Pulse width or 1/4f
5C
---------------------------------------------=
AN-3002 APPLICATION NOTE
2
REV. 4.00 4/30/02
The resulting LED forward current that will keep the output
circuitry conducting is shown as the result of proper design.
Figure 3. Pulse Train Waveforms
A desired sine wave to be present is shown in Figure 4.
The resulting LED forward current that will keep the output
circuitry conducting is shown as the result of proper design.
Figure 4. Sine Wave Waveforms
Matrices Opto-Coupling
With the low input LED current advantage of the 6N139, the
ability to drive matrices with but one TTL output is now pos-
sible as shown in Figure 5.
Figure 5. Opto-Coupling out of Matrices
Non-Loading Line Receiver
For virtual non-loading, the 6N139 is compatible with the
differential amplifier circuit of Figure 6.
Figure 6. Differential Amplifier Drive
For a virtual no-load optoisolator circuit use:
X=Non-latching output circuitry to follow
LED=Input diode of 6N139
Current requirement at “in” will be less than 20 micro
Amperes.
Example:
If “V
REF
” is made to be +1.4 Volts and the R
E
is 1.2 K
Ω
, the
circuit will respond nicely to TTL “0” and “1” levels. That is,
a “0” at “In” will cause LED current resulting in the conduc-
tion of the output circuitry. Conversely, a “1” at “In” will
result in no LED current. Notice that depending upon which
collector the LED is in series with it will give the option of
LED current flowing with a “0” or a “1” at “In”.
6N139 Output Circuitries
The following are two examples of 6N139 output circuitry.
One latching (Figure 7); the other non-latching (Figure 8),
but both capable of driving a TTL gate directly.
Referring to Figure 7 and assuming that the “RESET” has
been actuated by a momentary ground and no input signal is
being received, all transistors shown are non-conducting
(Output high, “1”). The arrival of an input signal will cause
all transistors to turn on. (Output low, “0”). The PNP transis-
tor, being turned on by the output transistor, will in turn latch
that same output transistor or until the “RESET” is again
initiated.
In Figure 8, where no signal is being received, the input
transistor is not conducting. The output transistor is very
slightly conducting. The 4.7M
Ω
resistor causing this slight
conduction will not bring the “Output” to a “0” level.
The purpose of this slight conduction is to reduce the
turn-on delay time. When a signal is received, both input
INPUT
(Volts)
+
t=0
OUTPUT
(Volts)
5.0
0.4
LED
(mA)
1.0
0.5
0
INPUT
(Volts)
+
0t=0
OUTPUT
(Volts)
5.0
0.4
LED
(mA)
1.0
0.5
0
1 of 16
ACTIVE
LOW
SCAN
CONTROL
16 X 16
MA
TRIX
5V
5.5KΩ
RESISTORS ALL ARE INPUT DIODES OF 6N139
IN
10KΩ
or
here
+V 6N139
AS EXAMPLE
VREF
VREF
RE
LED
0.5 mA
X
1mA
+V
APPLICATION NOTE AN-3002
REV. 4.00 4/30/02
3
and output transistors are turned on causing the “Output” to a
logic “0” state. The 4.7 M
Ω
resistor will now tend to reduce
the output transistor’s turn-off time.
If you have not looked over the 6N139 specification sheet,
you may not be totally aware of the current capabilities of
Fairchild Semiconductor optocouplers.
Figure 7. Latching Output Circuit for 6N139
Figure 8. NON-Latching Output Circuit for 6N139
4.7KΩ
4.7KΩ
4.7KΩ
4.7K
+V
(5V)
6N139
OUTPUT TO TTL
GROUND
FOR RESET*
*Normally OPEN momentary push-button
or
TTL output with open collector
4.7KΩ
4.7KΩ
4.7MΩ
+V
(5V)
6N139
OUTPUT TO TTL
AN-3002 APPLICATION NOTE
4/30/02 0.0m 001
Stock#AN300000xx
2002 Fairchild Semiconductor Corporation
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