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Application Note AN-3001
Optocoupler Input Drive Circuits
An optocoupler is a combination of a light source and a
photosensitive detector. In the optocoupler, or photon
coupled pair, the coupling is achieved by light being
generated on one side of a transparent insulating gap and
being detected on the other side of the gap without an
electrical connection between the two sides (except for a
minor amount of coupling capacitance). In the Fairchild
Semiconductor optocouplers, the light is generated by an
infrared light emitting diode, and the photo-detector is a
silicon diode which drives an amplifier, e.g., transistor. The
sensitivity of the silicon material peaks at the wavelength
emitted by the LED, giving maximum signal coupling.
dropped across the resistor at the desired IF, determined from
other criteria. A silicon diode is shown installed inversely
parallel to the LED. This diode is used to protect the reverse
breakdown of the LED and is the simplest method of achiev-
ing this protection. The LED must be protected from exces-
sive power dissipation in the reverse avalanche region. A
small amount of reverse current will not harm the LED, but it
must be guarded against unexpected current surges.
The forward voltage of the LED has a negative temperature
coefficient of 1.05 mV/°C and the variation is shown in
Figure 5.
Where the input to the optocoupler is a LED, the input
characteristics will be the same, independent of the type of
detector employed. The LED diode characteristics are shown
in Figure 1. The forward bias current threshold is shown at
approximately 1 volt, and the current increases exponen-
tially, the useful range of IF between 1 mA and 100 mA
being delivered at a VF between 1.2 and 1.3 volts. The
dynamic values of the forward bias impedance are current
dependent and are shown on the insert graph for RDF and
∆R as defined in the figure. Reverse leakage is in the nano-
ampere range before avalanche breakdown.
The brightness of the IR LED slowly decreases in an expo-
nential fashion as a function of forward current (IF) and time.
The amount of light degradation is graphed in Figure 6
which is based on experimental data out to 20,000 hours.
A 50% degradation is considered to be the failure point.
This degradation must be considered in the initial design of
optoisolator circuits to allow for the decrease and still remain
within design specifications on the current-transfer-ratio
(CTR) over the design lifetime of the equipment. Also, a
limitation on IF drive is shown to extend useful lifetime of
the device.
The LED equivalent circuit is represented in Figure 2, along
with typical values of the components. The diode equations
are provided if needed for computer modeling and the con-
stants of the equations are given for the IR LED’s. Note that
the junction capacitance is large and increases with applied
forward voltage. An actual plot of this capacitance variation
with applied voltage is shown on the graph of Figure 3. It is
this large capacitance controlled by the driver impedance
which influences the pulse response of the LED. The capaci-
tance must be charged before there is junction current to
create light emission. This effect causes an inherent delay of
10-20 nanoseconds or more between applied current and
light emission in fast pulse conditions.
In some circumstances it is desirable to have a definite
threshold for the LED above the normal 1.1 volts of the
diode VF. This threshold adjustment can be obtained by
shunting the LED by a resistor, the value of which is
determined by a ratio between the applied voltage, the
series resistor, and the desired threshold. The circuit of
Figure 7 shows the relationship between these values.
The calculations will determine the resistor values required
for a given IFT and VA. It is also quite proper to connect
several LED’s in series to share the same IF. The VF of the
series is the sum of the individual VF’s. Zener diodes may
also be used in series.
Where the input applied voltage is reversible or alternating
and it is desired to detect the phase or polarity of the input,
the bipolar input circuit of Figure 8 can be employed. The
individual optocouplers could control different functions or
be paralleled to become polarity independent. Note that in
this connection, the LED’s protect each other in reverse bias.
The LED is used in the forward biased mode. Since the
current increases very rapidly above threshold, the device
should always be driven in a current mode, not voltage
driven. The simplest method of achieving the current drive is
to provide a series current-limiting resistor, as shown in
Figure 4, such that the difference between VAPP and VF is
REV. 4.00 4/30/02
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