HCNR200-300 Datasheet, PDF (16/19 Pages) AVAGO TECHNOLOGIES LIMITED – High-Linearity Analog Optocouplers

Theory of Operation
Figure 1 illustrates how the HCNR200/201 high-linearity
optocoupler is configured. The basic optocoupler con-
sists of an LED and two photodiodes. The LED and one of
the photodiodes (PD1) is on the input leadframe and the
other photodiode (PD2) is on the output leadframe. The
package of the optocoupler is constructed so that each
photodiode receives approximately the same amount of
light from the LED.
An external feedback amplifier can be used with PD1 to
monitor the light output of the LED and automatically
adjust the LED current to compensate for any non-linear-
ities or changes in light output of the LED. The feedback
amplifier acts to stabilize and linearize the light output
of the LED. The output photodiode then converts the
stable, linear light output of the LED into a current, which
can then be converted back into a voltage by another
amplifier.
Figure 12a illustrates the basic circuit topology for
implementing a simple isolation amplifier using the
HCNR200/201 optocoupler. Besides the optocoupler,
two external op-amps and two resistors are required.
This simple circuit is actually a bit too simple to function
properly in an actual circuit, but it is quite useful for ex-
plaining how the basic isolation amplifier circuit works (a
few more components and a circuit change are required
to make a practical circuit, like the one shown in Figure
12b).
The operation of the basic circuit may not be immedi-
ately obvious just from inspecting Figure 12a, particu-
larly the input part of the circuit. Stated briefly, amplifier
A1 adjusts the LED current (I_F), and therefore the current
in PD1 (I ), to maintain its “+” input terminal at 0 V. For
PD1
example, increasing the input voltage would tend to in-
crease the voltage of the “+” input terminal of A1 above
0 V. A1 amplifies that increase, causing I to increase, as
F
well as I . Because of the way that PD1 is connected,
PD1
I_PD1 will pull the “+” terminal of the op-amp back toward
ground. A1 will continue to increase I until its “+” termi-
F
nal is back at 0 V. Assuming that A1 is a perfect op-amp,
no current flows into the inputs of A1; therefore, all of the
current flowing through R1 will flow through PD1. Since
the “+” input of A1 is at 0 V, the current through R1, and
therefore I_PD1 as well, is equal to V_IN/R1.
Essentially, amplifier A1 adjusts I so that
F
I = V /R1.
PD1 IN
Notice that I depends ONLY on the input voltage and
PD1
the value of R1 and is independent of the light output
characteristics of the LED. As the light output of the
LED changes with temperature, amplifier A1 adjusts I
F
to compensate and maintain a constant current in PD1.
Also notice that I is exactly proportional to V , giving
PD1
IN
a very linear relationship between the input voltage and
the photodiode current.
The relationship between the input optical power and
the output current of a photodiode is very linear. There-
fore, by stabilizing and linearizing I , the light output of
PD1
the LED is also stabilized and linearized. And since light
from the LED falls on both of the photodiodes, I_PD2 will be
stabilized as well.
The physical construction of the package determines the
relative amounts of light that fall on the two photodiodes
and, therefore, the ratio of the photodiode currents. This
results in very stable operation over time and tempera-
ture. The photodiode current ratio can be expressed as a
constant, K, where
K = I /I .
PD2 PD1
Amplifier A2 and resistor R2 form a trans-resistance am-
plifier that converts I back into a voltage, V , where
PD2
OUT
V = I *R2.
OUT PD2
Combining the above three equations yields an overall
expression relating the output voltage to the input volt-
age,
V_OUT/V_IN = K*(R2/R1).
Therefore the relationship between V and V is con-
IN
OUT
stant, linear, and independent of the light output
characteristics of the LED. The gain of the basic isolation
amplifier circuit can be adjusted simply by adjusting the
ratio of R2 to R1. The parameter K (called K in the electri-
3
cal specifications) can be thought of as the gain of the
optocoupler and is specified in the data sheet.
Remember, the circuit in Figure 12a is simplified in order
to explain the basic circuit operation. A practical circuit,
more like Figure 12b, will require a few additional compo-
nents to stabilize the input part of the circuit, to limit the
LED current, or to optimize circuit performance. Example
application circuits will be discussed later in the data
sheet.
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