HCNR200 datasheet(13/16 Pages) HP

13 / 16 page

1-430

Theory of Operation

Figure 1 illustrates how the

HCNR200/201 high-linearity

optocoupler is configured. The

basic optocoupler consists 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 photo-

diode 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-linearities 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

explaining 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 immediately obvious

just from inspecting Figure 12a,

particularly the input part of the

circuit. Stated briefly, amplifier

A1 adjusts the LED current (IF),

and therefore the current in PD1

(IPD1), to maintain its “+” input

terminal at 0 V. For example,

increasing the input voltage would

tend to increase the voltage of the

“+” input terminal of A1 above 0

V. A1 amplifies that increase,

causing IF to increase, as well as

IPD1. Because of the way that PD1

is connected, IPD1 will pull the “+”

terminal of the op-amp back

toward ground. A1 will continue

to increase IF until its “+”

terminal 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 IPD1 as

well, is equal to VIN/R1.

Essentially, amplifier A1 adjusts IF

so that

IPD1 = VIN/R1.

Notice that IPD1 depends ONLY on

the input voltage and 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, ampli-

fier A1 adjusts IF to compensate

and maintain a constant current

in PD1. Also notice that IPD1 is

exactly proportional to VIN, giving

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. Therefore, by stabilizing

and linearizing IPD1, the light

output of the LED is also

stabilized and linearized. And

since light from the LED falls on

both of the photodiodes, IPD2 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

temperature. The photodiode

current ratio can be expressed as

a constant, K, where

K = IPD2/IPD1.

Amplifier A2 and resistor R2 form

a trans-resistance amplifier that

converts IPD2 back into a voltage,

VOUT, where

VOUT = IPD2*R2.

Combining the above three

equations yields an overall

expression relating the output

voltage to the input voltage,

VOUT/VIN = K*(R2/R1).

Therefore the relationship

between VIN and VOUT is constant,

linear, and independent of the

light output characteristics of the

LED. The gain of the basic isola-

tion amplifier circuit can be

adjusted simply by adjusting the

ratio of R2 to R1. The parameter

K (called K3 in the electrical

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 opera-

tion. A practical circuit, more like

Figure 12b, will require a few

additional components to stabilize

the input part of the circuit, to

limit the LED current, or to

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