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Document Number 83622

Rev. 1.4, 26-Oct-04

VISHAY

IL300

Vishay Semiconductors

Operation Description

A typical application circuit (Figure 1) uses an opera-

tional amplifier at the circuit input to drive the LED.

The feedback photodiode sources current to R1 con-

nected to the inverting input of U1. The photocurrent,

The magnitude of this current is directly proportional

to the feedback transfer gain (K1) times the LED drive

LED current to force sufficient photocurrent to keep

the node voltage (Vb) equal to Va.

The output photodiode is connected to a non-invert-

ing voltage follower amplifier. The photodiode load

resistor, R2, performs the current to voltage conver-

sion. The output amplifier voltage is the product of the

output forward gain (K2) times the LED current and

the ratio of the product of the output forward gain (K2)

times the photodiode load resistor (R2) to the product

of the feedback transfer gain (K1) times the input

resistor (R1). This reduces to

The overall transfer gain is completely independent of

the LED forward current. The IL300 transfer gain (K3)

is expressed as the ratio of the output gain (K2) to the

feedback gain (K1). This shows that the circuit gain

becomes the product of the IL300 transfer gain times

the ratio of the output to input resistors

K1-Servo Gain

K2-Forward Gain

K3-Transfer Gain

The Transfer Gain is the ratio of the Forward Gain to

the Servo gain, i.e., K3 = K2/K1.

∆K3-Transfer Gain Linearity

The percent deviation of the Transfer Gain, as a func-

tion of LED or temperature from a specific Transfer

Gain at a fixed

LED current and temperature.

Photodiode

A silicon diode operating as a current source. The out-

put current is proportional to the incident optical flux

supplied by the LED emitter. The diode is operated in

the photovoltaic or photoconductive mode. In the pho-

tovoltaic mode the diode functions as a current

source in parallel with a forward biased silicon diode.

The magnitude of the output current and voltage is

dependent upon the load resistor and the incident

LED optical flux. When operated in the photoconduc-

tive mode the diode is connected to a bias supply

which reverse biases the silicon diode. The magni-

tude of the output current is directly proportional to the

LED incident optical flux.

LED (Light Emitting Diode)

An infrared emitter constructed of AlGaAs that emits

at 890 nm operates efficiently with drive current from

500

µA to 40 mA. Best linearity can be obtained at

drive currents between 5.0 mA to 20 mA. Its output

flux typically changes by - 0.5 % /°C over the above

operational current range.

Application Circuit

Figure 1. Typical Application Circuit

iil300_01

8

7

6

5

K1

1

2

3

4

K2

R1

R2

IL300

Vb

Va

+

-

U1

Vin

lp 1

-

U2

+

lp 2

Vout

VCC

VCC

VCC

VCC

IF

Vc

+