The TIL300 precision linear optocoupler consists of an infrared LED irradiating an isolated feedback photodiode
and an output photodiode in a bifurcated arrangement. The feedback photodiode captures a percentage of the
flux of the LED and generates a control signal that can be used to regulate the LED drive current. This technique
is used to compensate for the nonlinear time and temperature characteristics of the LED. The output-side
photodiode produces an output signal that is linearly proportional to the servo-optical flux emitted from the LED.
A typical application circuit (shown in Figure 1) uses an operational amplifier as the input to drive the LED. The
feedback photodiode sources current through R1, which is connected to the inverting input of the input
operational amplifier. The photocurrent IP1 assumes a magnitude that satisfies the relationship IP1 = VI/R1. The
magnitude of the current is directly proportional to the LED current through the feedback transfer gain
K1(VI/R1 = K1 × IF). The operational amplifier supplies LED current to produce sufficient photocurrent to keep
the node voltage Vb equal to node voltage Va.