iC-OV

5-BIT OPTO ENCODER

Rev C0, Page 7/9

Fig. 1: LED Current Control and Monitor

DESCRIPTION OF FUNCTIONS

LED Current Control

The integrated LED current control with driver stage keeps the photoelectric current of the reference photodiode

DREF constant. This compensates for ageing, dirt and the decline in LED efficiency as the temperature rises.

The photoelectric current of the reference DREF and the dark current of compensation diode DDC are amplified

in the current control. The amplified currents are subtracted from one another, yielding the actual value for the

driver’s negative input to trigger the LED.

At the same time, resistor RSET at the pin ISET sets the setpoint for the positive input of the driver - the voltage

at pin ISET is kept at a constant approx. 1.22V.

If there is an optical feedback loop from the LED to reference photodiode DREF, the driver changes the current

through the LED until the actual value at the negative input corresponds to the setpoint at the positive input.

The photoelectric current through the reference DREF and, therefore, the illuminance for the system as a whole

are kept constant. A monitor circuit detects the violation of the control range and indicates this via the fault

message circuit (block: Error).

The series resistor RLED connected in series to the LED limits the current through the diode and establishes the

operating limit of the control.

The optical feedback between LED and reference photodiode should suffice to yield an LED current of less than

8mA at room temperature. The power driver then also possesses sufficient current reserve to correct the

dropping efficiency of the LED at high temperatures.