iC-LV

5-BIT OPTO ENCODER

Rev A3, Page 8/12

DESCRIPTION OF FUNCTIONS

Illumination control

The integrated LED power control with a driver stage

keeps the photocurrent of reference photosensor

DREF constant. This compensates for aging, dirt and

drops in efficiency of the transmitting LED with rises

in temperature.

The photocurrent of reference sensor DREF and the

dark current of compensation sensor DD are ampli-

fied in the receiver. The amplified currents are sub-

tracted from one another, yielding an actual value to

feed the LED power control. The current adjusted by

resistor R1 at pin RSET generates the bias for the

control; the voltage at pin RSET is kept constant (see

Electrical Characteristics No. E01). If pin RSET re-

mains open an internal bias current is used which is

equivalent to an external resistor of ca. 24 k .

If there is an optical feedback loop from the LED to

reference sensor DREF the power driver alters the

LED current until the optical power received complies

with the given setpoint. The photocurrent generated

by reference sensor DREF

B and thus also the level

of illumination for the overall system

B is kept con-

stant.

A monitoring circuit detects when and if the LED con-

trol range is overshot or undershot and signals this by

switching error output NERR to low and via the error

bit during serial communication (when SEEN is high

and no synchronization is selected).

Resistor R3 connected in series to the transmitting

LED limits the current and governs the operating lim-

its of the LED power control.

At the same time the amplified dark current of com-

pensation sensor DD and the reference photocurrent

of sensor DREF are added together. The resulting

current, named INREF, is used to provide the switch-

ing threshold for the track comparators. This enables

operation of iC-LV with an external light source in-

stead of using power-controlled LED.

Track evaluation

The switching threshold supplied to the track com-

parators lies at half of INREF, ie. in the center be-

tween a full light and no light condition, and adjusts

automatically to changes in illumination. This enables

the device to be operated without the LED power

control with a constant illumination level only. The

hysteresis of the current comparators is also

photocurrent tracked and increases noise immunity.

The most significant bit (MSB) can be inverted by

connecting pin NDIR to ground (GND). If the pin re-

mains open, an internal pull-up current source gener-

ates a high level. When Gray code discs are used,

inverting the MSB track is tantamount to changing the

direction of rotation.