IC-LV Datasheet, PDF (8/12 Pages) IC-Haus GmbH – 5-BIT OPTO ENCODER

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 kΩ.
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.