IC-LV_16 Datasheet, PDF (10/15 Pages) IC-Haus GmbH – 5-BIT OPTO ENCODER

iC-LV
5-BIT OPTO ENCODER
DESCRIPTION OF FUNCTIONS
Rev C1, Page 10/15
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 tempera-
ture.
The photocurrent of reference sensor DREF and the
dark current of compensation sensor DD are amplified
in the receiver. The amplified currents are subtracted
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 remains open an
internal bias current is used which is equivalent to an
external resistor of ca. 24 kΩ.
Track evaluation
The switching threshold supplied to the track compara-
tors lies at half of INREF, ie. in the center between 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 con-
necting pin NDIR to ground (GND). If the pin remains
open, an internal pull-up current source generates a
high level. When Gray code discs are used, inverting
the MSB track is tantamount to changing the direction
of rotation.
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 ref-
erence sensor DREF - and thus also the level of illumi-
nation for the overall system - is kept constant.
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 limits 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 switching thresh-
old for the track comparators. This enables operation
of iC-LV with an external light source instead of using
power-controlled LED.