IC-OG_12 Datasheet, PDF (8/10 Pages) IC-Haus GmbH – 8-BIT DIFFERENTIAL SCANNING OPTO ENCODER

iC-OG
preliminary
8-BIT DIFFERENTIAL SCANNING OPTO ENCODER
DESCRIPTION OF FUNCTIONS
Rev D1, Page 8/10
LED current control
The integrated LED current control with a driver stage
controls the LED in accordance with the sum of the
photocurrents from the tracks 0 and 1. Compensation
is made for age and dirt, as well as for the reduced
efficiency of the LED caused by rises in temperature.
Figure 1: LED current control and monitoring.
The photodiodes DPO, DNO, DP1 and DN1 act as ref-
erence diodes. The sum is output via a current sink to
the comparison point pin CSR. Simultaneously, the re-
sistor R1 at pin ISET (the voltage at the ISET pin is kept
at a constant of approximately 1.22 V) supplies a ref-
erence current for the current source from VCC, which
also works towards the comparison point pin CSR. The
comparison point also receives the amplified current
from the compensation diode DDC in order to com-
pensate for dark currents and for the amplifier input
currents.
If there is an optical feedback from the LED to the refer-
ence photodiodes, the voltage at the CSR pin adjusts
to satisfy the needs of the power driver for the required
transmit current at pin LED. In this instance, the ratio
between I(ISET) and the sum of the photodiode cur-
rent ISUM is constant (Electrical Characteristics Nos.
705 and 706). The current flowing through the resistor
R1 is the setpoint for the control and directly presets
the desired level of illumination.
An internal capacitor ensures that the control is stable.
The comparison point pin CSR is lead out additionally,
enabling an external capacitor C2 to be connected to
adapt the control behavior. Lower values for R1 require
larger values for C2, which also improve the power-
supply rejection ratio for the control. Values from 10 nF
upwards are recommended.
A resistor in series with the LED limits the current in
pin LED and sets the operating limits of the control.
The optical feedback between the LED and the refer-
ence photodiodes should be good enough to establish
an LED current of less than 15 mA at room tempera-
ture. The power driver needs to have a sufficient cur-
rent reserve to correct the LED’s decline in efficiency
even at high temperatures.
Control Monitor and Error Message Output
The control monitor observes the potential at the CSR
pin. Voltages which bring the power driver to saturation
or off-state are recognized and indicated at the open-
collector output by NER = low.