IC-WGBLCCWGC2 ICHAUS [IC-Haus GmbH], IC-WGBLCCWGC2 Datasheet - Page 9

• Manufacturer Part Number: IC-WGBLCCWGC2
• Description: 14-BIT DIFFERENTIAL SCANNING OPTO ENCODER
• Manufacturer: ICHAUS [IC-Haus GmbH]

LED Current Control
The integrated LED current control with driver stage keeps the photocurrent of the reference photodiode DSR
constant. Compensation is made for aging and dirt as well as for the decline in the efficiency of the transmit
LED as the temperature rises.
Figure 1: LED current control and control monitor
The photocurrent in the reference photodiode DSR is amplified by the differential amplifier of the LED current
control and output to the comparison point pin CSR via a current sink. Simultaneously the resistor R1 at pin
RSR - the voltage at pin RSR is kept at a constant approx. 1.22V - supplies a reference current for the current
source from VCC which also works on the comparison point, pin CSR. To compensate for the dark current of
the reference diode and the amplifier input currents, the comparison point also receives the amplified current
of compensation diode DR.
If there is an optical feedback from the LED to the reference photodiode DSR, a voltage develops at pin CSR
which is just high enough to satisfy the needs of the power driver for the required transmit current at pin LED.
In this case the current ratio between I(RSR) and reference photodiode current I(DSR) is constant (electrical
characteristics No.707). The current through resistor R1 is the setpoint for the control and presets the desired
illuminance directly.
The capacitor at pin CSR ensures the stability of the control. The value selected for it should be higher than
10nF; lower values for R1 require larger values for CSR, which also improve the power supply rejection for the
control.
A resistor in series with the transmit LED limits the current in the LED pin and establishes the operating limits
of the control.
The optical feedback between LED and reference photodiode should be so good that an LED current of less
than 15mA develops at room temperature. Only then the power driver does have enough current reserve to also
correct the declining efficiency of the LED for high temperatures too. If higher LED currents are required, the
base of an external transistor can be connected to LGND to form a three-fold Darlington stage (increases the
saturation voltage at error message output NERR).
iC-WG
14-BIT DIFFERENTIAL SCANNING OPTO ENCODER
DESCRIPTION OF FUNCTIONS
Rev B0, Page 9/11