IC-WGBLCCWGC2+WG1R 参数 Datasheet PDF下载

iC-WG

14-BIT DIFFERENTIAL SCANNING OPTO ENCODER

Rev B0, Page 9/11

DESCRIPTION OF FUNCTIONS

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).