iC-OG

8-BIT DIFFERENTIAL SCANNING OPTO ENCODER

Rev C1, Page 7/9

ELECTRICAL CHARACTERISTICS

Operating Conditions: VCC= 5V ±10%, Tj= -40..125 °C, unless otherwise noted.

Item

Symbol

Parameter

Conditions

Tj

Fig.

Unit

EC

Min.

Typ.

Max.

Fig. 1: LED Current Control and Monitoring

Control Monitor NER

801 Vs()

Saturation Voltage lo

I(NER)= 3.2mA

0.27

0.4

V

802 Isc()

Short-Circuit Current lo

V(NER)= VCC

15

27

mA

803 I0()

Collector Off-state Current

NER: off, V(NER)= 0..6V

10

µA

DESCRIPTION OF FUNCTIONS

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.

The photodiodes DPO, DNO, DP1 and DN1 act as reference diodes. The sum is output via a current sink to

the comparison point pin CSR. Simultaneously, the resistor R1 at pin ISET (the voltage at the ISET pin is kept

at a constant of approximately 1.22V) supplies a reference 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 compensate for dark currents and for the amplifier input currents.

If there is an optical feedback from the LED to the reference 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

voltage ratio between I(ISET) and the sum of the photodiode current ISUM is constant (Electrical

Characteristics Nos. 706 and 707). 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 in the

BLCC20 package, enabling an external capacitor C2 to be connected to adapt the control behaviour. Lower

values for R1 require larger values for C2, which also improve the power-supply rejection ratio for the control.

Values from 10nF upwards are recommended.