General Note

TT Electronics reserves the right to make changes in product specification without

notice or liability. All information is subject to TT Electronics’ own data and is

considered accurate at time of going to print.

TT Electronics | OPTEK Technology

2900 E. Plano Parkway, St 200, Plano, TX 75074, USA | Ph: +1 972 323 2200

www.ttelectronics.com | sensors@ttelectronics.com

Rev C, 4/27/2023

Page 26

© TT electronics plc

FlexSense® I-Series Programmable

Incremental Encoder

FS210/FS310 Datasheet

LED Drive Control

The LEDK pin of the FS210/FS310 is intended to be connected to the cathode of the LED acting as the light source for the

encoder system. The LED is external for the FS210 and is in-package for the FS310. The LED drive can be regulated either at

a fixed current level or in a closed control loop, as selected by bit 7 of the ILED control register. The drive current can be

disabled completely by writing the ILED control register to 0x00.

In a transmissive-mode design the LED is placed on the opposite side of the code wheel from the PCB containing all the

other components in the encoder module, with the LEDK pin of the FlexSenseTM device connected off-board to the cathode

of the LED. The OP207CL IR LED from TT Electronics is recommended for use with the FS210.

In open-loop control, the current sunk by the LEDK pin is kept first-order constant at the level coded by the least-significant

7 bits of the ILED control register. ILED<6:0> is taken to represent an unsigned binary-weighted integer in the range [0,

127], and the drive current is fixed at 0.6mA/LSB nominally. Therefore, the optical power density incident to the

FS210/FS310 varies directly with the LED optical output power at this fixed drive current – both with temperature (LED

optical output power at a given drive current decreases with increasing temperature and vice-versa) and with LED

degradation (LED optical output power at a given drive current at constant temperature decreases slowly over the

operating lifetime of the LED).

In closed-loop control, the current sunk by the LEDK pin is varied in order to maintain nearly constant quadrature-track

signal voltage amplitude at all operating conditions – i.e. to compensate automatically for the degradation of LED output

optical power over operating life and for the heavy negative temperature dependence of LED output optical power at a

given drive current – without requiring any explicit user action. At the onset of closed-loop control, the LEDK drive current

is initialized to 7/8 (0.875) of the value encoded by ILED<6:0>. The LEDK drive current is decreased by 0.6mA if the

quadrature-track differential output voltage amplitude is greater than the level encoded by the LEDLP control register, and

the LED drive current is increased by 0.6mA if the quadrature-track differential output voltage amplitude is less than the

level encoded by LEDLP and the drive current does not exceed the level encoded by ILED<6:0>. In no case can the LEDK

drive current be greater than the level encoded by ILED<6:0>. The LEDK drive current is updated approximately either once

every sixteen seconds or once every second, depending on the state of the FAST bit of the LEDLP control register. The value

of the actual LEDK drive current can be read back through the LEDLV status register.

In closed-loop control, if the actual LEDK drive current is greater than 7/8 of the level encoded by ILED<6:0>, an alarm

condition is present: bit 6 of the interrupt mask/status register is set, and an interrupt is generated if not masked by bit 2 of

the interrupt mask/status register.

LED Drive Current Control Register (ILED: [0x0886])

Table 22. ILED register definition

MSB

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

LSB

C_OZ

ILK