Siemens SXG 75 Service Repair Documentation page 56

11.2.2 Illumination
The keypad backlight is supplied by 10 white LEDs. These are arranged as two chains of 5 LEDs,
both connected to a high voltage supply at 18.75V (VREG_BCKLT). The supply is generated by
switching regulator N900, and is enabled when BACKLIGHT_EN is high.
The current through each LED chain is set to approximately 3mA using the current-limiter circuit of
V2000/R2017/R2018. The backlight brightness is varied by turning the 6mA load current on and off
using a pulse-width modulated switching signal, LCD_BCKLT_PWM.
Keypad backlight
The keypad backlight is driven in the same manner as the LCD backlight. In the case of the keypad,
the current is limited to (0.7V/110R = ) 6.4mA.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 56 of 73