Keypad - Siemens SXG 75 Service Repair Documentation

When the LEDs are being driven, LCD_BCLKT_PWM is high, so V2200(a) is ON. Current therefore
flows through R2225. When the voltage across R2225 reaches approximately 0.7V, transistor
V2200(b) turns ON, and starts to turn V2200(a) OFF. An equilibrium is then reached, where the LED
current is set to 0.7V/39R= 18mA.
The forward voltage of the backlight LEDs is typically 3.3V at 18mA. So the voltage seen at the
collector of V2200(a) is typically (18.75-4*3.3=) 5.55V. However, due to possible tolerances on
components and supply voltages, the voltage may acceptably vary between about 2.8 and 7.1V.
Backlight brightness is varied by switching LCD_BCKLT_PWM on and off using a Pulse-width
modulated signal. LCD_BCKLT_PWM is generated by the General Purpose MN Timer on the
MSM6250. The timer output, GP_MN (GPIO[13]) is configured to supply a Pulse-Width-Modulated
signal. The LCD backlight LEDs are ON while the timer output is high, so brightness varies with the
duty cycle of the timer.
The LCD_BCKLT_PWM signal also varies the brightness of the keypad backlight, so the LCD and
keypad backlights will always vary in brightness together.

11.2 Keypad

The keypad assembly interfaces to the main PCB via the 16-way connector X2000. The Keypad
contains 28 keys which are read as a Matrix, and a separate key which is read directly by the
PM6650 in order to provide a power-on function.
11.2.1 Keypad mapping
The keypad mapping and interfacing with the MSM6250 is illustrated in the following figure. This
also shows the positions of the keys on the phone housing.
The Keypad is read as a matrix. The KYPD_9, KYPD_11, KYPD_13, KYPD_15 KYPD_17, and
KYPD_MEMO lines are driven by the MSM6250. They are either floating (inactive), or driven low
(active) one at a time. The KEYSENSE_N(0:4) lines are inputs to the MSM6250. These lines are
read to detect the position of the key being pressed.
The END/Power key is connected directly to the PHONE_ON input to the PM6650 (KPDPWR_N
pin). This key is read independently of the matrix to allow it to also be used to power-on the phone.
The PM6650 PHONE_ON input has an internal pull-up resistor which is always active while battery
voltage is available.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 54 of 73