Motorola T2290, Service Manual

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TDMA T2290/T2297: B+ Discharge/Backlight Driver
B40
Description
The B+ discharge circ uit consists of Q1574 and it's related components. This circuit is used in conjuncti on with the Power Cut process.
The Power Cut process will initiate an automatic power up cycle if battery power is disrupted for less than 3 seconds. During a Power Cut transistion, the phone will power
up automatically without activating the wake-up tone. Also, the phone will wake up to the same state that is had prior to the power disruption. The customer will never real-
ize that a power disruption occurred. If power is disrupt ed and not re-covered within 3 sec onds, the phone will re main off.
Capacitor C1563(GCAP section, page B25) is used to maintain the SRAM_VCC supply active when a power disrupti on occurs. SRAM_VCC is the supply voltage to main-
tain the RAM(U1300) active durin g a power disruption. During a power disruption everything except SRAM_VCC, will be powered off. In this condtion the supply 2.75V
is off. This will allow Q1574A to be switched on, allowing the gate of Q1574B to be pulled high by the source SRAM_VCC. Q1574B will be tu rned on when a high state is
present at the gate. This will then ground R1573 and allow the B+ line to discharge through R1573. The process of discharging the B+ line help s in eliminati ng any parasiti -
cal voltages on the B+ line wh en the power is re-covered prior to the 3 second time out.
When the phone is powered on, 2.75V will be present. This will switched Q1574A off and allow the gate of Q1574B to be pulled low by R1573. A low state at the gate of
Q1574B will turn it off and not al low R1573 to be grounded.
The BACKLIGHT line is used to activate the keypad backlights. A high state at BACKLIGHT will activate Q1810 and complete the backlight circui t path to ground. The
LEDs will then turn on. Q1809 is used as an overvoltage protection. If the voltage at the base of Q1809 is high than the voltage at the base of Q1810, Q1809 will be
switched on. This will force the BACKLIGHT line to ground and disable Q1810. The backlight circuit path will no longer have its path to ground and the backlights won't
activate.
The ACC_STATE lin e is used to in dicate what type of accessory is attached to the phone. The accessory will place a specific load on the MANUEL_TEST line that is
unique to other types of accessories.
SRAM_VCC
2.75V
Q1574A
Q1574B
B+
R1573
R1573
BACKLIGHT
BKLT_BIAS
Q1809
Q1810
R10134