KX-TCA175RU - Panasonic Telephone Service Manual (repair manual). Page 7

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Handset consists of the following ICs as shown in Block Diagram (Handset) (P.6).

• BBIC (Base Band IC): IC3

- All data signals (forming/analyzing ACK or CMD signal)
- All interfaces (ex: Key, Detector Circuit, Charge, LCD, LED, SRAM, Flash ROM)

• SRAM: IC1

- Application data and code are temporarily stored.

• Flash ROM (substitute EEPROM): IC2

- Source program data and temporary user data are stored.

• AMPLIFIER IC: IC5

- Single OP_AMP for SPEAKER

• RF  Module:

- PLL Oscillator
- Detector
- Compress/Expander
- Amplifier for reception
- Amplifier for transmission

When power on the Handset, the voltage is as follows;

BATTERY(3.5 V to 4.2V: Battery+) 

→ Q8 (1.8V) → Q10 (3.0V)

The reset signal is generated with IC3 and Q17.
Power for RF IC (IC601) is generated with IC603.
5.0V for LCD back-light is generated with IC4.
Battery (3.5V to 4.2V) is used for RF Module, Amplifier IC (IC605), DC/DC Converter (IC4).
1.8V is used only for BBIC (IC3).
3.0V is used for BBIC (IC3), LCD, SRAM (IC1), Flash ROM (IC2), Red LED, Green LED, Key back-light.