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This circuit is for absorbing surge voltage generated by the transformer.

The control circuit amplifies the output with increased voltage detected in the error detecting circuit. Then it drives the main tran-
sistor.
In this power supply, the duty ratio is defined by changing the ON period of the main transistor.
This is shown as follows.
When the output voltage of the 24V circuit increases, the current of the photo coupler PC101 increases, the pulse width of the
output control IC becomes narrower and the ON period of Q101 becomes shorter.

The highest drain current (Q101) is limited by a limiter circuit (IC101) of 24V. The 24V output is limited by this circuit.

If the 24V output increases because the error detecting circuit or control circuit is broken, IC101 will recognize this signal and
output becomes 0V.

Refer to Power Supply Board Section (P.126).

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KX-FC228UA-T / KX-FC228RU-T : CORDLESS HANDSET BLOCK DIAGRAM

SPEAKER

RECEIVER

MIC

CHARGE
CIRCUIT

Q4,Q5,Q9
R7

CHARGE
CONTACTS

CHARGE(+)

CHARGE(-)

CHARGE

CHARGE DETECT

EEPROM

SCL

SDA

SWITCHED
SUPPLY

    Q1

Q1

BATTERY
TERMINAL

D7

VBAT1

DC/DC-SWITCH

J1

J2

2.5V

Q3

1.8V

Q2

4.0V

2.5V

1.8V

LCD

RS

RESET

CS

SI

 SCL

CPU

Analog

Front

End

D/A

A/D

30

32

66

67
72

62

52

45

44

 58

56

  59 

 60

6

  9

10

77

ADPCM

Codec

Filter

DSP

Speech

Decoding

Speech

Encoding

BMC

Burst

Decoding

Burst

Encoding

RF

Interface

RF

Module

23

22

RXDA

TXDA

19

26

25

24

RSSI

SYDA
SYEN
SYCL

16

17

XTAL

X1

10.368
MHz

BATTERY

61

ON SWITCH

KEYPAD

ROWS

COLUMNS

1,12,40,76

27,28,34,35,36,74

BBIC

 11

LCD-BACK  LIGHT

KEY-BACK  LIGHT

Ringer LED

 7

75

46

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Handset consists of the following ICs as shown in Block Diagram (P.46).

• DECT BBIC (Base Band IC): IC1

- All data signals (forming/analyzing ACK or CMD signal)
- All interfaces (ex: Key, Detector Circuit, Charge, DC/DC Converter, EEPROM, LCD)

• RF Module: IC4

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

• EEPROM: IC3

- Temporary operating parameters (for RF, etc.)

When power on the Handset, the voltage is as follows;
BATTERY(2.2 V ~ 2.6 V: J1) 

→ F1, L1, D1 → Q2 (1.8 V), Q3 (2.5 V), Q1 (4.2 V)

The Reset signal generates IC1 (66 pin) and 1.8 V.

When charging the handset on the Base Unit, the charge current is as follows;
CN202 (2) [6V] 

→ CN411 (2) → CN407 (2) → CN527 (2) → IC7 (2-3) → R787 → R786 → Q548 → R783 → R55, R56 → 

CHARGE+(Base) 

→ CHARGE+(Handset) → L4 → Q4 → D7→ F1 → BATTERY+ ... Battery ... 

BATTERY- 

→ R45 → GND → L5 → CHARGE-(Handset)→ CHARGE-(Base) → GND → DC-(GND)

In this way, the BBIC on Handset detects the fact that the battery is charged.
The charge current is controlled by switching Q5 of Handset.

"Battery Low" and "Power Down" are detected by BBIC which check the voltage from battery.
The detected voltage is as follows;

• Battery Low

Battery voltage: V(Batt)   2.25V ± 50mV
The BBIC detects this level and "

" starts flashing.

• Power Down

Battery voltage: V(Batt)   2.0V ± 50mV
The BBIC detects this level and power down.

The hands-free loudspeaker at SP+ and SP- is used to generate the ring alarm.

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