Circuit Description Of Liu Pwb - Sharp FO-DC600 Service Manual

FO-DC600U

[4] Circuit description of LIU PWB

(1) LIU block operational description
1) Block diagram
SURGE
DIODE
PROTECTION
LINE
HI BRIDGE
CIRCUIT
LO
HS
DETECTOR
CI
EXT.TEL
DETECTOR
2) Circuit description
The LIU PWB is composed of the following 9 blocks.
1. Surge protection circuit
2. On-hook status detection circuit
3. Dial pulse generation circuit
4. CML relay
5. Matching transformer
6. Hybrid circuit
7. Signal selection
8. CI detection circuit
9. Power supply and bias circuit
LIU
TXOUT
ACTIVE
CHOKE
RXIN
+5V
CML0
XEXHS0
XCI0
5 – 16
CONTROL
TXA2
TXOUT
TXA1
HI
RXIN
LO
VREF
SPKR
CML0
XEXHS0
XCI0
LO
HI
HI
LO
SPEAKER
+5V
+24VA
DG
Fig. 8
3) Block description
1. Surge Protection circuit
This circuit protects the circuit from the surge voltage occurring on the
telephone line.
• The AR1 protect the circuit from the 390V or higher line surge
voltages.
• The AR2 and AR3 protect the circuit from the 500V or higher verti-
cal surge voltages.
• The ZD6 and ZD7 control the voltage generated on the secondary
side of matching transformer to 2V.
2. On-hook status detection circuit
The on-hook status detection circuit detects the Status of the push
speaker key, and the status of the hook of a telephone externally con-
nected.
• External telephone hook status detection circuit (XEXHS0)
This circuit comprises the photo-coupler PC3, resistors R13 and R12,
Zener diodes ZD1 and ZD2.
When an external telephone is connected and enters the on-hook
mode, the LED of photo-coupler PC3 emits light and the light receiv-
ing element tuns on. The status signal XEXHS0 is input to the pin10
of (SUB ASIC) (IC21: Control PWB).
XEXHS0 LOW : EXT. TEL OFF-HOOK
XEXHS0 HIGH: EXT. TEL ON-HOOK
3. Dial pulse generation circuit
The pulse dial generation circuit comprises CML.
CML ON: Make
CML OFF: Break
MODEM
(FM336PLUS)
RIN
CML0
XESHS0
XCI0
MONSEL
HI
BZOUTEN
LO
BZCONT
SPMUTE
VOLC
VOLB
VOLA
SUB ASIC