CPC5710N

R03

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7

3. Applications

3.1 Line-In-Use (LIU) and Line Polarity Detector

This circuit performs two phone line interface

functions, LIU and line polarity detection. The LIU

output is logic high if the tip to ring voltage is less than

approximately 17V. This will occur whenever a parallel

device on the line such as a telephone or FAX is taken

off hook. This circuit can also be used to detect for the

presence or loss of battery.

The Polarity output indicates the polarity of the phone

line. Whenever tip is positive with respect to ring,

Polarity out will be a logic high.

Figure 4. LIU and Polarity Detection Application Circuit

+V

+V

U1 CPC5710

LIU

Polarity

R1-4

8.06M

1%

1206

Ω

R5

604K

Ω

R17

4.7M

Ω

R15

10K

Ω

R14

4.7M

Ω

R16

10K

Ω

R6

4.7M

Ω

TIP

RING

Unless noted otherwise:

All resistors 1%, 1/16 W.

All capacitors 16 V 10%.

Switch shown

in Linear Mode

IN+

IN-

LINOUT+

+

+

-

-

-

+

1.625 V

1.25 V

2 V

0 V

A=6

V

3

6

7

1

2

5

4

GND

8

GND

+

-

+

-

50k

Ω

R1

R2

R3

R4

R9

499K

Ω

R13

20K

Ω

R12

20K

Ω

R11

499K

Ω

R8

20K

Ω

+V

1/4 U2

LM339

IN-

IN+

OUT

+V

1/4 U2

LM339

IN-

IN+

OUT

+V

1/4 U2

LM339

IN-

IN+

OUT

+V

R10

21K

Ω

R7

21K

Ω

C1

1nF

LIN/CMP

CMPOUT

LINOUT-