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APPLICATION NOTE

3/13

2. Application circuit: CLP270M in line card

Figure 3 shows the topology of a protected analog subscriber line at the exchange side. The CLP270M is

connected to the ring relay via two balanced Rp resistors, and to the Subscriber Line Interface Circuit. A

second device is located near the SLIC : it can be either a LCP1511D, a THDT series or a LCDP1511D.

These two devices are complementary and their functions are explained below :

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The first stage based on CLP270M manages the high power issued from the external surges. When

used in ringing mode, the CLP270M operates in voltage mode and provides a symmetrical and

bidirectional overvoltage protection at +/- 270 V on both TIP and RING lines. When used in speech

mode, the CLP270M operates in current mode and the activation current of the CLP270M is adjusted

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The second stage is the external voltage reference device which defines the firing threshold voltage

during the speech mode and also assumes a residual power overvoltage suppression. This protection

stage can be either a fixed or programmable breakover device. The THDTxx family acts as a fixed

breakover device while the LCP1511D or the LCDP1511D operates as a programmable protection.

Thanks to this topology, the surge current in the line is reduced after the CLP270M. Because the

remaining surge energy is low, the power ratings of Rp, the ring relay contacts and the external voltage

reference circuit can be downsized. This results in a significant cost reduction.

Rsense

Rp

1

2

Overvoltage

reference

(+/- 270 V)

Overvoltage

detector

Overcurrent

detector

OR

TIPL

TIPS

FS

SW3 SW1

Fuse

TIP

GND

1/2 CLP270M

V

1

2

3

-270

+270

A1

Fig. 4: Switching by voltage during ringing mode.

3. Ringing mode

In ringing mode (Ring relay in position 2), the only protection device involved is the CLP270M.

In normal conditions, the CLP270M operates in region 1 of A1 curve, and is idle.

If an overvoltage occuring between TIP (or RING) and GND reaches the internal overvoltage reference

(+/- 270 V), the CLP270M acts and the line is short-circuited to GND. At this time the operating point

moves to region 2 for positive surges (region 3 for negative surges). Once the surge current falls below

For surges occuring between TIP and RING, the CLP270M acts in the same way. This means that the

CLP270M ensures a tripolar protection.

When used alone, the CLP270M acts at the internal overvoltage reference level (+/- 270 V). Furthermore, it

is possible to adjust this threshold level to a lower voltage by using:

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