ST AN1220 APPLICATION NOTE

AN1220

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®

APPLICATION NOTE

LINECARD PROTECTION BY OVERVOLTAGE

A.S.D

INTRODUCTION

This device includes a primary protection level and is suitable for main distribution frames and line cards.
This protection concept is explained and, in addition, the CLP270M performances are analysed when
facing different surges as described in the BELLCORE GR 1089 recommendations.

Figure 1 is a simplified block diagram of a subscriber line protection that is commonly used.

Fig. 1: Subscriber line protection topology

ä

Telecommunication

line

AND OVERCURRENT WITH A CLP270M

P.Merceron / A.Bremond

“PRIMARY PRO TECTION”

CLP270M

MDF

EXCHANGE

“SECONDARY PROTECTION”

SLIC

LINE CARD

“SECONDARY PROTECTION”

Telecommunication

line

CLP270M

MDF

EXCHANGE

THDTxx

or

LCP1511D

or

LCDP1511D

LINE CARD

SLIC

This shows two different topologies :

n

A“primaryprotection”locatedontheMainDistributionFrame(MDF) eliminates coarsely the high energy
environmental disturbances (lightning transients and AC power mains disturbances)

n

A “secondary protection” located on the line card includes a primary protection level (first stage) and a
residual protection (second stage) which eliminates finely the remaining transientsthat have not been totally
suppressed by the first stage.

January 2000

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APPLICATION NOTE

STMICROELECTRONICS CLP270M CONCEPT

1. Evolution of the SLIC protection

Fig. 2: Line card protection

Over the years, the silicon protection
performances have considerably changed.

The first generation of products like SMTHBTxx
and SMTHDTxx offered fixed overvoltage
protectionagainst surges oneither TIP orRING line
in four packages.

The following generation like THBTxx and

Programmable

thanks to any

external v o ltage

reference

+I

I

SWON

Programmable

thanks to an

external resistor

V

THDTxx still offered fixed overvoltage protection
against surges on both TIP and RING lines in two
packages.

The next step was the introduction of the LCP
devices which brought the advantage of full
programmable voltage.

-I

SWON

Line card
operating
conditions

Today, the CLP270M combines the features of all
the previous generations. In addition to that, it offers an overcurrent detection when operating in speech
mode and also a Failure Status output signal.

The figure 2 summarizes the firing modes of the CLP270M which basically holds the SLIC inside its correct
voltage and current values.

Fig. 3 : CLP270M in line card

er oltage

erence

I

-Vbat

TIP

-Vbat

(*)

SLIC

RING

Rp

Rp

1

2

1

2

External

voltage

reference

Ring

Generator

(*) LCP1511D orTHDT series or LCDP1511D

TIP

RING

Fuse

Rsense

TIPL TIPS

Overcurrent

detector

detector

detector

RINGS

Ov v

ref

(+/- 270V)

Overvoltage

reference

(+/- 270 V)

OR

SW3 SW1

FS GND

Fuse

SW4 SW2

OR

Overcurrent

RINGL

Rsense

Overvoltage

Overvoltage

detector

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APPLICATION NOTE

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 :

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

n

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 incurrent mode and the activation current of the CLP270Mis adjusted

sense

.

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

n

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.

Fig. 4: Switching by voltage during ringing mode.

Fuse

TIP

ILG

Rsense

TIPL TIPS

1/2 CLP270M

Overcurrent

detector

V

Overvoltage

Overvoltage

detector

OR

SW3 SW1

FS

reference

(+/- 270V)

LG

GND

1

Rp

2

-270

I

LG

A1

2

1

+270

3

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.
Ifan overvoltage occuring between TIP (or RING)and GND reaches theinternal 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 negativesurges). Once the surge current falls below
the switch off current I

, the device returns to its initial state (region 1).

SWOFF

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

V

Whenused alone, theCLP270M acts atthe internal overvoltagereference level (+/-270 V). Furthermore,it
is possible to adjust this threshold level to a lower voltage by using:

n

up to 4 fixed external voltage reference (VZ1to VZ4) (see fig. 5a, here-below).

n

external reference supplies, Vb1and Vb2(see fig. 5b, on next page).

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APPLICATION NOTE

Fig. 5a: Method to adjust the reference voltage.

Fuse

TIP

Rsense

1

Rp

TIPL TIPS

Overcurrent

detector

Overvoltage

detector

Overvoltage

detector

RING

OR

SW3 SW1

FS GND

SW4 SW2

RINGL

OR

Overcurrent

detector

Rsense

Fuse

Fig. 5b: Method to adjust the reference voltage.

Fuse

TIP

Rsense

TIPL TIPS

RINGS

Overvoltage

reference

(+/- 270 V)

Overvoltage

reference

(+/- 270 V)

2

VZ1

VZ2

VZ3

VZ4

1

Rp

2

1

Rp

2

Overcurrent

detector

VB1

VB2

1

2

RING

Overvoltage

OR

SW3 SW1

FS GND

SW4 SW2

RINGL

OR

Overcurrent

detector

detector

Overvoltage

detector

RINGS

Overvoltage

reference

(+/- 270 V)

Overvoltage

reference

(+/- 270 V)

Rsense

Fuse

Rp

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