ST AN2456 APPLICATION NOTE

AN2456

Application note

Protection of single battery voltage SLICs for new networks in US market

Introduction

Despite the widespread proliferation of digital technologies, analog networks remain the most commonly used telecommunication means in the world. Thanks to its simple and cheap technology, POTS (plain old telephone set) are still utilized extensively.

Figure 1. Schematic of new networks

Central Office / LT

Long lines

(Twisted pair)

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Short lines

(Twisted pair)

Optical

Optical Network

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Central Office / LTCentral Office / LT

Figure 1 shows a schematic of modern telecom networks that are expected to keep

growing. These networks are essentially made up of two kinds of lines, classical long lines (several kilometers long) that directly connect the Central Office (CO) to the subscriber, and short lines (few tens of meters in length) that connect Remote Central Offices (RCO), Optical Network Units (ONU) or Radio Network Units (RNU) to subscriber terminals. In these networks SLICs can be present in either conventional Central Offices or any of the above mentioned remote locations or in Remote Terminals (RT) in the customer premises which connect to POTS through short lines. The link between these remote locations and the CO is established by a long high speed digital link e.g. coaxial cable, fiber optic cable, Wireless Local Loop (WLL). This disparity in line length has given rise to SLICs of two types. Protection of both kinds of SLICs is a very important aspect in the design of the system. The purpose of this application note is to propose a complete protection solution for short line SLICs located in RT in customer premises.

November 2006 Rev 1 1/15

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Contents AN2456

1 Telecom disturbances

Figure 2 shows the different disturbances that may appear on the telecom line. Among

these, lightning and power crossing are the main considerations for wireline applications. These disturbances are well defined in the country standards.

Figure 2. Line disturbance causes

Subscriber

Location

Remote

Terminal

SLIC

Protection

Short

line

POTS

Atmospheric effects (Lightning)

50/60Hz mains disturbances

Electrostatic discharges

Central office

As the purpose of this application note is to illustrate the protection topology needed for short line SLICs located in RT in customer premises in the US, the standard to be considered for lightning and power crossing tests is the Telcordia GR1089 Intrabuilding (equipment port type 4).

The Telcordia GR1089 specifies two acceptance criteria:

● First level: the equipment shall continue to operate properly after the test.

● Second level: the equipment may be damaged, but should not become a fire or

electrical hazard.

For intrabuilding equipment, however, only first level acceptance criterion need to be satisfied for lightning tests.

Ta bl e 1 and Tab le 2 give the list of lightning and power crossing tests in the Telcordia

GR1089 intrabuilding standard for equipment port type 4.

Table 1. Telecordia GR1089 intrabuilding lightning tests

Voltage and

Surge

Peak voltage

(V)

1 ±800 100 2/10 1 M (on tip and ring)

2 ±1500 100 2/10 1 L

1. M: metallic; L: longitudinal

Peak current

(A)

current

waveforms

(µs)

Repetitions of

each polarity

Tes t

configuration

(1)

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Telecom disturbances AN2456

Table 2. Telecordia GR1089 2nd level intrabuilding power crossing test

(equipment port type 4)

Tes t Tes t for

Voltage

(V

rms

Current

)

(A

rms

)

Duration

Test

configuration

(1)

1 Secondary contact 120 25 15 minutes M (on tip and ring), L

1. M: metallic; L: longitudinal

Table 3. IEC61000-4-2 ESD surge standard

Contact discharge Air discharge

Level Test voltage (kV) Level Test voltage (kV)

1212

2424

3638

48415

(1)

X

1. “X” is a level to be defined.

Special X

(1)

Special

Ta bl e 3 shows the most commonly used worldwide standard for ESD. Generally, Level 4 is

required. Such tests appear in the Telcordia GR1089 standard and do apply to equipment port type 4.

The next section presents the protection concept of the LCP152xx used to protect short line SLICs with a single battery voltage.

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AN2456 LCP152xx concept

2 LCP152xx concept

Figure 3. LCP152xx concept behavior

GND

L 1

L 2

Fuse1

-Vbat

Fuse2

TIP

ID1

T1

IG

Gate

C

T1

Th1

RING

D1

ID1

GND

V Tip

V Ring

Figure 3 shows the protection circuit using the LCP152xx crowbar concept that can be used

for protection of single battery high voltage SLICs. It’s noteworthy that the schematic does not show series resistors that would normally be used on wires L1 and L2. This configuration permits the negative firing threshold V

of the LCP152xx to be programmed at

g

the negative battery voltage of the SLIC (up to -150 V), while the positive clamping threshold is at the GND.

Figure 4. LCP152xx electrical characteristics

I

Vg

Vf

Ih

V

It can be seen from the characteristics of the device (shown in Figure 4) that the device operates asymmetrically.

Under normal operating conditions i.e. when the voltage on the line is between 0 and –V the LCP152xx is transparent to the application.

For positive surges on either wire (e.g. L1), the diode D1 clamps the surge voltage to its forward voltage (Vf).

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