ST AN2053 APPLICATION NOTE

AN2053

Application note

SLIC protection for both classical and new networks

Introduction

Even with booming digital technologies, telecom analog lines remain the most used link in the world. The market opening to new operators, reserved so far to national telecom administration, makes an increase of new applications using this simple and cheap way to supply speech information. POTS (plain old telephone set) is still alive.

Figure 1. Proposed solutions for the subscriber

Central office

	Pots
	Suscriber house
	Pots
Long line
Short line	High speed to
	Pots coupling
High speed digital link

Figure 1 shows possibilities subscribers already have got and which will be in a growing phase in the near future. This will split the SLIC (Subscriber Line Interface Circuit) in two different types according to the application:

■The long lines using the classical copper twisted pairs up to several kilometers long

■The short lines (only a few tens of meter long)

In the second case shown at the bottom of Figure 1, the long distance carrying of the signal is assumed by modern digital supports like optical fibers, coax, RF link etc.

For both of these applications the protection needs remain one of the major issues of the system design, so STMicroelectronics, which is one of the major players in the world of telecom protection, already proposes optimized solutions for these two topologies.

June 2011

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www.st.com

Protect against what?	AN2053

1 Protect against what?

Telecommunication lines are submitted mainly to two kinds of disturbances. The first one is linked to atmospheric effects while the second one is produced by the 50/60 Hz mains network (see Figure 2). These disturbances are well defined in individual country standards and Table 1 shows the main standards in use.

Figure 2. Main telecom line disturbance causes

Atmospheric effects	Central office
	ESD
	50/60 Hz mains effects

Figure 3 shows an example of lightning surge definition. This is given by the ITU-T K20 standard (International Telecommunication Union). This simulation is based on the discharge of a 20 µF capacitance through resistances. The 20 µF capacitance and the 50 Ω resistance define the surge wave duration while the 15 Ω resistance and the 0.2 µF capacitance manage the rise time. In this case the surge is defined as a 10/700 µs wave. The tests shall be managed in both transversal and longitudinal modes.

Figure 3. ITU-T K20 lightning surge test definition

			R3 = 25 Ω
				Coupling	A		Equipment
		Surge				Equipment
				Coupling			under test
		Surge		network		under test
		generator		network	B
							E
		generator
	R2 = 15 Ω	Figure 1
Uc		K20	transversal test
20 µF	0.2 µF
	R1 = 50 Ω		R4 = 25 Ω		A		Equipment
				Coupling
		Surge	R5 = 25 Ω				under test
				network
	K20 surge generator
		generator			B		E
		K20	longitudinal test

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AN2053	Protect against what?

Figure 4 shows the ITU-T requirement for both the mains induction and contact test circuits. This simulation is based on the application of 50/60 Hz through resistance during a programmed duration (i.e. 0.2 s for induction and 15 min. for contact).

Figure 4. ITU-T K20 power induction and power contact surge test definition

				10 Ω
				160 Ω	T1	A
				600Ω
R = 600 Ω
		A Equipment
Surge				S		Equipment
generator	Ω	under test
						under test
R = 600		B E		10 Ω
Uac			Uac
				160 Ω
Timing circuit			Timing circuit		T2	B E
				600 Ω
K20 power induction surge generator			K20 power contact surge generator

Table 1.	Main line card lightning surge standards
Country		Standard	Surge	Waveform	Current (A)
			voltage (V)
Worldwide		ITU-T K20	1500	10/700 µs	37.5
Worldwide		IEC 61000-4-5	1000/4000	10/700 µs	25/100
Worldwide		IEC 61000-4-5	1000/4000	1.2/50 µs	25/100
USA		GR-1089 Core (Telcordia)	2500	2/10 µs	500
USA		GR-1089 Core (Telcordia)	1000	10/1000 µs	100

	Table 2.	GR-1089 Core Intra-building lightning surge standard
				Surge Current per	Repetitions
	Test	Surge Voltage (V)	Waveform		Each
				conductor (A)
					polarity
	1	±800	2/10 µs	100	1
	2	±1500	2/10 µs	100	1

Table 1 and 2 show the main worldwide lightning surge standards. Table 1 is dedicated to classical wired telecom line cards while the Table 2 is dedicated to intra-building applications. The main worldwide standards for the 50/60 Hz disturbances can be defined by 2 parameters: the applied voltage, between 60 to 1000 V and the test duration, between 0.2 s to 15 min. This type of disturbances obliges the designer to put series elements, like PTC or a fuse between line and protection devices.

Section 2 presents the protection concept used to protect both short and long lines.

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LCP concept	AN2053

2 LCP concept

Figure 5. LCP15xx concept behavior

Rs1
L 1				TIP
				V Tip
	Ig			ID1
		T1	Th1	D1
-Vbat		Gate		GND
GND
C
Rs2				RING
L 2
				V Ring

Figure 5 shows the classical protection circuit using the LCP15xx crowbar concept. This topology has been developed to protect the new high voltage SLICs. It allows the system to be programmed for the negative firing threshold while the positive clamping value is fixed at GND.

When a negative surge occurs on one wire (L1 for example), a current Ig flows through the base of the transistor T1 and then injects a current in the gate of the thyristor Th1. Th1 turns on and all the surge current is short circuited to ground. After the surge, when the current flowing through Th1 becomes lower than the holding current Ih, then Th1 switches off.

When a positive surge occurs on one wire (L1 for example) the diode D1 conducts and the surge current is short circuited to ground.

In order to minimize the remaining voltage across the SLIC inputs during the surge, a 4 point structure has been implemented (Pins 1 and 8 for TIP / Pins 4 and 5 for RING). This fact allows the board designer to connect the track as designed in Figure 6. With such a PCB

design, extra voltages caused by track stray inductance and current slope (Ldi/dt) remain located on the line side of the LCP and do not affect its SLIC side.

The capacitor C is used to speed up the crowbar structure firing during the fast negative surge edges. This allows the dynamic breakover voltage at the SLIC Tip and Ring inputs to be minimized during fast strikes. Please note that this capacitor is generally present around

the SLIC -Vbat pin. So to be efficient it has to be moved as close as possible to the LCP15xx Gate pin and to the reference ground track (or plan) (see Figure 6). Optimized value for C is

220 nF.

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AN2053	LCP concept

Figure 6. Example of PCB layout based on LCP15xx protection

To line side

2 2 0 n F

GND

To SLIC side

The series resistors Rs1 an Rs2 designed in Figure 5 represent the fuse resistors or the PTC which are mandatory to withstand the power contact or the power induction tests imposed by the different country standards. Taking into account this fact, the actual lightning surge current flowing through the LCP is equal to:

Isurge = Vsurge / (Rg + Rs)

With:

●Vsurge = peak surge voltage imposed by the standard

●Rg = series resistor of the surge generator

●Rs = series resistor of the line card (e.g. PTC)

For a line card with 30 Ω of series resistors which has to be qualified under GR-1089 Core 1000 V, 10/1000 µs surge, the actual current through the LCP1521S is:

Isurge = 1000 / (10 + 30) = 25 A

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