LCDP1521

5 Technical information

7/11

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 fires and

all the surge current flows through the ground. After the surge when the current flowing through

Th1 becomes less negative 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 flows through the ground.

The capacitor C is used to speed up the crowbar structure firing during the fast surge edges.

This mimimizes the dynamic breakover voltage at the SLIC Tip and Ring inputs during fast

So, to be efficient, it has to be as close as possible to the LCDP1521 Gate pin and to the

reference ground track (or plan). The optimized value for C is 220 nF.

The series resitors Rs1 and Rs2 in Figure 2 represent the fuse resistors or the PTC which are

mandatory to withstand the power contact or the power induction tests imposed by the various

country standards. Taking into account this fact, the actual lightning surge current flowing

through the LCDP is equal to:

Example: For a line card with 60

Ω of series resistors which has to be qualified under GR1089

Core 1000V 10/1000µs surge, the actual current through the LCDP1521 is equal to:

The LCDP1521 is particularly optimized for the new telecom applications such as the fiber in

the loop, the WLL, the remote central office. In this case, the operating voltages are smaller

than in the classical system. This makes the high voltage SLICs particularly suitable. The

schematics of Figure 3 shows the topologies most frequently used for these applications.

Figure 3.

Protection of high voltage SLICs

I surge = V surge / (Rg + Rs)

With

V surge = peak surge voltage imposed by the standard.

Rg = series resistor of the surge generator

Rs = series resistor of the line card (equivalent to PTC + R on Figure 3.)

I surge = 1000 / (10 + 60) = 14A

LCDPxxxx

SLIC 1

-Vbat

PTC or Fuse

PTC or Fuse

R

R

Line 1

PTC or Fuse

PTC or Fuse

Line 2

RING

TIP

SLIC 2

R

R