LCDP1521

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TECHNICAL INFORMATION

GND

Gate

TIP

RING

GND

-Vbat

C

Rs1

Rs2

L 1

L 2

V Tip

Th1

D1

T1

IG

ID1

Fig. A1: LCDP1521 concept behavior.

Figure A1 shows the classical protection circuit using the LCDP1521 crowbar concept. This topol-

ogy has been developed to protect the new high voltage SLICs. It allows to program 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 fires and all the surge current

flows through the ground. After the surge when the current flowing through Th1 becomes less negative

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 allows to minimize the dynamical breakover voltage at the SLIC Tip and Ring inputs during fast

strikes. Note that this capacitor is generally present around the SLIC - Vbat pin.

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

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

The series resitors Rs1 and Rs2 designed in figure A1 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:

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+Ron Fig. A2)

e.g. For a line card with 60

Ω of series resistors which has to be qualified under GR1089 Core 1000V

10/1000

µs surge, the actual current through the LCDP1521 is equal to:

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

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 A2 gives the

most frequent topology used for these applications.