DocID17500 Rev 4

5/10

LCP12

Technical information

10

2

Technical information

Figure 7. LCP12 concept behavior

Figure 7 shows the classical protection circuit using the LCP12 crowbar concept. This

topology has been developed to protect two-battery voltage SLICs. It allows both positive

and negative firing thresholds to be programmed. The LCP12 has two gates (Gn and Gp).

Gn is biased to negative battery voltage -Vbat, while Gp is biased to the positive battery

voltage +Vb.

When a negative surge occurs on one wire (L1 for example), a current IGn flows through the

base of the transistor T1 and then injects a current in the gate of the thyristor Th1 which

turns-on. All the surge current flows through the ground. After the surge, when the current

When a positive surge occurs on one wire (L1 for example), a current IGp flows through the

base of the transistor T2 and then injects a current in the gate of the thyristor Th2 which

fires. All the surge current flows through the ground. After the surge, when the current

and the same Figure 4 also applies.

The capacitors Cn and Cp are used to speed up the crowbar structure firing during the fast

rise or fall edges. This allows minimization of the dynamic breakover voltage at the SLIC TIP

and RING inputs during fast surges. Please note that these capacitors are generally

available around the SLIC. To be efficient they have to be as close as possible to the LCP12

gate pins (Gn and Gp) and to the reference ground track (or plan). The optimized value for

Cn and Cp is 220 nF.

The series resistors Rs shown in Figure 7 represent the fuse resistors or the PTCs which

are needed to withstand the power contact or the power induction tests imposed by the

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

through the LCP12 is equal to:

I surge = Vsurge / (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 (e.g. PTC)

Cp

Gn

TIP

RING

Gp

GND

-Vbat

+Vb

Cn

Rs1

Rs2

L 1

L 2

GND

V Tip

Th1

Th2

T1

T2

IGn

IGp

V Ring