LCP02-150B1

6/8

Figure 5 shows the classical protection circuit using the LCP02 crowbar concept. This topology has been

developped to protect the new two-battery voltage SLICs. It allows both positive and negative firing thresholds

to be programmed. The LCP02-150B1 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 fires. All the surge current

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

as per figure2.

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 flowing through Th2 becomes less positive

temperature dependant and the same figure 2 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 to minimize the dynamical 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 LCP02-150B1 gate pins (Gn and Gp) and to the reference ground

track (or plan). The optimized value for Cn and Cp is 220nF.

The series resistors Rs1 and Rs2 designed in figure 5 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 LCP02-150B1 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)

e.g. : For a line card with 50

Ω of series resistors which has to be qualified under Bellcore 1000V 10/1000µs

surge, the present current through the LCP02-150B1 is equal to :

I surge = 1000 / (10 + 50) = 17A

TECHNICAL INFORMATION

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

Fig. 5: LCP02 concept behavior.