Datasheet

Technical information LCDP1521

6/12 Doc ID 8627 Rev 4

4 Technical information

Figure 6. LCDP1521 concept behavior

Figure 6 shows the classic protection circuit using the LCDP1521 crowbar concept. This

topology has been developed to protect the new high voltage SLICs. This supports the

programming of 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 I

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 I

, 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 minimizes the dynamic breakover voltage at the SLIC Tip and Ring inputs during fast

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

BAT

pin.

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 resistors Rs1 and Rs2 in Figure 6 represent the fuse resistors or the PTC which

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

Table 7. Test circuit component values

Pulse (µs) V

(V) (μF) (nF) (μH) (Ω)(Ω)(Ω)(Ω)(A)(Ω)

10 700 1500 20 200 0 50 15 25 25 10 110

1.25015001 33 0 761325251560

2 10 2500 10 0 1.1 1.3 0 3 3 10 245

VRing

GND

Gate

TIP

RING

GND

-Vbat

Rs1

Rs2

L 1

L 2

V Tip

Th1

ID1