Datasheet
DocID16804 Rev 3 5/10
LCP1521S Technical information
10
2 Technical information
Figure 4. LCP concept behavior
Figure 4 shows the classical protection circuit using the LCP crowbar concept. This topology
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 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.
Figure 5. Example of PCB layout based on LCP1521S protection
Figure 5 shows the classical PCB layout used to optimize line protection.
The capacitor C is used to speed up the crowbar structure firing during the fast surge edges.
This allows minimization of the dynamic 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 LCP Gate pin and from the
reference ground track (or plan) (see Figure 5). The optimized value for C is 220 nF.
VRing
GND
Gn
TIP
RING
GND
-Vbat
C
Rs1
Rs2
L 1
L 2
VTip
Th1
D1
T1
IG
ID1
To
line side
22
0
nF
GND
To
SLIC side