LCDP1521_06 Datasheet, PDF (7/11 Pages) STMicroelectronics – Dual line programmable transient voltage suppressor for SLIC protection

LCDP1521
5 Technical information
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.
The capacitor C is used to speed up the crowbar structure firing during the fast surge edges.
This mimimizes 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 resitors Rs1 and Rs2 in Figure 2 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 + R on Figure 3.)
Example: For a line card with 60 Ω of series resistors which has to be qualified under GR1089
Core 1000V 10/1000µs 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 3 shows the topologies most frequently used for these applications.
Figure 3. Protection of high voltage SLICs
-Vbat
PTC or Fuse
LCDPxxxx
R
TIP
Line 1
R
PTC or Fuse
SLIC 1
RING
R
PTC or Fuse
Line 2
R
PTC or Fuse
SLIC 2
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