DocID17500 Rev 4 5/10
LCP12 Technical information
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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
flowing through Th1 becomes less negative than the negative holding current I
H-
, Th1
switches off. This holding current I
H-
is temperature dependent as per Figure 4
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 than the positive holding current I
H+
, Th2
switches off. This holding current I
H+
, typically 20 mA at 25 °C, is temperature dependent
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
