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
than the negative holding current Ih-, Th1 switches off. This holding current I
H-
is temperature dependant
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
than the positive holding current Ih+, Th2 switches off. This holding current I
H+
, typically 20mA at 25°C, is
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 50Ω 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.
