LCP02-150M datasheet(6/10 Pages) STMICROELECTRONICS

6 / 10 page

Technical information

LCP02-150M

6/10

Technical information

Figure 5.

LCP02 concept behavior

Figure 5. shows the classical protection circuit using the LCP02-150M 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 LCP02-150M 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. The entire surge current flows through the ground. After the surge, when the current

flowing through Th1 becomes less negative than the negative holding current, Th1 switches

off. This holding current IH- is temperature dependant as per Figure 2.

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. The entire 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 IH+ is temperature dependant and is equal to 30 mA at

25° C.

The capacitors CN and CP are used to speed up the crowbar structure firing during the fast

surge rise or falling 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-

150M 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 Rs1 and Rs2 represent the fuse, 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 LCP02-150M is equal to:

I surge = Vsurge / (Rg + Rs)

Where:

Vsurge = peak surge voltage imposed by the standard.

Rg = series resistor of the surge generator

Rs = series resistor of the line card (e.g. PTC)

The LCP02-150M topology is particularly optimized for the new telecom applications such

as cable modem, fiber in the loop, WLL systems, and decentralized central office for

example. The schematics of Figure 6. and Figure 7. give the 2 most frequent topologies

used for these emergent applications.

TIP

RING

GND

-Vbat

+Vb

Rs1

Rs2

L 1

L 2

GND

V Tip

Th1

Th2

Ign

Igp

V Ring

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