AN2064 - APPLICATION NOTE

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Figure 3 shows the classical protection topology used to protect one section of an analog subscriber line

card. A first stage uses both series overcurrent protection devices (fuses or PTCs) and TRISIL protectors

(TRISIL 1, threshold voltage = +/- 270V for US) allowing the ring relay to be protected against full lightning

and power contact surges. A second level (TRISIL 2, threshold voltage = 0/-Vbat) allows the SLIC to be

fine tuned protected. The presence of longitudinal series resistors makes the current capability rate of this

TRISIL to be adjusted.

Figure 4 shows the protection topology generally used to protect the ADSL modem (or line cards when

use of a line transformer). This modem is connected to the analog line card by means of a splitter stage

and then receives the same operating voltages. Please note that series resistors are generally not per-

mitted in such an application. Protection is provided by the TRISIL (TRISIL 1, threshold voltage = +/-

270V), which assumes the full lightning current while the series overcurrent protection devices (fuses or

PTCs) allow the module to be well protected against 50/60Hz power contact.

3. Series protection

As previously mentioned, the use of series overcurrent protection devices is mandatory to protect sub-

scriber line cards against 50/60Hz power contacts. As far as power contact is concerned, standards re-

quire the equipment to withstand several tests with different acceptance criteria. First level criterion for

US standards (or A criterion for European requirements) requires the equipment to be fully operational

after tests while second level (or B criterion) allows the system to be out of order but no fire or smoke is

permitted. Table 2 shows AC power fault requirements of the US Telcordia GR1089 standard.

Figure 4: CO ADSL modem protection circuit

Tip

Ring

TRISIL 1

TRISIL 1

C

TRISIL 2

Transformer

Fuse or PTC

Fuse or PTC

ADSL CO Modem

Secondary protection Fuse or PTC and TRANSIL 1

Fuse or PTC