AN2064 - APPLICATION NOTE

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1. TRISIL™ selection

Transient voltage suppressor (TRISIL) selection has to take into account the two working modes it will

meet during its life. The first mode is the normal operating mode where the protection device has to be

transparent, that means no impact on the speech or data signal. The second mode is the suppression

mode where the TRISIL has to eliminate all dangerous transient voltage surges.

During normal operation, we have to focus on voltages and currents managed in the line. These values

depend on the specific countries where equipment is located and the type of signal being managed

(analog or digital). For example, in the US the nominal battery voltage is -56.6V and the ringing voltage

between +155.5V and -268.7V in ringing mode. In digital networks the voltages can be the same as those

used in analog networks, as is the case with ADSL. Frequently, ringing is managed by digital code where

only the battery voltage is present (generally -100V), as is the case in ISDN applications. When the

telephone is picked up, the loop current increases and indicates to the CO to stop the ring signal or to wait

for dialing signals. Call connection occurs when the loop current exceeds a few milliamps. Analog CO

systems may use series resistors while digital systems do not. These resistors (10

Ω to 100Ω depending

on the applicable country standard) are used to manage line longitudinal balancing while the use of any

series resistance is forbidden in ADSL system. From these requirements we can conclude that the TRISIL

threshold voltage has to be higher than 268.8V for US analog and ADSL networks (190V for Europe) while

the TRISIL leakage current has to be lower than 1mA. The right choice for US is 270V (200V for Europe)

and the leakage current is less than a few µA.

When considering the suppression mode, we have to take into account that telecommunication lines can

be subjected mainly to two kinds of disturbances. The first disturbance is linked to atmospheric effects

while the second disturbance is produced by contact or proximity with the 50/60Hz mains network. These

disturbances are well defined in standards, which can be worldwide or dedicated to a specific country.

Figure 2: One wire telecom protection topology

Line side

Series element

Transient Voltage Suppressor

or

TRISIL

Protected side