AN2456

SLIC protection

7/15

3

SLIC protection

Figure 6.

Protection diagram for short line SLICs like STLC3055N located in remote

terminals

Figure 6 gives the protection circuit of a single battery voltage short line SLIC located in RT

in the US. In this particular application note the STLC3055N has been chosen as an

example of a single battery high voltage SLIC. High voltage SLICs like this, are well suited to

short line applications that require smaller battery and ringing voltages, as compared to long

line applications. Since the ringing signal is smaller, it is generated by the SLIC itself

(without external ring generator). These SLICs are capable of higher voltages (~ -70 V

during ringing) than classical long line SLICs (which generate ~ -56.6 V constant) and are

therefore called high voltage SLICs.

At any given time the line card could be in one of three operating modes i.e. idle mode,

ringing mode or speech mode. The protection circuit used must protect the line card during

all three operating modes. During idle mode there is only the battery voltage on the line and

when in speech or ringing mode the corresponding signal is superimposed on the battery

voltage. The LCP1521S assumes this protection function for all three operating modes.

The fuse TCP500mA from Cooper-Bussman along with a 10

Ω resistor is the serial

protection used to protect the LCP1521S and the EUT against power crossing disturbances.

The 50

Ω resistor is used to limit the current entering the SLIC.

Rin

g

Gen

.

SLI

C

STLC3055

N

-Vba

TIP

S

RING

S

220 nF

G

TIPL

RINGL

LCP1521S

Ring

Gen

.

SLIC

STLC3055N

-Vbat

TIPS

RINGS

TCP500 mA

220 nF

G

10 W

TCP500 mA

50 W

50 W

10 W

-Vbat

VTip

VRing