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STLC3055N

3.2.4 Ringing

When this mode is selected STLC3055N self generate an higher negative battery (-70V typ.) in order to

allow a balanced ringing signal of typically 65Vpeak.

In this condition both the DC and AC feedback are disabled and the SLIC line drivers operate as voltage

buffers. The ring waveform is obtained toggling the D2 control bit at the desired ring frequency. This bit in

fact controls the line polarity (0=direct; 1=reverse). As in the ACTIVE mode the line voltage transition is

performed with a ramp transition, obtaining in this way a trapezoidal balanced ring waveform (see fig.8).

The shaping is defined by the CREV external capacitor.

Selecting the proper capacitor value it is possible to get different crest factor values.

The following table shows the crest factor values obtained with a 20Hz and 25Hz ring frequency and with

1REN. These value are valid either with European or USA specification

Figure 8. TIP/RING Typical Ringing Waveform

Table 9. :

(*) Distorsion already less than 10%.

The ring trip detection is performed sensing the variation of the AC line impedance from on hook (relatively

high) to off-hook (low). This particular ring trip method allows to operate without DC offset superimposed

on the ring signal and therefore obtaining the maximum possible ring level on the load starting from a given

negative battery. It should be noted that such a method is optimised for operation on short loop applica-

tions and may not operate properly in presence of long loop applications (> 500

Ω ). Once ring trip is de-

tected, the DET output is activated (logic level low), at this point the card controller or a simple logic circuit

should stop the D2 toggling in order to effectively disconnect the ring signal and then set the STLC3055N

in the proper operating mode (normally ACTIVE).

3.2.4.1 Ring Level in Presence of More Telephone in Parallel

As already mentioned above the maximum current that can be drawn from the Vpos supply is controlled

and limited via the external RSENSE. This will limit also the power available at the self generated negative

battery.

If for any reason the ringer load will be too low the self generated battery will drop in order to keep the

power consumption to the fixed limit and therefore also the ring voltage level will be reduced.

Ω the peak current from Vpos is limited to about 900mA,

which correspond to an average current of 700mA max. In this condition the STLC3055N can drive up to

CREV

CREST FACTOR @20Hz

CREST FACTOR @25Hz

22nF

1.2

1.26

27nF

1.25

1.32

33nF

1.33

Not significant (*)

GND

TIP

RING

dV/dT set

by CREV

2.5V typ.

65V

typ.

VBAT

2.5V typ.