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STLC3075

3.2.4 Ringing

When this mode is selected STLC3075 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 loop are disabled and the SLIC line drivers operate as volt-

age 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.

Figure 8. TIP/RING typical ringing waveform

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:

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 applications and may not

operate properly in presence of long loop applications (> 500

Ω).

Once ring trip is detected, 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 STLC3075 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 is too low the self generated battery will drop in order to keep the power

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.