Functional description

STLC3055N

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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 Figure 7). 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 20 Hz and 25 Hz ring

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

Figure 7.

TIP/RING typical ringing waveform

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

STLC3055N in the proper operating mode (normally Active).

Ring level in presence of more telephone in parallel

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

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.

about 900 mA, which correspond to an average current of 700 mA max. In this condition the

STLC3055N can drive up to 3REN with a ring frequency fr=25 Hz (1REN = 1800

Ω + 1.0 µF,

European standard).

Table 9.

CREST factor

CREV

CREST factor @20 Hz

CREST factor @25 Hz

22nF

1.2

1.26

27nF

1.25

1.32

33nF

1.33

1.

Distortion already less than 10%.

GND

TIP

RING

dV/dT set

by CREV

2.5V typ.

65V

typ.

VBAT

2.5V typ.