STLC3085

Functional description

15/28

Figure 6.

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.

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 STLC3085 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 consumption to the fixed limit and therefore also the ring voltage level will be

reduced.

limited to about 770mA, which correspond to an average current of 600mA max. In this

condition the STLC3085 can drive up to 2REN with a ring frequency fr=25Hz (1REN = 1800

Ω +

1.0

µF, European standard).

USA standards.

CREV

CREST FACTOR @20Hz

CREST FACTOR @25Hz

22nF

1.2

1.26

27nF

1.25

1.32

33nF

1.33

(1) Distorsion already less than 10%.

GND

TIP

RING

dV/dT set

by CREV

2.5V typ.

59V

typ.

VBAT

2.5V typ.