STLC3055N_09 Datasheet, PDF (14/34 Pages) STMicroelectronics – WLL and ISDN-TA subscriber line interface circuit

Functional description
STLC3055N
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
GND
TIP
2.5V typ.
65V
typ.
dV/dT set
by CREV
RING
VBAT
2.5V typ.
Table 9. CREST factor
CREV
CREST factor @20 Hz
CREST factor @25 Hz
22nF
1.2
27nF
1.25
33nF
1.33
1.26
1.32
Not significant ⁽¹⁾
1. Distortion 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
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
is controlled and limited via the external R_SENSE. 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.
In the typical application with R_SENSE = 110 mΩ the peak current from Vpos is limited to
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).
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