STLC3080 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STLC3080' PDF : 23 Pages View PDF
STLC3080
APPENDIX D
RINGING MODE AND RING TRIP DETECTION
In ringing mode the STLC3080 provides:
- Relay driver capability (relay is synchronized
with low level of CKRING)
- Ring-Trip detection
The monitor of the line state is performed by
sensing the line current converted into a voltage
drop across the RR resistor connected in series
to the line. This voltage is read via RS1and RST2
input pins of a differential stage that identifies,
during the ringing phase, the ON/OFF HOOK
state of the line (see Fig. D1).
The Ring-Trip condition is detected by sensing
the DC component of the line current, rejecting
the AC component. With RR = 600Ω the Ring-
Trip threshold is: Iline>7.5mA
When the Ring-Trip is detected, the STLC3080:
- deactivates the ringing relay RELR (if CKRING
is low);
- indicates the ring-trip detection by setting
DET=low;
- forces the Active state.
The information at RELR and DET pins is lached
and it doesn’t change opening the current loop.
To reset the latched informations the Active or
On-Hook Transmission mode have to be entered
(in general changing the device mode the latched
information is removed).
Although the ring-trip detection sets DET to signal
the line status, there is a substantial difference re-
spect to the on/off-hook detection. In Ringing
mode on-hook condition, an AC current is present
on the line. The ring-trip detector rejects the AC
component by integrating the line current: the de-
tection threshold can be reached only if the line
current has a DC component higher than the
threshold. As a consequence the response is not
immediate (as it is for off-hook in Active state): it
takes an amount of time that is dependent on the
DC current value (i.e. on the line length). The AC
rejection and the delay depend on the CRT ca-
pacitor value (see Fig.D1).
When the voltage on the capacitor exceeds 3V,
the Ring-Trip is detected (see fig.D3). CRT
should be selected in order to avoid that during
one half sinewave cycle, in on-hook, its voltage
VCRT exceeds 3V (ring-trip threshold). The mini-
mum value of CRT can be carried out with the fol-
lowing formula:
Ccrt > 6µF/Fring
With Fring = 20Hz, you obtain a Ccrt = 390nF.
When the CRT capacitor is selected, it must be
considered that it is also used for the rejection of
the common mode current. In this case the mini-
mum value of the CRT capacitor can be carried
out with the following formula:
Ccrt > (Ip/Fl) ⋅ 560µF
Where Ip is the peak of the longitutudinal current
and Fl is the frequency of this current. With Ip =
25mA @ Fl = 50Hz you obtain 330nF.
For this reasons the suggested value for typical
central office application is 470nF.
Figure D1.ring trip circuit block diagram.
ILINE
RS1
RR
RS2
VCC
Ith=7.5uA(typ)
IRING
ICRT
COMP
3Volt
DET
CRT
IRING=ILINERS1/RR
VB
RS1=RS2
20/23
Share Link: 
