STLC3080 datasheet(20/23 Pages) STMICROELECTRONICS

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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 = 600

Ω 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 > 6

µF/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.

APPENDIX D

RINGING MODE AND RING TRIP DETECTION

RS1

RS2

CRT

ILINE

ICRT

VCC

Ith=7.5uA(typ)

DET

3Volt

COMP

IRING

IRING=ILINERS1/RR

RS1=RS2

Figure D1.ring trip circuit block diagram.

STLC3080

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