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ATTL7554BP

L7554 Low-Power SLIC

Agere -> LSI Corporation 

Data Sheet

March 1997

L7554 Low-Power SLIC

Applications (continued)

dc Applications (continued)

Ring Trip Detection

The ring trip circuit is a comparator that has a special in-

put section optimized for this application. The equiva-

lent circuit is shown in Figure 31, along with its use in an

application using unbalanced, battery-backed ringing.

PHONE

HOOK SWITCH

RLOOP

RC PHONE

RTS1

402 Ω

RTS2

274 kΩ

VRING

RTSP

2 MΩ

CRTS2

0.27 µF

RTSN

2 MΩ

VBAT

RTSP +

IP = IN

CRTS1

0.022 µF

IN

+

–

7V

–

RTSN 15 kΩ

NRDET

Figure 31. Ring Trip Equivalent Circuit and

Equivalent Application

12-3014 (C)

The comparator input voltage compliance is VCC to

VBAT, and the maximum current is 240 µA in either

direction. Its application is straightforward. A resistance

(RTSN + RTS2) in series with the RTSN input establishes a

current that is repeated in the RTSP input. A slightly

lower resistance (RTSP) is placed in series with the RTSP

input. When ringing is being injected, no dc current

flows through RTS1, so the RTSP input is at a lower

potential than RTSN. When enough dc loop current

flows, the RTSP input voltage increases to trip the com-

parator. In Figure 31, a low-pass filter with a double

pole at 2 Hz was implemented to prevent false ring trip.

The following example illustrates how the detection cir-

cuit of Figure 31 will trip at 12.5 mA dc loop current us-

ing a –48 V battery.

IN = –--2--7-.--2--–-8---9-(--–---k4---Ω8----)

= 17.9 µA

The current IN is repeated as IP in the positive compar-

ator input. The voltage at comparator input RTSP is:

VRTSP = VBAT + ILOOP(dc) × RTS1 + IP × RTSP

Using this equation and the values in the example, the

voltage at input RTSP is –12 V during ringing injection

(ILOOP(dc) = 0). Input RTSP is, therefore, at a level of 5 V

below RTSN. When enough dc loop current flows

through RTS1 to raise its dc drop to 5 V, the comparator

will trip. In this example,

ILOOP(dc) = 4----05---2---V---Ω---

= 12.5 mA

Ring Ground Detection

Pin ICM sinks a current proportional to the longitudinal

loop current. It is also connected to an internal compar-

ator whose output is pin RGDET. In a ground start

application where Tip is open, the ring ground current

is half differential and half common mode. In this case,

to set the ring ground current threshold, connect a

resistor RICM from pin ICM to VCC. Select the resistor

according to the following relation:

RICM(kΩ) = V--I--RC---GC--(--×m----1-A-2---)0--

The above equation is shown graphically in Figure 18.

It applies for the case of Tip open. The more general

equation can be used in ground key application to

detect a common-mode current ICM:

RICM(kΩ) = V-I-C--C-M--C-(--m-×----6A--0-)-

Lucent Technologies Inc.

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