Data Sheet
March 1997
L7554 Low-Power SLIC
The current IN is repeated as IP in the positive compar-
ator input. The voltage at comparator input RTSP is:
Applications (continued)
dc Applications (continued)
Ring Trip Detection
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,
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
5 V
ILOOP(dc) = ----------------
402 Ω
RTSP
+
–
RTSP
RC PHONE
2 MΩ
IP = IN
IN
NRDET
RTS1
402 Ω
CRTS1
0.022 µF
CRTS2
0.27 µF
+
–
7 V
= 12.5 mA
RTS2
RTSN
RTSN
15 kΩ
2 MΩ
274 kΩ
VRING
VBAT
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:
12-3014 (C)
Figure 31. Ring Trip Equivalent Circuit and
Equivalent Application
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.
VCC × 120
IRG(mA)
RICM(kΩ) =
----------------------
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:
VCC × 60
ICM(mA)
R ICM(kΩ) =
-------------------
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.
–7 – (–48)
IN = --------------------------
2.289 kΩ
= 17.9 µA
Lucent Technologies Inc.
23
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