Data Sheet
May 2001
L8576B Dual Ringing SLIC
CF1 and CF2 must exhibit a stable capacitance value
over its voltage range to ensure a properly shaped
waveform. Do not use a ceramic capacitor for CF1 and
CF2; use a capacitor with a polyester, polypropylene,
polycarbonate, or polystyrene dielectric.
Applications (continued)
dc Design (continued)
Off-Hook Detection
The loop closure comparator has built-in longitudinal
rejection, eliminating the need for an external 60 Hz fil-
ter. The loop closure detection threshold is internally
set at 12 mA.
80
60
40
20
0
–20
–40
–60
Power Ringing
The L8576B is designed to generate a balanced trape-
zoidal power ring signal to tip and ring. Because the
L8576B generates the power ringing signal, no ring
relay is needed in this mode of operation. Alternatively,
the L8576B SLIC can be used in a battery-backed,
unbalanced ringing application. In this case, the ring
signal is generated by a central ring generator and is
bused to individual tip/ring pairs. A ringing relay is used
during ringing to disconnect the L8576B from, and
apply the ring generator to, the tip and ring pair.
–80
0.00 0.04 0.08 0.12 0.16 0.20
0.02 0.06 0.10 0.14 0.18
TIME (s)
12-3346a (F)
Notes:
Slew rate = 5.65 V/ms.
trise = tfall = 23 ms.
pwidth = 2 ms.
period = 50 ms.
This section discusses in detail the use of the L8576B
in the balanced mode of operation.
Figure 18. Ringing Waveform Crest Factor = 1.6
Crest Factor
80
60
40
20
The balanced ring signal is generated by simply tog-
gling between the powerup forward and reverse battery
states. The state change is done by applying a square
wave (whose frequency is the desired ring frequency)
to logic input B2. Capacitors CF1 and CF2 and resistor
RRNG are used to control or ramp the speed of the tran-
sition of the battery reverse, thus shaping the balanced
ring signal. Setting capacitor CF1 = CF2 = 0.22 µF and
setting RRNG to 28.7 kΩ provides a crest factor of 1.3
for a 20 Hz ring frequency. This satisfies the Telcordia*
GR-909 requirement of ringing waveform crest factor
between 1.2 and 1.6. Crest factor is defined as the
peak to rms voltage ratio of the ring signal. Ringing
waveforms of crest factors 1.6 and 1.2 are shown in
Figures 18 and 19. The crest factor can be adjusted by
the value of RRNG and will be influenced slightly by the
value of VBAT. The CF1 and CF2 capacitors should not
be changed because these affect the dc feedback loop
stability in current limit. An RRNG value of 22.6 kΩ will
lower the crest factor to about 1.2 with a –65 V or
–72 V battery for a 20 Hz ring frequency. Likewise, an
RRNG value of 34.8 kΩ will raise the crest factor to
about 1.4. For ring frequencies greater than 20 Hz, the
RRNG value should be lowered until the desirable crest
factor is achieved. Note the RRNG is common to both
sections of the device.
0
–20
–40
–60
–80
0.00 0.04 0.08 0.12 0.16 0.20
0.02 0.06 0.10 0.14 0.18
TIME (s)
12-3347a (F)
Notes:
Slew rate = 10.83 V/ms.
trise = tfall = 12 ms.
pwidth = 13 ms.
period = 50 ms.
Figure 19. Ringing Waveform Crest Factor = 1.2
* Telcordia is a registered trademark of Telcordia Technologies, Inc.
18
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
