L8575 Datasheet, PDF (27/36 Pages) Agere Systems – Dual-Resistive,Low-Cost Subscriber Line Interface Circuit(SLIC)

Advance Data Sheet
March 1997
L8575
Dual-Resistive, Low-Cost SLIC
dc Characteristics
I/V Characteristics
Resistors R1 and R2 are the dc feed resistors. R1 is
connected from battery to Ring, and R2 is connected
from Tip to ground. The dc loop current is fed to the
subscriber loop via these resistors. These resistors will
set the dc I/V template for the line circuit, with the I/V
template being linear with a –1/600 Ω slope. No con-
stant current region at short dc loops is provided by
resistors R1 and R2 or the L8575 SLIC.
The dc Tip/Ring voltage under open loop conditions is
3.36 V less than battery. In order to drive an on-hook ac
signal, the Tip and Ring voltage must be set to a value
less than the battery voltage. The amount that the open
loop voltage (VOC) is decreased relative to the battery
(VBAT) is referred to as the overhead voltage (VOH). This
overhead voltage is due to 5.6 mA of bias current flow
from both the Tip and Ring current drive amplifier’s flow
through resistors R2 and R1, respectively. Thus, the
overhead is given by:
VOH = (R1 x 5.6 mA) + (R2 x 5.6 mA)
VOH = (300 x 5.6 mA) + (300 x 5.6 mA) = 3.36 V
The nominal dc I/V template for the L8575 SLIC is
shown in Figure 11.
Loop Length
The loop range equation is given by:
RL = --V----^B----^A---^T--I--_L--^–----V-----^O---^H--- – R1 – R2
Where,
RL is the dc resistance of the subscriber loop.
IL is the dc loop current.
|VBAT| is the magnitude of the battery voltage.
VOH is the overhead voltage—nominal 3.36 V.
R1 = R2 = dc feed resistors = 300 Ω.
Thus, for a nominal –48 V battery with a minimum
18 mA loop requirement, the loop range will be:
RL = --4---8-----0-V--.--0--^–-1---8-3---.-A-3---6------V-- – 300 Ω – 300 Ω
RL = 1880 Ω
80
70
60
50
40
30
20
10
0
0
5 10 15 20 25 30 35 40 45 50
VT-R (V)
VOC
(44.7)
VBAT
(48)
12-3430(F).r1
Figure 11. L8575 SLIC I/V Template
Lucent Technologies Inc.
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