L8576B Datasheet, PDF (17/26 Pages) Agere Systems – Dual Ringing SLIC

Data Sheet
May 2001
L8576B Dual Ringing SLIC
Applications (continued)
dc Design
Battery Feed
The dc feed characteristic can be described by:
VT/R = ⁽----^V----^B---^A--^T------^–---^V----^O---^H---⁾---^×----^R-----^L
RL + 2RP + Rdc
IL = -----^V---^B---^A---^T-----^–----^V---^O---^H----
RL + 2RP + Rdc
Where:
IL = dc loop current.
VT/R = dc loop voltage.
|VBAT| = battery voltage magnitude.
VOH = overhead voltage. This is the difference between
the battery voltage and the open loop tip/ring
voltage.
RL = loop resistance, not including protection resistors.
RP = protection resistor value.
Rdc = SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 17.
50
45
40
35
30
25
20
15
10
5
0
0
1
6 kΩ
VBAT = –65 V
VBAT = –48 V
–1
Rdc
10
20
30
40
50
60
LOOP VOLTAGE (V)
12-3503.a (F)
Figure 17. Loop Current vs. Loop Voltage
Starting from the on-hook condition and going through
to a short circuit, the curve passes through two regions:
Region 1: On-hook and low loop currents. In this
region, the slope corresponds to the dc resistance of
the SLIC, Rdc (typically 60 Ω). The open circuit voltage
is the battery voltage less the overhead voltage of the
device, VOH (default is 7.1 V typical). These values are
suitable for most applications.
Region 2: Current limit. The dc current is limited to a val-
ue determined by external resistor RPROG. This region of
the dc template has a high resistance (6 kΩ).
Calculate the external resistor as follows:
RPROG (kΩ) = 3.5 x (ILIM – 9.2) mA^*
To control power dissipation, it is recommended that a
51.1 kΩ RPROG resistor be used to set a default current-
limit value of 24 mA.
RPWR
The RPWR resistors dissipate the excess power associ-
ated with a single power supply, short-loop application.
The resistor provides VBAT to tip and ring amplifiers.
There is one resistor associated with each channel.
The value of RPWR is dependent upon the battery
potential and the current-limit value. The value of RPWR
can be determined by using the following equation:
RPWR = --^V----^B--^A---^T-----^–----²---²--^.--³-
ILIM – 0.003
Power dissipation of the resistor is:
WRPWR = (ILIM – 0.003)² RPWR.
For the recommended –68.5 V VBAT and 24 mA ILIM
design, a 2.2 kΩ, 2 W resistor is suitable. 2 W resistors
are available as surface-mount components.
Overhead Voltage
In order to drive an on-hook ac signal, the SLIC must
set up the tip and ring voltage to a value less than the
battery voltage. The amount that the open loop voltage
is decreased relative to the battery is referred to as the
overhead voltage and is expressed as:
VOH = |VBAT| – (VPT – VPR)
Without this buffer voltage, amplifier saturation will
occur and the signal will be clipped. The L8576 is auto-
matically set at the factory to allow undistorted on-hook
transmission of a 3.17 dBm signal into a 900 Ω loop
impedance.
The drive amplifiers are capable of 4 Vrms minimum
(VAMP). So, the maximum signal the device can guaran-
tee is:
V T/R
=
4
V


--_Z---_T---_/-_R^Z----^T-₊-^/-^R--₂---_R----_P-_
For normal forward or reverse battery operation, over-
head voltage is internally set to about 8 V. In ringing
mode, the overhead voltage is automatically decreased
to about 4 V to permit passage of a larger ring signal.
* During the balanced ringing mode, the current limit is increased
from the value predicted by this equation by a factor of 3.5.
Agere Systems Inc.
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