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| 型號(hào)： | L8576B |
| 英文描述： | Dual Ringing SLIC |
| 中文描述： | 雙振鈴用戶接口 |
| 文件頁(yè)數(shù)： | 17/26頁(yè) |
| 文件大小： | 583K |
| 代理商： | L8576B |
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Agere Systems Inc.
17
Data Sheet
May 2001
L8576B Dual Ringing SLIC
Applications
(continued)
dc Design
Battery Feed
The dc feed characteristic can be described by:
(
--------------------------------------------
=
Where:
I
L
= dc loop current.
V
T/R
= dc loop voltage.
|V
BAT
| = battery voltage magnitude.
V
OH
= overhead voltage. This is the difference between
the battery voltage and the open loop tip/ring
voltage.
R
L
= loop resistance, not including protection resistors.
R
P
= protection resistor value.
R
dc
= SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 17.
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, R
dc
(typically 60
). The open circuit voltage
is the battery voltage less the overhead voltage of the
device, V
OH
(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 R
PROG
. This region of
the dc template has a high resistance (6 k
).
Calculate the external resistor as follows:
R
PROG
(k
) = 3.5 x (I
LIM
– 9.2) mA
*
To control power dissipation, it is recommended that a
51.1 k
R
PROG
resistor be used to set a default current-
limit value of 24 mA.
R
PWR
The R
PWR
resistors dissipate the excess power associ-
ated with a single power supply, short-loop application.
The resistor provides V
BAT
to tip and ring amplifiers.
There is one resistor associated with each channel.
The value of R
PWR
is dependent upon the battery
potential and the current-limit value. The value of R
PWR
can be determined by using the following equation:
R
PWR
=
Power dissipation of the resistor is:
W
RPWR
= (I
LIM
– 0.003)
2
R
PWR
.
For the recommended –68.5 V V
BAT
and 24 mA I
LIM
design, a 2.2 k
, 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:
V
OH
=
|
V
BAT
|
– (V
PT
– V
PR
)
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
loop
impedance.
The drive amplifiers are capable of 4 Vrms minimum
(V
AMP
). So, the maximum signal the device can guaran-
tee is:
=
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.
I
L
R
L
+
R
dc
+
2R
P
---------------------------------
=
V
T/R
V
OH
–
)
R
dc
R
L
×
R
L
2R
P
+
+
10
20
30
40
50
0
LOOP VOLTAGE (V)
15
50
60
0
10
5
45
40
35
30
25
20
L
V
BAT
= –65 V
V
BAT
= –48 V
1
6 k
–1
Rdc
I
LIM
0.003
–
----------------------------
V
T/R
4 V
+
Z
T/R
2R
P
--------------------------
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