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| 型號： | L7597 |
| 廠商： | Lineage Power |
| 英文描述： | Resistive Subscriber Line Interface Circuit (SLIC), Ring Relay, and Protector (SRP) for Short Loop and TA-909 Applications(用于短環(huán)路和TA-909的電阻性用戶線接口、振鈴繼電器和保護器) |
| 中文描述： | 電阻用戶線接口電路（SLIC），環(huán)繼電器和保護（SRP）的短環(huán)路和TA - 909應用（用于短環(huán)路和電訊- 909的電阻性用戶線接口，振鈴繼電器和保護器） |
| 文件頁數(shù)： | 27/34頁 |
| 文件大小： | 564K |
| 代理商： | L7597 |
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Lucent Technologies Inc.
27
Advance Data Sheet
March 1997
Relay, and Protector (SRP) for Short Loop and TA-909 Applications
L7597 Resistive Subscriber Line Interface Circuit (SLIC), Ring
dc Characteristics
V-I Characteristics
Resistors R
1
and R
2
are the dc feed resistors. R
1
is
connected from battery to Ring, and R
2
is connected
from Tip to Ground. The dc loop current is fed to the
subscriber loop via these resistors. In short loop appli-
cations, to minimize power consumption, the lower-
voltage battery is used to supply dc current to the
subscriber loop and this current is limited by the inter-
nal current-limit circuitry. In the talk state, the lower-
voltage battery is applied to the loop and the dc cur-
rent-limit circuitry is active. When the lower-voltage bat-
tery is used, the loop length must be short enough
such that the battery voltage less overheads is capable
of supplying sufficient dc loop current to allow for
proper operation of the handset. Typically a minimum
18 mA is required. In this mode of operation, the V-I
template is restricted to short loop lengths and the cur-
rent is limited to 25 mA for all allowed loop lengths.
Loop Length
The loop range when operating from the lower voltage
battery in current limit is given by:
R
L
= ({|V
BAT
| –V
OHLIM
}/I
LIMIT
) – R
1
– R
2
Where R
L
is the dc resistance of the subscriber loop,
specified in TA-909 as 450
minimum. V
OHLIM
is the
overhead or drop associated with the current-limit
circuit; typically 2.2 V I
LIMIT
is the current-limit value
(25 mA).
|
V
BAT
|
is the magnitude of the lower voltage
battery; recommended 25.5 V.
R
1
= R
2
= dc feed resistors = 200
R
L
= ({25.5 V – 2.2 V}/0,025 A) – 200
– 200
=
532
R
L
= 532
≥
450
ac Design
Codec Features and Selection Summary
There are four key ac design parameters:
I
Termination impedanceis the impedance looking
into the 2-wire port of the line card. It is set to match
the impedance of the telephone loop in order to mini-
mize echo return to the telephone set.
I
Transmit gainis measured from the 2-wire port to the
PCM highway.
I
Receive gainis done from the PCM highway to the
transmit port.
I
Hybrid balance networkcancels the unwanted
amount of the receive signal that appears at the
transmit port.
At this point in the design, the codec needs to be
selected. The discrete network between the SLIC and
the codec can then be designed. Below is a brief codec
feature and selection summary.
First-Generation Codecs
These perform the basic filtering, A/D (transmit), D/A
(receive), and
μ
-law/A-law companding. They all have
an op amp in front of the A/D converter for transmit
gain setting and hybrid balance (cancellation at the
summing node). Depending on the type, some have
differential analog input stages, differential analog out-
put stages, and
μ
-law/A-law selectability. This genera-
tion of codec has the lowest cost. It is most suitable for
applications with fixed gains, termination impedance,
and hybrid balance.
Second-Generation Codecs
This class of devices includes a microprocessor inter-
face for software control of the gains and hybrid bal-
ance. The hybrid balance is included in the device. ac
programmability adds application flexibility and saves
several passive components. It also adds several I/O
latches that are needed in the application. It does not
have the transmit op amp, since the transmit gain and
hybrid balance are set internally.
Third-Generation Codecs
This class of devices includes the gains, termination im-
pedance, and hybrid balance—all under microproces-
sor control. Depending on the device, it may or may not
include latches.
In the codec selection, increasing software control and
flexibility are traded for device cost. To help decide, it
may be useful to consider the following:
I
Will the application require only one value for each
gain and impedance
I
Will the board be used in different countries with dif-
ferent requirements
I
Will several versions of the board be built If so, will
one version of the board be most of the production
volume
I
Does the application need only real termination
impedance
I
Does the hybrid balance need to be adjusted in the
field
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