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參數資料
| 型號： | LUCL8574DP-DT |
| 英文描述： | L8574D Resistive Subscriber Line Interface Circuit(SLIC), Ring Relay,and Protector(SRP)for Long Loop and TR-57 Applications |
| 中文描述： | L8574D電阻用戶線接口電路（SLIC），環(huán)繼電器和保護（SRP）的為遠距離和TR - 57應用 |
| 文件頁數： | 25/36頁 |
| 文件大?。?/td> | 489K |
| 代理商： | LUCL8574DP-DT |
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Lucent Technologies Inc.
25
Data Sheet
October 1998
Relay, and Protector (SRP) for Long Loop and TR-57 Applications
L8574D Resistive Subscriber Line Interface Circuit (SLIC), Ring
Applications
(continued)
Protection
(continued)
sense inputs, TS and RS, are protected with diodes to
battery (V
BAT1
) and V
CCA
and the series high-value
external resistors which connect them to Tip and Ring.
Because the battery noise cancellation input CBN is
connected to pin V
BF
through a 0.1
μ
F capacitor, it must
also be protected. Internally, it is protected with an 8 V
zener diode connected to V
CCA
. An external resistor of
at least 3 kW (5 kW is recommended) is required to
limit the surge current. No external protection device is
required.
Tip/Ring Drivers
The L8574 has two Tip/Ring drivers with outputs called
PT and PR. Each driver operates as a current source
capable of sinking or sourcing adequate ac signal cur-
rent plus the dc bias current that is required during on-
hook transmission.
Receive Interface
The receive interface circuitry couples the differential
signal on receive inputs IRCV and RCVN to the Tip/
Ring drivers. Input IRCV is a low-impedance (<5
)
current input while RCVN is a high-impedance voltage
input. Internal feedback forces the voltage at IRCV to
be equal to RCVN so that a voltage applied to RCVN
causes a current flow out of IRCV which equals that
voltage divided by the impedance connected from
IRCV to AGND (assuming the input voltage is refer-
enced to AGND).
The receive interface and Tip/Ring
drivers provide a
current gain of 200; i.e., a differential output current
flows from PT to PR which is 200 times the current
flowing into IRCV. The receive interface also provides a
level shift since the inputs, IRCV and RCVN, are refer-
enced to analog ground, while the outputs, PT and PR,
swing between V
CCA
and V
BAT1
. The receive interface
ensures that the input current is not converted to a
common-mode current at PT and PR.
Transmit Interface
The transmit interface circuitry interfaces the differen-
tial voltage on Tip and Ring to transmit output XMT. The
Tip/Ring differential voltage (both ac and dc) appears
on output XMT with a gain of 0.5. The transmit interface
uses an operational amplifier with four external resis-
tors to perform a differential to single-ended conver-
sion. The operational amplifier inputs are TS and RS.
Output XMT is referenced to ground (AGND). The lon-
gitudinal balance and gain accuracy at XMT depends
on the matching of the external resistors (0.35%).
Because a large dc potential exists at XMT, a capacitor
must be used to couple the ac signal to the low-voltage
codec circuitry.
Battery Noise Cancellation
The battery noise cancellation circuit senses the ac
noise on the battery via the capacitor connected from
input CBN to V
BF
. It couples this noise, 180
°
out of
phase, to the Ring current drive amplifier. This cancels
the battery noise that is coupled to the Ring through
the feed resistor connected to V
BF
.
Additionally, it ensures longitudinal balance, which
depends only on the matching of the battery feed resis-
tors by creating an ac ground at V
BF
with respect to sig-
nals on the Ring lead.
For the cancellation to operate properly, both the phase
and gain must be accurate. The battery noise cancella-
tion gain is a transconductance which is equal to 100
divided by the resistor connected from RGBN to
ground (AGND). This value must be equal to the recip-
rocal of the dc feed resistor (1/200). That is:
100/R
9
= 1/200
R
9
= 20 k
It is advantageous if resistors R
9
and R
1
are matched
and tracked thermally, i.e., located on the same film
integrated circuit (FIC).
PSRR can be improved by adding a 47 pF capacitor in
series with a 50 k
resistor from R
GBN
to ground.
Also, to implement the battery noise cancellation func-
tion, connect the following circuit from CBN to V
BF
and
analog ground.
5-5278a (F)
Figure 8. Implementing the Noise Cancellation
Function
V
BF
CBN
301 k
0.1
μ
F
5.11 k
0.01
μ
F
L8574
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