LUCL8574DP-D AGERE [Agere Systems], LUCL8574DP-D Datasheet - Page 25

• Manufacturer Part Number: LUCL8574DP-D
• Description: L8574D Resistive Subscriber Line Interface Circuit(SLIC), Ring Relay,and Protector(SRP)for Long Loop and TR-57 Applications
• Manufacturer: AGERE [Agere Systems]

Data Sheet
October 1998
Applications
Protection
sense inputs, TS and RS, are protected with diodes to
battery (V
external resistors which connect them to Tip and Ring.
Because the battery noise cancellation input CBN is
connected to pin V
also be protected. Internally, it is protected with an 8 V
zener diode connected to V
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
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-
Lucent Technologies Inc.
BAT1
) and V
(continued)
CCA
BF
(continued)
CCA
through a 0.1 F capacitor, it must
and V
and the series high-value
BAT1
CCA
Relay, and Protector (SRP) for Long Loop and TR-57 Applications
. The receive interface
. An external resistor of
L8574D Resistive Subscriber Line Interface Circuit (SLIC), Ring
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
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
Additionally, it ensures longitudinal balance, which
depends only on the matching of the battery feed resis-
tors by creating an ac ground at V
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
R
It is advantageous if resistors R
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
Also, to implement the battery noise cancellation func-
tion, connect the following circuit from CBN to V
analog ground.
Figure 8. Implementing the Noise Cancellation
9
= 20 k
0.01 F
9
= 1/200
Function
BF
. It couples this noise, 180 out of
0.1 F
301 k
5.11 k
9
BF
and R
.
BF
GBN
with respect to sig-
to ground.
1
are matched
V
CBN
BF
L8574
BF
5-5278a (F)
and
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