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22
Lucent Technologies Inc.
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)
Resistor Module
The L8574 requires certain external resistors at the Tip and Ring interface. Because of matching and protection
requirements, one of the most economical options recommended to implement these resistors is in a thick-film
resistor module. A schematic and a brief description of the function of each of these resistors is given in Figure 6.
Note that Microelectronic Modules Corporation MMC* A31A8574AA and MMCA11A8574AA thick-film resistor
modules are application-specific resistor modules designed for use with the L8574 SLIC. The values, tolerance,
matching, and power rating of the MMCA31A8574AA and MMCA11A8574AA modules are given in Table 16.
Resistors R
1
and R
2
are the dc feed resistors. R1 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. The resistors set the dc feed resis-
tance, which is R
1
+ R
2
(400 = 200 + 200).
Resistors R
1
and R
2
also provide a common-mode impedance of (200
||
200) 100
. These resistors will primarily
determine the longitudinal balance of the line circuit; thus, they must be matched appropriately to meet longitudinal
balance requirements (0.35% for 50 dB and 0.2% for 58 dB). Also, they have a significant impact on the termination
impedance of the SLIC. Feedback using external components (external components when a first- or second-gener-
ation codec is used) allows the user to set the termination impedance at 600
, or most ITU-T recommended ter-
mination impedances.
Under normal operating conditions, the current through resistors R
1
and R
2
is limited by the current-limit circuitry to
25 mA. Thus, the 2 W rating of resistors R
1
and R
2
in MMCA31A8574AA and MMCA11A8574AA is adequate for
normal operation. The power rating of these resistors is discussed more in the Protection section of this data sheet.
*
MMCis a registered trademark of Microelectronic Modules Corporation. For additional information, contact Microelectronic Modules Corpo-
ration (MMC), 2601 S. Moorland Rd., New Berlin, WI 53151 U.S.A.: Tel. 414-785-6506, FAX 414-785-6516, e-mail sales@mmccorp.com.