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| 型號： | L8575 |
| 英文描述： | Dual-Resistive,Low-Cost Subscriber Line Interface Circuit(SLIC) |
| 中文描述： | 雙電阻，低成本用戶線接口電路（SLIC） |
| 文件頁數(shù)： | 18/36頁 |
| 文件大小： | 633K |
| 代理商： | L8575 |
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L8575
Dual-Resistive, Low-Cost SLIC
Advance Data Sheet
March 1997
18
Lucent Technologies Inc.
Applications
(continued)
Resistor Module
(continued)
Resistors R
3
through R
6
set the gain of the SLIC in the
transmit (2-wire to 4-wire) direction. This is shown in
Figure 5.
5-3422(F)
Figure 5. L8575 SLIC Dual-Resistive Matching
Requirements
The matching of resistors R
3
through R
6
will determine
the gain accuracy of the SLIC; therefore, these resis-
tors must also be matched accordingly. The matching
requirements are given in Table 10.
Because of the high resistance values, the normal
operating power of resistors R
3
through R
6
will be rela-
tively low. Given design margin and thick film technol-
ogy capabilities, a power rating of 250 mW for these
resistors is not unreasonable.
Resistors R
7
and R
8
are used to couple the PT and PR
current drive amplifiers to Tip and Ring. Since the PT
and PR drive amplifiers are current sources, the value
of the series resistance does not affect the loop length
or other performance of the SLIC, and may be arbi-
trarily high for protection purposes. A value of 1.4 k
is
adequate for protection purposes.
Under normal operating conditions, these resistors will
see the battery voltage less the Tip/Ring voltage.
Assuming a Tip/Ring voltage of 6 V (representative of a
short into a handset), the nominal continuous operating
power of R
7
and R
8
is given by:
(48 V – 6 V)
2
/2.8 k
= 0.630 W
per R
7
and R
8
resistor pair
630 mW/2 = 315 mW
per resistor (R
7
and R
8
)
Hence, the operating power rating of 500 mW for R
7
and R
8
. This is the nominal rating for R
7
and R
8
under
normal operating conditions. Again, the ability of these
resistors to withstand fault conditions depends on the
power rating.
Resistor R
9
is also included on the thick film resistor
module. This resistor is used to set the gain of the bat-
tery noise cancellation circuit. See the Battery Noise
Cancellation section of this data sheet for design equa-
tions to set the value of R
9
.
Protection
Because of the resistive feed architecture, a simple
inexpensive protection scheme that does not require a
separate external protection device may be used. The
MMCA31A8575AA resistor module has specifications
which are qualified to Bellcore GR-CORE-1089,
UL*1459, UL497A, FCC part 68.302 (d) & (e) and REA
form 397G, ITU-T K20, and ITU-T K21.
Lightning and power cross protection are provided by
the two dc feed resistors, R
1
and R
2
, in the external
resistor module. Under fault conditions, these resistors
serve as fault current-limiting resistors. Fault current is
steered to ground and to battery via resistors R
2
and
R
1
, respectively. Thus, the battery design must be such
that the various specified faults can be applied to the
battery through 300
,
without damaging the battery or
the line circuit.
Resistors R
1
and R
2
need to be designed to survive
lightning surges and to dissipate power associated with
a Ring ground dc fault and specified ac power cross
faults—both a sneak under and full surge type fault.
Under certain sustained fault conditions, R
1
and R
2
could fail when they are required to survive. For this
reason, a per-channel fault detector is included on the
L8575 SLIC.
When the voltage across either R
1
and/or R
2
is greater
than a nominal 36 V, the fault detect bit (FLT) in the
serial data output will go high. The control logic on the
line card detects FLT is high, and opens an external
electromechanical relay (EMR) to isolate the resistors
from the loop, enabling the resistors to service
extended power cross. (Note the EMR is the test in or
test out EMR, and this relay is driven by one of the
internal relay drivers on the L8575 SLIC.)
A delay of 10 ms to 30 ms is provided (using an exter-
nal capacitor on pin CFLT) in the fault detector. This
prevents transients on the Tip and Ring from tripping
the fault detector when a fault is not present.
* ULis a registered trademark of Underwriters Laboratories, Inc.
–
+
TIP
RING
R
5
200 k
R
6
200 k
R
3
100 k
XMT
R
4
100 k
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| L85J250E | 制造商:Ohmite Mfg Co 功能描述: |
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| L8605 | 制造商:Leviton Manufacturing Co 功能描述: |
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