L7554 AGERE [Agere Systems], L7554 Datasheet - Page 21

• Manufacturer Part Number: L7554
• Description: L7554 Low-Power SLIC
• Manufacturer: AGERE [Agere Systems]

Data Sheet
March 1997
Applications
dc Applications
In addition to the required peak signal level, the SLIC
needs about 2 V from each power supply to bias the
amplifier circuitry. It can be thought of as an internal
saturation voltage. Combining the saturation voltage
and the peak signal level, the required overhead can be
expressed as:
where V
between the Tip/Ring amplifiers and V
R
output series resistance of each internal amplifier.
Z
Example 1, On-hook Transmission of a Meter Pulse:
Signal level: 2.2 Vrms into 200
35
I
signal)
Lucent Technologies Inc.
V
=
LOOP
T/R
P
O H
( ) is the protection resistor value, and 40
( ) is the ac loop impedance.
V
protection resistors
= 0 (on-hook transmission of the metering
=
S A T
V
Figure 26. SLIC 2-Wire Output Stage
SAT
T/R
V
V
+
–
OH
S A T
+
is the combined internal saturation voltage
=
[Z
=
1
+
9.4 V
T/R
R
R
+
4.0
P
P
]
1
2 R
----------------------------- -
(continued)
+
+
(continued)
R
R
2 R
----------------------------- -
P
Z
1
OC
OC
+
T R
+
/2
/2
40
P
2 35
--------------------------- -
Z
+
T R
V
200
40
AMP
+
–
+
40
2 Z
--------------- -
1000
v
SAT
T R
T R
2 2.2
(4.0 V typ.).
10
dBm 20
is the
12-2563 (C)
Accounting for V
overhead of 9.9 V would ensure transmission of an
undistorted 2.2 V metering signal.
Adjusting Overhead Voltage
To adjust the open loop 2-wire voltage, pin DCR is
programmed at the midpoint of a resistive divider from
ground to either –5 V or V
overhead voltage will be independent of the battery
voltage. Figure 27 shows the equivalent input circuit to
adjust the overhead.
Figure 27. Equivalent Circuit for Adjusting the
The overhead voltage is programmed by using the fol-
lowing equation:
–5 V
V
OH
Overhead Voltage
=
R1
R2
=
= 6.5 – 4 V
6.5
6.5
SAT
–
+
DCR
tolerance of 0.5 V, a nominal
4
25 k
20
–
5
------------------------------------- -
R
DCR
L7554 Low-Power SLIC
BAT
2
R
30%
+
. In the case of –5 V, the
1
------------------------------------- -
R
R
2
R
1
+
25 k
1
R
25 k
1
25 k
25 k
12-2562 (C)
21