ATTL7554BP Agere Systems, ATTL7554BP Datasheet - Page 20

• Manufacturer Part Number: ATTL7554BP
• Description: L7554 Low-Power SLIC
• Manufacturer: Agere Systems

L7554 Low-Power SLIC
Applications
dc Applications
Battery Feed
The dc feed characteristic can be described by:
where:
I
V
|V
V
R
R
Rdc = SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 25.
Note: V
20
20
L
V
T/R
OH
L
P
BAT
= dc loop current.
T R
= loop resistance, not including protection resistors.
= protection resistor value.
= overhead voltage. This is the difference between
= dc loop voltage.
| = battery voltage magnitude.
Figure 25. Loop Current vs. Loop Voltage
BAT
the battery voltage and the open loop Tip/Ring
voltage.
=
50
40
30
20
10
0
------------------------------------------- -
= –48 V; I
0
V
R
B A T
L
+
2R
I
LIM
–
L
10
(continued)
V
=
P
= 22 mA; R
O H
+
10 k
---------------------------------- -
R
Rdc
1
L
V
LOOP VOLTAGE (V)
+
B A T
20
R
2R
L
DC1
P
–
+
V
= 113 .
Rdc
O H
30
R
–1
DC1
40
I
LIM
12-3050 (C)
50
Starting from the on-hook condition and going through
to a short circuit, the curve passes through two regions:
Region 1; On-hook and low loop currents. The slope
corresponds to the dc resistance of the SLIC, R
fault is 113
battery voltage less the overhead voltage of the device,
V
for most applications, but can be adjusted if needed. For
more information, see the sections entitled Adjusting dc
Feed Resistance and Adjusting Overhead Voltage.
Region 2; Current limit. The dc current is limited to a val-
ue determined by external resistor R
the dc template has a high resistance (10 k ).
Calculate the external resistor as follows:
Overhead Voltage
In order to drive an on-hook ac signal, the SLIC must
set up the Tip and Ring voltage to a value less than the
battery voltage. The amount that the open loop voltage
is decreased relative to the battery is referred to as the
overhead voltage. Expressed as an equation,
Without this buffer voltage, amplifier saturation will
occur and the signal will be clipped. The 7551 is auto-
matically set at the factory to allow undistorted on-hook
transmission of a 3.17 dBm signal into a 900
impedance. For applications where higher signal levels
are needed, e.g., periodic pulse metering, the 2-wire
port of the SLIC can be programmed with pin DCR.
The drive amplifiers are capable of 4 Vrms minimum
(V
has a worst-case value of 46 . So, the maximum sig-
nal the device can guarantee is:
Thus, R
next step is to determine the amount of overhead volt-
age needed. The peak voltage at output of Tip and
Ring amplifiers is related to the peak signal voltage by:
OH
AMP
(default is 6.5 V typical). These values are suitable
). Referring to Figure 26, the internal resistance
P
vamp = v
V
35
T/R
R
typical). The open-circuit voltage is the
V
PROG
OH
=
allows 2.2 Vrms metering signals. The
4 V
= |V
T/R
(k ) = 1.67 I
BAT
1
---------------------------------------- -
Z
+
T/R
| – (V
2 R
----------------------------- -
+
Lucent Technologies Inc.
Z
PT
2 R
P
Z
T/R
LIM
+
T R
– V
40
PROG
P
(mA)
+
PR
46
)
. This region of
March 1997
Data Sheet
DC1
loop
(de-