Data Sheet
March 1997
L7554 Low-Power SLIC
Accounting for VSAT tolerance of 0.5 V, a nominal
overhead of 9.9 V would ensure transmission of an
undistorted 2.2 V metering signal.
Applications (continued)
dc Applications (continued)
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 VBAT. In the case of –5 V, the
overhead voltage will be independent of the battery
voltage. Figure 27 shows the equivalent input circuit to
adjust the overhead.
RP
ROC/2
ROC/2
+
+
VT/R
[ZT/R]
VAMP
–
–
RP
R1
25 kΩ ± 30%
12-2563 (C)
DCR
R2
Figure 26. SLIC 2-Wire Output Stage
–5 V
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:
12-2562 (C)
Figure 27. Equivalent Circuit for Adjusting the
Overhead Voltage
Λ
2(RP +40 Ω)
VOH = VSAT + 1 +
v
T ⁄ R
-----------------------------
ZT ⁄ R
The overhead voltage is programmed by using the fol-
lowing equation:
2(RP +40 Ω)
2 ZT ⁄ R
VSAT + 1 +
--------------- × 10dBm ⁄ 20
1000
=
-----------------------------
VOH = 6.5 – 4 VDCR
ZT ⁄ R
where VSAT is the combined internal saturation voltage
between the Tip/Ring amplifiers and VSAT (4.0 V typ.).
RP (Ω) is the protection resistor value, and 40 Ω is the
output series resistance of each internal amplifier.
ZT/R (Ω) is the ac loop impedance.
||
R1 25 kΩ
= 6.5 – 4 –5 ×
-------------------------------------
||
R2 +R1 25 kΩ
||
R1 25 kΩ
Example 1, On-hookTransmission of a Meter Pulse:
= 6.5 + 20
-------------------------------------
+ R1 ||
R2
25 kΩ
Signal level: 2.2 Vrms into 200 Ω
35 Ω protection resistors
ILOOP = 0 (on-hook transmission of the metering
signal)
2(35 + 40)
VOH = 4.0 + 1 + --------------------------- 2(2.2)
200
= 9.4 V
Lucent Technologies Inc.
21
AGERE [ AGERE SYSTEMS ]
