Data Sheet
March 1997
L7554 Low-Power SLIC
Starting from the on-hook condition and going through
to a short circuit, the curve passes through two regions:
Applications (continued)
dc Applications
Region 1; On-hook and low loop currents. The slope
corresponds to the dc resistance of the SLIC, RDC1 (de-
fault is 113 Ω typical). The open-circuit voltage is the
battery voltage less the overhead voltage of the device,
VOH (default is 6.5 V typical). These values are suitable
for most applications, but can be adjusted if needed. For
more information, see the sections entitled Adjusting dc
Feed Resistance and Adjusting Overhead Voltage.
Battery Feed
The dc feed characteristic can be described by:
( VBAT – VOH) × RL
VT ⁄ R =
--------------------------------------------
RL + 2RP + Rdc
Region 2; Current limit. The dc current is limited to a val-
ue determined by external resistor RPROG. This region of
the dc template has a high resistance (10 kΩ).
V BAT – VOH
I L =
----------------------------------
RL + 2RP + Rdc
Calculate the external resistor as follows:
where:
IL = dc loop current.
RPROG (kΩ) = 1.67 ILIM (mA)
VT/R = dc loop voltage.
|VBAT| = battery voltage magnitude.
VOH = overhead voltage. This is the difference between
the battery voltage and the open loop Tip/Ring
voltage.
RL = loop resistance, not including protection resistors.
RP = protection resistor value.
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,
Rdc = SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 25.
VOH = |VBAT| – (VPT – VPR)
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 Ω loop
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.
50
40
30
1
10 kΩ
I
LIM
The drive amplifiers are capable of 4 Vrms minimum
(VAMP). Referring to Figure 26, the internal resistance
has a worst-case value of 46 Ω. So, the maximum sig-
nal the device can guarantee is:
20
–1
RDC1
10
0
ZT/R
VT/R = 4 V
-----------------------------------------
0
10
20
30
40
50
ZT/R + 2(RP + 46)
LOOP VOLTAGE (V)
Thus, RP ≤ 35 Ω allows 2.2 Vrms metering signals.The
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:
Note: VBAT = –48 V; ILIM = 22 mA; RDC1 = 113 Ω.
12-3050 (C)
Λ
Figure 25. Loop Current vs. Loop Voltage
Λ
2(RP +40 Ω)
-----------------------------
ZT ⁄ R
vamp = vT/R 1 +
20
Lucent Technologies Inc.
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