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ATTL7554BP

L7554 Low-Power SLIC

Agere -> LSI Corporation 

L7554 Low-Power SLIC

Data Sheet

March 1997

Applications (continued)

dc Applications

Battery Feed

The dc feed characteristic can be described by:

VT ⁄ R = (----V----B---A--T-----–----V----O---H---)---×----R----L-

RL + 2RP + Rdc

IL = -----V----B---A--T-----–----V----O---H----

RL + 2RP + Rdc

where:

IL = dc loop current.

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.

Rdc = SLIC internal dc feed resistance.

The design begins by drawing the desired dc template.

An example is shown in Figure 25.

50

40

30

1

10 kΩ

20

10

ILIM

–1

RDC1

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, 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.

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).

Calculate the external resistor as follows:

RPROG (kΩ) = 1.67 ILIM (mA)

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,

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.

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:

0

0

10

20

30

40

50

LOOP VOLTAGE (V)

Note: VBAT = –48 V; ILIM = 22 mA; RDC1 = 113 Ω.

12-3050 (C)

Figure 25. Loop Current vs. Loop Voltage

V T/R

=

4

V





--Z---T--/-R------+---Z-2--T-(-/-RR---P----+----4---6---)-

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:

Λvamp = ΛvT/R1 + 2----(--R----PZ----+T---⁄-4R--0----Ω-----)

20

Lucent Technologies Inc.

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