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ATTL7554AP

L7554 Low-Power SLIC

Agere -> LSI Corporation 

Data Sheet

March 1997

L7554 Low-Power SLIC

Applications (continued)

dc Applications (continued)

+

VT/R

–

RP

[ZT/R]

ROC/2

+

VAMP

–

RP

ROC/2

12-2563 (C)

Figure 26. SLIC 2-Wire Output Stage

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.

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.

R1

25 kΩ ± 30%

DCR

R2

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:

VOH = VSAT + 1 + 2----(--R----PZ----+T---⁄-4R---0---Ω-----)vΛT ⁄ R

= VSAT + 1 + 2----(--R----PZ----+T---⁄-4R---0---Ω-----) 2---1--Z-0--T-0--0⁄--R-- × 10dBm ⁄ 20

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.

Example 1, On-hook Transmission of a Meter Pulse:

Signal level: 2.2 Vrms into 200 Ω

35 Ω protection resistors

ILOOP = 0 (on-hook transmission of the metering

signal)

–5 V

12-2562 (C)

Figure 27. Equivalent Circuit for Adjusting the

Overhead Voltage

The overhead voltage is programmed by using the fol-

lowing equation:

VOH = 6.5 – 4 VDCR

= 6.5 – 4–5 × -R---2--R--+--1--R--|-|-1--2--|5-|---2k---5Ω----k--Ω----

=

6.5

+

20





-R---2--R--+--1---R-|-|--1-2--|5-|---2k--5-Ω---k---Ω----

VOH = 4.0 + 1 + 2----(---3---25---0--+-0----4---0----) 2(2.2)

= 9.4 V

Lucent Technologies Inc.

21

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