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ATTL7557AAU

Low-Power SLICs with Battery Switch

Agere -> LSI Corporation 

Data Sheet

January 2000

L7556, L7557 Low-Power SLICs

with Battery Switch

Applications (continued)

dc Applications (continued)

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 device 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:

VT ⁄ 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:

Λ

Vamp

=

Λ

V

T/R





1

+

-2---(--R----P-Z---+T---⁄-4-R--0----Ω----)

VOH

=

VSAT

+





1

+

-2---(--R----P-Z---+T---⁄-4R---0----Ω----)-

Λ

VT

⁄

R

where VSAT is the combined internal saturation voltage

between the tip/ring amplifiers and VBAT (5.4 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)

VOH

=

5.4

+





1

+

-2----(--3---2-5---0--+--0---4----0----)

2 (2.2) = 10.8 V

Accounting for VSAT tolerance of 0.5 V, a nominal

overhead of 11.3 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 voltage.

R1

R2

–5 V

25 kΩ ± 30%

DCR

RP

+

VT/R [ZT/R]

–

RP

40 Ω

+

VAMP

40 Ω –

12-2560.e (F)

Figure 26. SLIC 2-Wire Output Stage

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 (F)

Figure 27. Equivalent Circuit for Adjusting the

Overhead Voltage

The overhead voltage is programmed by using the fol-

lowing equation:

VOH = 7.9 – 4 VDCR

=

7.9

–

4





–5

×





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





= 7.9 + 20R----2--R--+--1---R-|-|--1-2--|-5|---2-k--5-Ω--k---Ω----

Lucent Technologies Inc.

21

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