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PBL386612

Subscriber Line Interface Circuit

Ericsson  

PBL 386 61/2

Preliminary

High-Pass Transmit Filter

When CODEC/filter with a singel 5 V power

supply is used, it is necessary to separate

the different signal reference voltages be-

tween the SLIC and the CODEC/filter. In

the transmit direction this can be done by

connecting a capacitor between the VTX

output of the SLIC and the CODEC/filter

input. This capacitor will also form, togeth-

er with RTX and/or the input impedance of

the CODEC/filter, a high-pass RC filter. It is

recommended to position the 3 dB break

point of this filter between 30 and 80 Hz to

get a fast enough response for the dc steps

that may occur with DTMF signaling.

Capacitor CLP

The capacitor CLP, which connects between

the terminals LP and VBAT, positions the

high end frequency break point of the low

pass filter in the dc loop in the SLIC. CLP

together with CHP and ZT (see section Two-

Wire Impedance) forms the total two wire

output impedance of the SLIC. The choice

of these programming components influ-

ence the power supply rejection ratio

(PSRR) from VBAT to the two wire side in

the low frequency range.

RFeed

[Ω]

RSG CLP CHP

[kΩ] [nF] [nF]

2•25

4.02 330 68

2•50

23.7 330 68

2•200

147 100 33

2•400

301 47 33

2•800

619 22 33

Table 1. RSG, CLP and CHP values for

different feeding characteristics.

Table 1 suggest values of CLP and CHP for

different feeding characteristics.

For values outside table 1, please con-

tact Ericsson Microelectronics for assist-

ance.

Adaptive Overhead Voltage, AOV

The Adaptive Overhead Voltage feature

minimises the power dissipation and at the

same time provides a flexible solution for

differing system requirements and possi-

ble future changes concerning voice, me-

tering and other signal levels. This is done

by using an overhead voltage which auto-

matically adapts to the signal level (voice +

metering). With the AOV-pin left open, the

PBL 386 61/2 will behave as a SLIC with

fixed overhead voltage for signals in the 0

- 20kHz frequency range and with an ampli-

Figure 11. The AOV funktion when the AOV-pin is left open. (Observe, burst

undersampled).

tude less than 3.1VPeak11. For signal ampli-

tudes between 3.1VPeak and 6.2VPeak, the

AOV-function will expand the overhead

voltage making it possible for the signal, Vt,

to propagate through the SLIC without dis-

tortion (see figure 11). The expansion of

the overhead voltage occurs instantane-

ously. When the signal amplitude decreas-

es, the overhead voltage returns to its initial

value with a time constant of approximately

one second.

If the AOV-pin is connected to AGND, the

overhead voltage will automatically be ad-

justed for signal levels between 0.6 VPeak

and 6.2 VPeak.

AOV In the Constant Current Region

When the overhead voltage is automatical-

ly increased, the apparent battery (VApp,

reference F in figure 13), will be reduced by

the signal amplitude minus 3.1 VPeak(11), (Vt

- 3.1(11)).

In the constant current region this change

will not affect the line current as long as

VTR < VApp - (ILConst • RFeed) - (Vt-3.1(11)),

(references A-C in figure 13).

AOV In the Resistive Loop Feed Region

The saturation guard will be activated when

the SLIC is working in the resistive loop

feed region, i.e.

VTR > VApp - (ILConst • RFeed) - (Vt - 3.1(11))

(references D in figure 13).

If the signal amplitude is greater than

3.1VPeak11 the line current, IL, will be re-

duced corresponding to the formula

∆IL = | (Vt - 3.1(11))/(RL + RFeed) |.

This reduction of line current will intro-

duce a transversal signal into the two-wire

which under some circumstances may be

audible (e g when sending metering sig-

nals > 3.1 VPeak without any speech signal

burying the transversal signal generated

from the linecurrent reduction).

The sum of all signals should not exceed

6.2 VPeak.

Line Feed

If VTR < VApp - (ILConst • RFeed), the PBL 386 61/

2 SLIC will emulate constant current feed

(references A-C in figure 13).

For VTR > VApp - (ILConst • RFeed) the PBL 386

61/2 SLIC will emulate resistive loop feed

programmable between 2•25 Ω12 and 2•900

Ω (references D in figure 13). The current

limitation region is adjustable between

0 mA and 65 mA13.

When the line current is approaching

open loop conditions, the overhead volt-

age is reduced. To ensure maximum open

loop voltage, even with telephone line leak-

age, this occurs at a line current of approx-

imately 5 mA (references E in figure 13).

After the overhead voltage reduction, the

line voltage is kept nearly constant with a

steep slope corresponding to 2 • 25 Ω(ref-

erence G in figure 13).

The open loop voltage, VTRMax, measured

between the TIPX and RINGX terminals is

tracking the battery voltage VBat (referenc-

es H in figure 13). VTRMax is programmable

by connecting the AOV-pin to AGND or by

12

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