PBL 386 11/2
High-Pass Transmit Filter
WhenCODEC/filterwithasingel5Vpower
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
theCODEC/filter, ahigh-passRCfilter. Itis
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
ThecapacitorCLP,whichconnectsbetween
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.
Figure 11. The AOV funktion when the AOV-pin is left open. (Observe, burst
undersampled).
tude less than 2.5VPeak11. For signal ampli- duced corresponding to the formula
tudes between 2.5VPeak and 5.0VPeak, the
AOV-function will expand the overhead
∆IL = | (Vt - 2.5(11))/(RL + RFeed) |.
This reduction of line current will intro-
voltagemakingitpossibleforthesignal, Vt, duce a transversal signal into the two-wire
to propagate through the SLIC without dis- which under some circumstances may be
tortion (see figure 11). The expansion of audible (e g when sending metering sig-
theoverheadvoltageoccursinstantaneous- nals > 2.5 VPeak without any speech signal
ly. When the signal amplitude decreases, burying the transversal signal generated
the overhead voltage returns to its initial from the linecurrent reduction).
RFeed
[Ω]
RSG
CLP
CHP
[kΩ] [nF] [nF]
4.02 330 68
23.7 330 68
147 100 33
301 47
619 22
valuewithatimeconstantofapproximately
one second.
The sum of all signals should not exceed
2•25
2•50
2•200
2•400
2•800
5.0 VPeak
.
IftheAOV-pinisconnectedtoAGND, the
overhead voltage will automatically be ad-
justed for signal levels between 0.6 VPeak
Line Feed
33
33
If VTR < VApp - (ILConst • RFeed), the PBL 386
11/2 SLIC will emulate constant current
feed (references A-C in figure 14).
and 5.0 VPeak
.
Table 1. RSG, CLP and CHP values for
different feeding characteristics.
For VTR > VApp - (ILConst • RFeed) the PBL 386
11/2 SLIC will emulate resistive loop feed
programmablebetween2•25Ω12 and2•900
Ω (references D in figure 14). The current
limitation region is adjustable between 0
mA and 65 mA13.
AOV In the Constant Current Region
Whentheoverheadvoltageisautomatical-
ly increased, the apparent battery (VApp
reference F in figure 14), will be reduced by
the signal amplitude minus 2.5 VPeak(11), (Vt
- 2.5(11)).
Table 1 suggest values of CLP and CHP for
different feeding characteristics.
For values outside table 1, please con-
tact Ericsson Microelectronics for assis-
tance.
,
When the line current is approaching
openloopconditions,theoverheadvoltage
is reduced. To ensure maximum open loop
voltage, even with telephone line leakage,
thisoccursatalinecurrentofapproximate-
ly5mA(referencesEinfigure14).Afterthe
overhead voltage reduction, the line volt-
age is kept nearly constant with a steep
slope corresponding to 2 • 25 Ω(reference
G in figure 14).
The open loop voltage, VTRMax, measured
between the TIPX and RINGX terminals is
tracking the battery voltage VBat (referenc-
es H in figure 14). VTRMax is programmable
by connecting the AOV-pin to AGND or by
leaving the AOV-pin open.
Intheconstantcurrentregionthischange
will not affect the line current as long as
VTR < VApp - (ILConst • RFeed) - (Vt- 2.5(11)),
(references A-C in figure 14).
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 11/2 will behave as a SLIC with
fixed overhead voltage for signals in the 0
-20kHzfrequencyrangeandwithanampli-
AOV In the Resistive Loop Feed Region
Thesaturationguardwillbeactivatedwhen
the SLIC is working in the resistive loop
feed region, i.e.
VTR > VApp - (ILConst • RFeed) - (Vt - 2.5(11)
(references D in figure 14).
)
If the signal amplitude is greater than
11
2.5VPeak the line current, IL, will be re-
12
ERICSSON [ ERICSSON ]
