PBL 386 15/1
High-Pass Transmit Filter
WhenCODEC/filterwithasingle5Vpower
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.
1 VPeak
2.50 V
2.50 V
2.50 V
Capacitor CLP
ThecapacitorCLP,whichconnectsbetween
the terminals LP and VBAT2, 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.
Figure 11. The AOV funktion when the AOV-pin is left open. (Observe, burst
undersampled).
Theopenloopvoltage, VTRMAX, measured
Duringoperationtheinfluenceoftheadap-
tive overhead function will not effect the
SLIC performance in the constant current
region of operation (see figure 11). If,
however, the SLIC is in the off-hook,
constant voltage region of operation then
the influence of the adaptive headroom will
be apparent as a slight decrease in line
between the TIPX and RINGX terminals
tracksthebatteryvoltageVBAT(references
J in figure 17). According to the formula:
RFEED
[Ω]
CLP
CHP
[nF]
68
[nF]
470
VTRMAX = | VBAT | -4.6
2•25
When the line current is approaching
voltage (and hence line current) as the openloopconditions(referencesGinfigure
SLIC adjusts to accommodate the larger 17) the overhead voltage is reduced. The
Table 1. CLP and CHP values.
(voice + metering) signal.
line voltage is kept nearly constant with a
steep slope corresponding to 2x25 Ω
(references H in figure 17), to ensure
maximum open loop voltage, even with a
leaking telephone line.
Adaptive Overhead Voltage, AOV
The Adaptive Overhead Voltage feature
minimizes the power dissipation and at the
same time provides a flexible solution for
different 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).
The PBL38615/1 will behave as a SLIC
with fixed overhead for signals in the 0-
20kHz range and with an amplitude less
than 1Vpeak. For signal amplitudes between
1VPeak and 2.5VPeak the adaptive overhead
function will expand the overhead voltage
making it possible for the signal to propa-
gate through the SLIC without distortion (
This is the total sum of voice and metering
signal). The expansion of the overhead
occurs instantaneously. When the signal
amplitudedecreases,theoverheadreturns
to its initial value with a time constant of
approximately one second (see figure 11).
Line Feed
If VTR < | VBAT2 | -5.7 approx (See formula
C in figure 17). the PBL 386 15/1 SLIC will
emulateconstantcurrentfeed. (references
A-C in figure 17). The constant current
regionisadjustablebetween18mAand30
mA.
Constant Current Region
The constant current (reference A-C in
figure17)isadjustedbyconnectingaresis-
tor, RLC, between terminal PLC and ground
according to the equation:
If VTR > | VBAT2 | -5.7 approx (See
formula C in figure 17). the PBL 38615/1
SLIC will emulate a constant voltage feed
with 2 x 25 Ω source impedance (refer-
ences C-E in figure 17). This section is
madeassteepaspossibletoswitchbattery
faster.
500 10.4 • In (ILProg • 32)
RLC
=
-
ILProg
ILProg
Can simplifies to:
500
RLC
=
ILProg
If the loop current is less than 5.5mA then
the SLIC will automatically switch to supply
the DC feed via Vbat rather than Vbat2
(referencesEinfigure17).Thiswillnotgive
any disturbances on the line.
12
ERICSSON [ ERICSSON ]
