PBL 386 11/2
12
Figure 11. The AOV funktion when the AOV-pin is left open. (Observe, burst
undersampled).
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 between 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, together 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 C
LP
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. C
LP
together with CHP and ZT (see section TwoWire Impedance) forms the total two wire
output impedance of the SLIC. The choice
of these programming components influence the power supply rejection ratio
(PSRR) from VBAT to the two wire side in
the low frequency range.
R
Feed
R
SGCLP
C
HP
[Ω][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 contact Ericsson Microelectronics for assistance.
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 possible future changes concerning voice, metering and other signal levels. This is done
by using an overhead voltage which automatically 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
- 20kHz frequency range and with an ampli-
tude less than 2.5V
Peak
11
. For signal ampli-
tudes between 2.5V
Peak
and 5.0V
Peak
, the
AOV-function will expand the overhead
voltage making it possible for the signal, Vt,
to propagate through the SLIC without distortion (see figure 11). The expansion of
the overhead voltage occurs instantaneously. When the signal amplitude decreases,
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 adjusted for signal levels between 0.6 V
Peak
and 5.0 V
Peak
.
AOV In the Constant Current Region
When the overhead voltage is automatically increased, the apparent battery (V
App
,
reference F in figure 14), will be reduced by
the signal amplitude minus 2.5 V
Peak
(11)
, (Vt
- 2.5
(11)
).
In the constant current region this change
will not affect the line current as long as
VTR < V
App
- (I
LConst
• R
Feed
) - (Vt- 2.5
(11)
),
(references A-C in figure 14).
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.
V
TR
> V
App
- (I
LConst
• R
Feed
) - (Vt - 2.5
(11)
)
(references D in figure 14).
If the signal amplitude is greater than
2.5V
Peak
11
the line current, IL, will be re-
duced corresponding to the formula
∆I
L
= | (Vt - 2.5
(11)
)/(RL + R
Feed
) |.
This reduction of line current will introduce a transversal signal into the two-wire
which under some circumstances may be
audible (e g when sending metering signals > 2.5 V
Peak
without any speech signal
burying the transversal signal generated
from the linecurrent reduction).
The sum of all signals should not exceed
5.0 V
Peak
.
Line Feed
If VTR < V
App
- (I
LConst
• R
Feed
), the PBL 386
11/2 SLIC will emulate constant current
feed (references A-C in figure 14).
For VTR > V
App
- (I
LConst
• R
Feed
) the PBL 386
11/2 SLIC will emulate resistive loop feed
programmable between 2•25 Ω12 and 2•900
Ω (references D in figure 14). The current
limitation region is adjustable between 0
mA and 65 mA13.
When the line current is approaching
open loop conditions, the overhead voltage
is reduced. To ensure maximum open loop
voltage, even with telephone line leakage,
this occurs at a line current of approximately 5 mA (references E in figure 14). After the
overhead voltage reduction, the line voltage is kept nearly constant with a steep
slope corresponding to 2 • 25 Ω(reference
G in figure 14).
The open loop voltage, V
TRMax
, measured
between the TIPX and RINGX terminals is
tracking the battery voltage V
Bat
(referenc-
es H in figure 14). V
TRMax
is programmable
by connecting the AOV-pin to AGND or by
leaving the AOV-pin open.