PBL 386 21/2
hybrid balance to accommodate different
line impedances without change of
hardware. In addition, the transmit and
receivegainmaybeadjusted.Please,refer
to the programmable CODEC/filter data
sheets for design information.
R
FB
R
TX
VTX
Longitudinal Impedance
V
T
PBL
38621/2
A feed back loop counteracts longitudinal
voltages at the two-wire port by injecting
longitudinal currents in opposing phase.
Thus longitudinal disturbances will
appear as longitudinal currents and the
TIPX and RINGX terminals will experience
very small longitudinal voltage excursions,
leaving metallic voltages well within the
SLIC common mode range.
Z
Z
Combination
CODEC/Filter
T
B
Z
RX
V
RX
RSN
Figure 9. Hybrid function.
recommended to position the 3 dB break
The SLIC longitudinal impedance per
wire, ZLoT and ZLoR, appears as typically
20Ω to longitudinal disturbances. It should
be noted that longitudinal currents may
exceed the dc loop current without distur-
bing the vf transmission.
point of this filter between 30 and 80 Hz to Battery Feed
get a faster response for the dc steps that
The PBL 386 21/2 SLIC emulate a battery
characteristic with current limitation
adjustable.Theopenloopvoltagemeasured
between the TIPX and RINGX terminals is
tracking the battery voltage VBat. The
signalling headroom, or overhead voltage
may occur at DTMF signalling.
Capacitor CLP
Capacitors CTC and CRC
ThecapacitorCLP,whichconnectsbetween
the terminals CLP 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 choise
of these programmable components have
an influence on the power supply rejection
ratio (PSRR) from VBAT to the two wire
side at sub-audio frequencies. At these
frequencies capacitor CLP also influences
the transversal to longitudinal balance in
theSLIC.Table1suggestsasuitablevalue
on CLP. The typical value of the transversal
to longitudinal balance (T-L bal.) at 200Hz
is given in table 1 for the chosen value on
CLP.
The capacitors designated CTC and CRC
in figure 11, connected between TIPX
and ground as well as between RINGX
and ground, can be used for RFI filtering.
The recommended value for CTC and CRC
is 2200 pF. Higher capacitance values
may be used, but care must be taken to
prevent degradation of either longitudinal
balance or return loss. CTC and CRC
contribute to a metallic impedance of
1/(π·f·CTC) = 1/(π·f·CRC), a TIPX to ground
impedance of 1/(2·π·f·CTC) and a RINGX
to ground impedance of 1/(2·π·f·CRC).
VTRO, is programmable with a resistor ROV
connected between terminal POV on the
SLIC and ground. Please refer to section
“Programmable overhead voltage(POV)”.
Thebatteryvoltageoverhead,VOH,depends
ontheprogrammedsignaloverheadvoltage
V
TRO. VOH defines the TIP to RING voltage
at open loop conditions according to
VTR(at IL = 0 mA) = |VBat| - VOH
Refer to table 2 for the typical value on
VOH
.
.
SLIC
VOH(typ) [V]
AC - DC Separation Capacitor, CHP
The high pass filter capacitor connected
between terminals HP and TIPX provides
the separation of the ac signal from the
dc part. CHP positions the low end
frequency response break point of the ac
loop in the SLIC. Refer to table 1 for a
recommended value of CHP.
Example: A CHP value of 47 nF will
position the low end frequency response
3dB break point of the ac loop at 5.6 Hz
(f3dB) according to f3dB = 1/(2 π RHP CHP)
where RHP = 600 kΩ.
PBL 386 21/2
2.5 +VTROprog
Table 2. Battery overhead.
T-L bal.
The current limit (reference A - C in figure
12) is adjusted by connecting a resistor,
RLC, between terminal PLC and ground
according to the equation:
RFeed RSG
CLP @ 200Hz CHP
[Ω]
[kΩ]
[nF]
[dB]
[nF]
2·25
0
150
-46
47
1000
ILProg + 4
Table 1. RSG, CLP and CHP values for cons-
tant current feeding characteristics.
RLC
=
where RLC is in kΩ for ILProg in mA.
For values outside table 1, please contact
Ericsson Microelectronics for assistance.
High-Pass Transmit Filter
The capacitor CTX in figure 11 connected
between the VTX output and the
CODEC/filter forms, together with RTX and/
or the input impedance of a programmable
CODEC/filter, a high-pass RC filter. It is
A second, lower battery voltage may be
connected to the device at terminal VBAT2
to reduce short loop power dissipation.
The SLIC automatically switches between
the two battery supply voltages without
need for external control. The silent battery
11
ERICSSON [ ERICSSON ]
