
64
The value of R
SG
should be calculated to allow maximum
loop length operation. This requires that the saturation guard
reference voltage be set as high as possible without clipping
the incoming or outgoing VF signal. A voltage margin of -4V
on tip and -4V on ring, for a total of -8V margin, is
recommended as a general guideline. The value of R
SG
is
calculated using Equation 6:
where:
V
BAT
= Battery voltage.
V
MAR
= Voltage Margin. Recommended value of -8V to
allow a maximum overload level of 3.1V peak.
For on-hook transmission R
L
=
∞
, Equation 6 reduces to:
5
BAT
SLIC in the Standby Mode
Overall system power is saved by configuring the SLIC in the
standby state when not in use. In the standby state the tip and
ringamplifiersaredisabledandinternalresistorsareconnected
between tip to ground and ring to V
BAT
. This connection
enables a loop current to flow when the phone goes off-hook.
The loop current detector then detects this current and the
SLIC is configured in the active mode for voice transmission.
The loop current in standby state is calculated as follows:
where:
I
L
= Loop current in the standby state.
R
L
= Loop resistance.
V
BAT
= Battery voltage.
(AC) Transmission Path
SLIC in the Active Mode
Figure 16 shows a simplified AC transmission model. Circuit
analysis yields the following design equations:
where:
V
TR
= Is the AC metallic voltage between tip and ring,
including the voltage drop across the fuse resistors R
F
.
V
TX
= Is the AC metallic voltage. Either at the ground
referenced 4-wire side or the SLIC tip and ring terminals.
I
M
= Is the AC metallic current.
R
F
= Is a fuse resistor.
Z
T
= Is used to set the SLIC’s 2-wire impedance.
V
RX
= Is the analog ground referenced receive signal.
Z
RX
= Is used to set the 4-wire to 2-wire gain.
E
G
= Is the AC open circuit voltage.
Z
L
= Is the line impedance.
(AC) 2-Wire Impedance
The AC 2-wire impedance (Z
TR
) is the impedance looking
into the SLIC, including the fuse resistors, and is calculated
as follows:
Let V
RX
= 0. Then from Equation 10
RSG
---------------------------------------------
VBAT
VMAR
–
(
)
1
)
+
16.66V
–
×
----------------------------------------------------------------------------------------+
17300
–
=
(EQ. 6)
R
SG
MAR
------------------–
17300
–
=
(EQ. 7)
I
L
V
L
3V
–
---------+
≈
(EQ. 8)
V
TR
V
TX
I
M
2R
F
+
=
(EQ. 9)
V
T
----------
V
RX
-----------
+
I
------------
=
(EQ. 10)
V
TR
E
G
I
M
Z
L
–
=
(EQ. 11)
V
TX
RSN
TIP
RING
I
M
1000
Z
TR
V
TR
E
G
-
V
TX
I
M
V
TX
Z
RX
1
HC5523
R
F
R
F
A = 4
+
-
+
-
+
+
-
Z
T
+
-
V
RX
+
-
A = 250
A = 250
I
M
Z
L
FIGURE 16. SIMPLIFIED AC TRANSMISSION CIRCUIT
V
TX
Z
T
I
------------
=
(EQ. 12)
HC5523