65
gives the relationship between the R
SG
resistor value and
the programmable saturation guard reference voltage:
where:
V
SGREF
= Saturation Guard reference voltage, and
R
SG
= Saturation Guard programming resistor.
When the Saturation guard reference voltage is exceeded,
the tip to ring voltage is calculated using Equation 4:
10
5
+
L
where:
V
TR
= Voltage differential between tip and ring, and
R
L
= Loop resistance.
For on-hook transmission R
L
=
∞
, Equation 4 reduces to:
5
SG
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, and
V
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:
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 ring amplifiers are disabled and internal resistors are
connected 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, and
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 resisto,
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, and
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:
Z
TR
is defined as:
V
M
Substituting in Equation 9 for V
TR
:
V
M
Substituting in Equation 12 for V
TX
:
V
SGREF
12.5
------------------
5
SG
+
=
(EQ. 3)
V
TR
R
L
16.66
5
R
SG
)
600
R
DC1
R
DC2
+
---------+
×
=
(EQ. 4)
V
TR
16.66
------------------
+
=
(EQ. 5)
R
SG
5
------------------------------------------
+
V
BAT
V
MARGIN
–
(
)
1
)
L
16.66V
–
×
----------------------------------------------------------------------------------------------+
=
(EQ. 6)
R
SG
5
BAT
MARGIN
------------------–
=
(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
Z
T
I
------------
=
(EQ. 12)
Z
TR
-----------
=
(EQ. 13)
Z
TR
----------
2R
-----------------------
I
M
M
+
=
(EQ. 14)
Z
TR
Z
------------
2R
F
+
=
(EQ. 15)
HC5526