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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
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 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 15 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, 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:
Substituting in Equation 9 for V
TR
:
Substituting in Equation 12 for V
TX
:
Therefore:
Equation 16 can now be used to match the SLIC’s
impedance to any known line impedance (Z
TR
).
Example:
Calculate Z
T
to make Z
TR
= 600
in series with 2.16
μ
F.
R
F
= 20
.
Z
T
= 560k
in series with 2.16nF.
(AC) 2-Wire to 4-Wire Gain
The 2-wire to 4-wire gain is equal to V
TX
/ V
TR
.
From Equations 9 and 10 with V
RX
= 0:
(AC) 4-Wire to 2-Wire Gain
The 4-wire to 2-wire gain is equal to V
TR
/V
RX
.
From Equations 9, 10 and 11 with E
G
= 0:
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
Z
T
I
------------
=
(EQ. 12)
Z
TR
V
M
-----------
=
(EQ. 13)
Z
TR
V
M
----------
2R
-----------------------
I
M
M
+
=
(EQ. 14)
Z
TR
Z
------------
2R
F
+
=
(EQ. 15)
Z
T
1000
Z
TR
2R
F
–
(
)
=
(EQ. 16)
Z
T
1000
600
j
ω
2.16
10
6
–
-----------------------------------------
2
20
–
+
=
A
2
4
–
V
TR
-----------
Z
1000
T
F
--------------------------+
=
=
(EQ. 17)
A
4
2
–
V
RX
-----------
Z
RX
-----------
–
Z
------------
2R
F
Z
L
+
+
--------------------------------------------
=
=
(EQ. 18)
HC5515