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| 型號(hào)： | RSLIC18 |
| 廠商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 2/10頁 |
| 文件大?。?/td> | 115K |
| 代理商： | RSLIC18 |
4-2
Functional Description
The functional blocks of the AC loop are the receiver, tip and
ring amplifiers, sense amplifier and transmit amplifier.
Receiver
The receiver provides the current summing node for the
voice signals from the CODEC (VRX) and the impedance
matching feedback (VTX). The current generated by each
voltage signal and internal 200k
resistor is summed and
mirrored to the Tip and Ring amplifier inverting inputs.
Positive voltages at the receive input (VRX) will source
current to the Tip amp and sink current from the Ring amp.
Tip and Ring Amplifiers
Both amplifiers are of voltage feedback design with a 200k
feedback resistor. The voltage and current relationships of
the receiver to Tip and Ring amplifier outputs is shown in
Figure 2.
The 20
resistors are the sense resistors which provide the
loop current information to the sense amplifier. The sense
resistor voltage drops are compensated by the feedback
loop of each amplifier.
Sense Amplifier
The sense amplifier is configured as a 4 input differential
amplifier with a voltage gain of 1/4. The differential input
pairs are connected across the internal 20
sense resistors.
Current flowing out of Tip and into Ring is the convention for
positive loop current (I
L
) flow. The sense connections across
the sense resistors form an inverting relationship between
the loop current flow and the output voltage of the sense
amp. Figure 3 shows the sense amplifier with connections to
the sense resistors.
The sense amplifier output voltage as a function of the Tip
sense connections can be found by applying superposition
to the circuit of Figure 3.
Simplifying the terms in parenthesis leads to the Tip sense
differential relationship of Equation 2.
The voltage across the Tip sense resistor is the loop current
multiplied by the sense resistor as shown in Equation 3.
The voltage at the Tip amplifier output (V
TO
) is more positive
than the Tip sense connection voltage (V
T
) as defined by the
loop current flow convention of Figure 3. Substituting the
loop current term of Equation 3 into the sense amplifier
output expression of Equation 2 yields:
Applying the same superposition analysis to the Ring sense
connections results in the complete sense amplifier output
expression of Equation 5.
The final step in defining the sense amplifier functionality is to
express the loop current in terms of the load impedance.
Since the loop from Tip to Ring represents a closed system
the loop current out of Tip equals the loop current into Ring.
Therefore the voltage across any impedance in the loop will
provide the loop current information. The loop impedance
between Tip and Ring (V
T
and V
R
) is the protection resistors
(2R
P
) and the load impedance (Z
L
). Using this voltage and
impedance relationship, the sense amplifier output voltage is
rewritten as shown in Equation 6. The last term of Equation 6
represent the voltage gain relationship of the sense amp.
Transmit Amplifier
The transmit amplifier is a voltage feedback design with the
noninverting terminal referenced to ground. It amplifies the
sense amplifier output to achieve impedance synthesis. The
output equation for the transmit amplifier is provided below.
The term RS is the external resistor used to program the
synthesized impedance of the device. The value of RS is
equal to 400 x Z
O
. In addition to impedance matching, the
transmit amplifier drives the CODEC transmit interface.
FIGURE 2. RECEIVE INTERFACE
TIP
RING
VRX
(+
V)
R
R
+
-
+
-
1:1
20
20
R
(+
V)
(-
V)
(+
I)
(+
I)
VB/2
FIGURE 3. SENSE AMPLIFIER SENSING CONNECTIONS
TIP
RING
+
-
+
-
+
-
20
20
R
4R
4R
4R
4R
R
(+
I
L
)
(+
I
L
)
V
SA
V
TO
V
T
V
RO
V
R
V
SA T
V
T
R
4R
||
--------+
1
-------
+
V
TO
-------
–
=
(EQ. 1)
V
SA T
1
4
--
V
T
V
TO
–
(
)
=
(EQ. 2)
V
TO
V
T
–
20I
L
=
(EQ. 3)
V
SA T
1
4
--
V
T
V
TO
–
(
)
1
4
--
20I
L
–
(
)
5
–
I
L
=
=
=
(EQ. 4)
V
SA
V
SA T
V
SA R
5
–
I
L
(
)
5
–
I
L
(
)
10I
L
–
=
+
=
+
=
(EQ. 5)
I
L
V
L
V
P
–
-------+
=
V
SA
V
T
V
R
–
(
)
P
+
L
------–
=
(EQ. 6)
V
TA
V
SA
R
S
–
------------
V
SA
400Z
O
–
---------------------
=
=
(EQ. 7)
Application Note 9824
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