ISL5586 Intersil Corporation, ISL5586 Datasheet - Page 14

• Manufacturer Part Number: ISL5586
• Description: Low Power Ringing SLIC
• Manufacturer: Intersil Corporation

Impedance and Gain Derivations
The feedback mechanism for monitoring the AC portion of
the loop current consists of two amplifiers, the sense
amplifier (SA) and the transmit amplifier (TA). The AC
feedback signal is used for impedance synthesis. A detailed
model of the AC feed back loop is provided below
Impedance Programming Resistor Derivation
The gain of the transmit amplifier, set by R
programmed resistance of the SLIC. For complex line
terminations R
(Figure 1). The capacitor C
the loop current. Figure 11 illustrates the impedance
synthesis loop. Note that the ground symbols shown in
Figures 11 through 14 represent AC grounds, not
necessarily actual DC potentials.
The receiver block provides a single-ended to differential
conversion with a voltage gain of 2. The voltage at Tip and
Ring due to the feedback from V
The Feedback amplifier (TA) provides the programmable
gain required for impedance synthesis to the Receiver block.
The output voltage (V
output voltage and the gain of the feedback amplifier, which
can be substituted for V
The sense amplifier shown in Figure 11 is configured as a 4
input differential amplifier with a gain of 3/4. The output
voltage, V
Ring sense resistors (20Ω each) which can also be
expressed in terms of loop current.
Substituting Equation 37 into Equation 35 and rearranging
terms yields Z
Rearranging and solving for R
relationship between the impedance programming resistor
and the programmed impedance.
4-WIRE TO 2-WIRE GAIN
The 4-wire to 2-wire gain is defined as the gain from the
differential receive input to the 2-wire load Z
function of the terminating impedance, synthesized
impedance and protection resistors and is illustrated in
Figure 12. The input current to the receiver block Irx4w
comes from the difference of the V
VTR
VTR
V
Z
R
0
SA
S
=
=
=
=
=
V
---------- -
133.3
IL
TR
–
–
–
2
2
2
SA
×
×
×
=
20
×
V
V
, is a function of the voltage across the Tip and
4
Z
ZO
SA
0
S
×
×
0
, the SLIC’s synthesized 2-wire impedance.
IL
20
is replaced with a complex network Z
×


×
×
------------
8KΩ
R
(
IL
3 4 )
ZO
S
⁄
×


ZO
) is a function of the Sense Amplifier
3
-- -
4
×
.
4-14
FB
------------
8KΩ
R
S
blocks the DC component of
s
, Equation 39 shows the
ZO
=
RX
60
is shown in Equation 35.
input current and the
×
------------
8KΩ
R
S
s
, determines the
L
. The gain is a
(EQ. 35)
(EQ. 36)
(EQ. 37)
(EQ. 38)
(EQ. 39)
S
V
amplifiers and yields the relationship shown in Equation 40.
The voltage V
voltage V
V
Equation 42.
Substituting Equation 42 into Equation 41 gives Equation 43.
The V
Equation 43 yielding Equation 44.
A loop equation can be derived for the 2-wire side that
replaces V
Expressing IL in terms of V
for V
and the output of the Receive amplifier.
The differential voice input is configured for a gain of 1.4.
The relationship between V
in Equation 47. Substituting for V
shown in Equation 48. Note that the differential voice input is
outside the impedance synthesis loop, so the gain of the
receive amplifier has no effect on the SLIC’s impedance.
When the combination of the device source impedance and
the protection resistors equal the terminating impedance, the
receive gain equals 2.92dB and is inverted with respect to
the 4-wire input.
2-WIRE TO 4-WIRE GAIN
The 2-wire to 4-wire gain (G
Tip and Ring terminals (V
V
V
V
V
V
V
V
V
------------------- -
V
ZO
SA
TR
Z0
SA
Z0
TR
2W
2W
rx
V
RX4W
2W
=
can be expressed in terms of loop current as shown in
2W
feedback current. This current is fed to the Tip and Ring
=
=
+
=
=
=
=
Z0
1.4
IL
–
–
–
–
–
–
V
IL
yields the relationship between the 2-wire voltage
2
IL
=
term in Equation 40 can now be replaced by
2
2V
×
SA.
SA
×
×
×
×
TR
2R
–
×
rx
(
(
2.8
2
V
V
2
V
×
p
rx
×
R XP
ZO,
×
as shown in the equation below.
rx
×
------------
8KΩ



R



20
=
20
–
----------------------------------------- -
Z
–
------------------------------------- -
Z
S
O
2 IL
L
V
–
is a function of the sense amplifier output
×
–
×


+
ZO
2V
+ 2R
V
3
-- -
4
3
-- -
4
Z
RXM
×
Z
)
×
Z
rx
0
L
----------- -
8KΩ
L
2
+
R
–
P
TR
×
2R
)
S
IL 4
+ Z
20
24
2W
) to the V
=
RX


P
) is defined as the gain from the



L
1.4
×



×
/Z
and the voice input is shown
3
-- -
4
20
L


×
RX
, rearranging, and solving
×
V
×


RX4W
TX
, the 4-2-Wire gain is
3
-- -
4
------------
8KΩ
R


S
×
differential output.




------------
8KΩ
R
S


(EQ. 40)
(EQ. 41)
(EQ. 42)
(EQ. 43)
(EQ. 44)
(EQ. 45)
(EQ. 46)
(EQ. 47)
(EQ. 48)