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ISL5585_04 Datasheet, PDF (13/24 Pages) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband (VOB)
ISL5585
Substituting Equation 19 into Equation 17 (VIN =0) and
defining ∆IM = -V2W/ZL results in Equation 37 for VTX.
VTX
=
V-----2---W---
2
-Z---L-----–----2----R-----P--
ZL
(EQ. 37)
Combining Equations 36 and 37 results in Equation 38.
G2-4
=
V-----T---X--
EG
=
–-2---(---Z----L-Z---+-L----2-–---R-2---P-R----+P-----Z----O-----)
=
–-2---(---Z----L----+-----2-Z---RO----P-----+-----Z----O-----)
(EQ. 38)
A more useful form of the equation is rewritten in terms of
VTX/V2W. A voltage divider equation is written to convert
from EG to V2W as shown in Equation 39.
V2W
=



Z----L---Z--+--O---Z---+-O-----2-+--R---2--P--R----P--
EG
(EQ. 39)
Substituting ZL = ZO + 2RP and rearranging Equation 39 in
terms of EG results in Equation 40.
EG = 2V2W
(EQ. 40)
Substituting Equation 40 into Equation 38 results in an
equation for 2-wire to 4-wire gain that’s a function of the
synthesized input impedance of the SLIC and the protection
resistors.
G2-4
=
-V----T----X--
V2W
=

–

(---Z----L----+-----2---Z-R---O-P------+----Z----O-----)
=
0.416
(EQ. 41)
If ZL is set to 600Ω, ZO is programmed with RS to be
498.76Ω (66.5kΩ/133.33), and RP is equal to 49.9Ω. This
results in a 2-wire to 4-wire gain of 0.416 or -7.6dB.
When the protection resistors are set to zero, the transmit
gain is -6dB.
Transhybrid Gain
The transhybrid gain is defined as the 4-wire to 4-wire gain
(G44).
G44
=
G42
×
G
24
=

–

R--R---I-S-N--



-Z---L-----+-----2--Z-R---O--P-----+-----Z----O-- 
(EQ. 42)
Understanding Phase Across the ISL5585
4-Wire to 2-Wire Phase
The phase of a signal through the ISL5585 is dependent
upon whether the source is driving the signal 4-wire to 2-wire
or 2-wire to 4-wire.
Figure 6 illustrates the phase of the input signal across the
ISL5585 when the signal is applied at the -IN pin of the
ISL5585 through the RIN resistor. The Transmit Amplifier
(TA) inverts the signal 180 degrees at the VTX pin. The
feedback around the tip amplifier inverts the signal again on
the tip lead. The input signal will cause AC loop current to
flow through the 20 Ω sense resistors in the direction from
V 1 to V2 and V3 to V4. This results in an inverted signal
(referenced from tip) on the VSA and thus the VFB pin. This
out of phase signal is the signal used by the feedback path to
match the line impedance of the 2-wire side.
2-Wire to 4-Wire Phase
Figure 7 Illustrates the phase of the input signal across the
ISL5585 when the signal is applied across tip and ring.
When you’re driving the 2-wire side with a source the
ISL5585 looks like a predetermined impedance
(programmed with resistor RS). The current flows through
the 20Ω sense resistors in the direction V2 to V1 and V4 to
V3. This results in a non-inverted signal (referenced from tip)
on the VSA and thus the VFB pin. This signal is then
inverted by the TA amplifier and the signal appearing on the
VTX putput is out of phase with the signal on tip.
Summary of the Phase Through the ISL5585
° 4-Wire to 2-Wire (VIN to V2W) is 180 out of phase
° 2-Wire to 4-Wire (V2w to VTX) is 180 out of phase
° 4-Wire to 4-Wire (VIN to VTX) is 180 out of phase
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