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HC5515 Datasheet, PDF (9/17 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby | |||
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HC5515
on tip and -4V on ring, for a total of -8V margin, is
recommended as a general guideline. The value of RSG is
calculated using Equation 6:


RSG=





ï£
--------------------------------------------------------------5-----â¢----1---0----5---------------------------------------------------------------
( VBAT
â
VMAR)
Ã

1
ï£
+
(---R----D-----C--6---1-0---0-+---R--R--L--D-----C-----2----)
â 16.66V
â


17300



(EQ. 6)
where:
VBAT = Battery voltage, and
VMAR = Voltage Margin. Recommended value of -8V to
allow a maximum overload level of 3.1VPEAK.
For on-hook transmission RL = â, Equation 6 reduces to:
RSG = --V----B----A----T------â----5-V----Mâ¢----1-A---0-R--5---â-----1---6---.--6---6----V--- â 17300
(EQ. 7)
SLIC in the Standby Mode
Overall system power is saved by conï¬guring the SLIC in the
standby state when not in use. In the standby state the tip
and ring ampliï¬ers are disabled and internal resistors are
connected between tip to ground and ring to VBAT. This
connection enables a loop current to ï¬ow when the phone
goes off-hook. The loop current detector then detects this
current and the SLIC is conï¬gured in the active mode for
voice transmission. The loop current in standby state is
calculated as follows:
IL â -R-V---L-B----+A----T1----8--â-0---0-3---â¦-V--
where:
IL = Loop current in the standby state,
RL = Loop resistance, and
VBAT = Battery voltage.
(EQ. 8)
(AC) Transmission Path
SLIC in the Active Mode
Figure 15 shows a simpliï¬ed AC transmission model. Circuit
analysis yields the following design equations:
VTR = VTX + IM ⢠2RF
(EQ. 9)
-V--Z--T--T-X-- + V-Z----RR----XX-- = 1----0I--M-0----0-
VTR = EG â IM ⢠ZL
(EQ. 10)
(EQ. 11)
where:
VTR = Is the AC metallic voltage between tip and ring,
including the voltage drop across the fuse resistors RF,
VTX = Is the AC metallic voltage. Either at the ground
referenced 4-wire side or the SLIC tip and ring terminals,
IM = Is the AC metallic current,
RF = Is a fuse resistor,
ZT = Is used to set the SLICâs 2-wire impedance,
VRX = Is the analog ground referenced receive signal,
ZRX = Is used to set the 4-wire to 2-wire gain,
EG = Is the AC open circuit voltage, and
ZL = Is the line impedance.
(AC) 2-Wire Impedance
The AC 2-wire impedance (ZTR) is the impedance looking
into the SLIC, including the fuse resistors, and is calculated
as follows:
Let VRX = 0. Then from Equation 10:
VTX = ZT ⢠1----0I--M-0----0-
(EQ. 12)
ZTR is deï¬ned as:
ZTR = V---I--TM---R---
(EQ. 13)
Substituting in Equation 9 for VTR:
ZTR = V---I--TM---X-- + 2----R-----FI--M---â¢----I--M---
Substituting in Equation 12 for VTX:
ZTR = 1----Z0---0-T---0- + 2RF
(EQ. 14)
(EQ. 15)
Therefore:
ZT = 1000 ⢠(ZTR â 2RF)
(EQ. 16)
Equation 16 can now be used to match the SLICâs
impedance to any known line impedance (ZTR).
Example:
Calculate ZT to make ZTR = 600⦠in series with 2.16µF.
RF = 20â¦.
ZT
=
1000
â¢

ï£
600
+
-j-Ï------â¢----2----.-1---1-6-----â¢----1---0----â---6-
â
2
â¢
20
ZT = 560k⦠in series with 2.16nF.
(AC) 2-Wire to 4-Wire Gain
The 2-wire to 4-wire gain is equal to VTX/ VTR.
From Equations 9 and 10 with VRX = 0:
A2 â 4 = V-V----TT---R-X-- = Z----T-----â-Z--1-T--0---â0---1-0--0---+0----02----R-----F-
(EQ. 17)
(AC) 4-Wire to 2-Wire Gain
The 4-wire to 2-wire gain is equal to VTR/VRX.
From Equations 9, 10 and 11 with EG = 0:
A4 â 2
=
-V----T----R--
VRX
=
â---Z----T---
ZRX
â¢
-1-------Z0------0--T------0------+----Z-2--L-R-----F-----+-----Z---L--
(EQ. 18)
63
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