HC5526_03 Datasheet, PDF (11/20 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5526
gives the relationship between the R_SG resistor value and
the programmable saturation guard reference voltage:
V_SGREF
=
12.5 + -⁵----^•-----¹---⁰----⁵-
R_SG
(EQ. 3)
where:
V_SGREF = Saturation Guard reference voltage, and,
R_SG = Saturation Guard programming resistor.
When the Saturation guard reference voltage is exceeded,
the tip to ring voltage is calculated using Equation 4:
V_TR
=
R_L
×
-------¹----⁶---^.--⁶---⁶-----+-----⁵-----^•----¹---⁰----⁵----^⁄---^R----^S----^G----------
R_L + (R_DC1 + R_DC2) ⁄ 600
(EQ. 4)
where:
V_TR = Voltage differential between tip and ring, and,
R_L = Loop resistance.
For on-hook transmission R_L = ∞, Equation 4 reduces to:
V_TR
=
16.66 + -⁵----^•-----¹---⁰----⁵-
R_SG
(EQ. 5)
The value of R_SG should be calculated to allow maximum
loop length operation. This requires that the saturation guard
reference voltage be set as high as possible without clipping
the incoming or outgoing VF signal. A voltage margin of -4V
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:
R_SG=
----------------------------------------------------------------⁵-----^•----¹----⁰---⁵-----------------------------------------------------------------
(
V_BAT

–
V_MARGIN
)
×


1
+
(_---R_{-----D----C-6---10----0}+----_R-R_{---L--D----C----2----})
– 16.66V
(EQ.
6)
where:
V_BAT = Battery voltage, and
V_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:
R_SG
=
------------------------------⁵----^•-----¹---⁰----⁵------------------------------
V_BAT – V_MARGIN – 16.66V
(EQ. 7)
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:
I_L
≈
--^V----^B----^A----^T------–-----³----^V--
R_L + 1800Ω
(EQ. 8)
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 16 shows a simplified AC transmission model. Circuit
analysis yields the following design equations:
V_TR = V_TX + I_M • 2R_F
(EQ. 9)
-^V----^T---^X-- + ^V-----^R----^X-- = ----^I--^M------
Z_T Z_RX 1000
V_TR = E_G – I_M • Z_L
(EQ. 10)
(EQ. 11)
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:
V_TX
=
Z_T
•
----^I--^M------
1000
(EQ. 12)
Z_TR is defined as:
Z_TR
=
V_{-----T---R---}
I_M
Substituting in Equation 9 for V_TR:
Z_TR
=
V_{-----T---X-- +} 2_----R_-----F-----•_----I_--M--
I_M
I_M
(EQ. 13)
(EQ. 14)
Substituting in Equation 12 for V_TX:
Z_TR
=
----^Z----^T----
1000
+
2R_F
(EQ. 15)
11