HC5523 Datasheet, PDF (10/18 Pages) Intersil Corporation – LSSGR/TR57 CO/Loop Carrier SLIC with Low Power Standby

HC5523
Z_TR is defined as:
Z_TR = ^V---_I--^TM---^R---
(EQ. 13)
Substituting in Equation 9 for V_TR
Z_TR = ^V---_I--^TM---^X-- + ²----^R-----^F_I--_M---^•----^I--^M---
(EQ. 14)
Substituting in Equation 12 for V_TX
Z_TR = ₁----^Z0---₀-^T---₀- + 2R_F
(EQ. 15)
Therefore
Z_T = 1000 • (Z_TR – 2R_F)
(EQ. 16)
Equation 16 can now be used to match the SLIC’s
impedance to any known line impedance (Z_TR).
Example:
Calculate Z_T to make Z_TR = 600Ω in series with 2.16µF.
R_F = 20Ω.
Z_T
=
1000
•


600
+
-_j-_ω------_•----₂----_.-₁---¹-₆-----_•----₁---₀----_–---₆- – 2 • 20_
Z_T = 560kΩ in series with 2.16nF
(AC) 2-Wire to 4-Wire Gain
The 2-wire to 4-wire gain is equal to V_TX/ V_TR
From Equations 9 and 10 with V_RX = 0
A_{2 – 4}
=
-^V----^T----^X--
V_TR
=
Z^----_T^-----⁄^-Z--1^-T--0^---⁄0^---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 V_TR/V_RX
From Equations 9, 10 and 11 with E_G = 0
A_{4 – 2} = _V-^V----_RT----^RX-- = –_Z---^Z-_R---^T--_X-- • -_1-------Z0------₀--_T------₀------+----^Z-₂--^L-_R-----_F-----+-----_Z---_L--
(EQ. 18)
For applications where the 2-wire impedance (Z_TR,
Equation 15) is chosen to equal the line impedance (Z_L), the
expression for A_4-2 simplifies to:
A_{4 – 2} = –_Z---^Z-_R---^T--_X- • ¹²--
(EQ. 19)
(AC) 4-Wire to 4-Wire Gain
The 4-wire to 4-wire gain is equal to V_TX/V_RX
From Equations 9, 10 and 11 with E_G = 0
A_{4 – 4}
=
-^V----^T----^X--
V_RX
=
–---^Z----^T----
Z_RX
•
-_1-------Z0------₀--_T-----^Z_-0----^L--+---+--₂---^2R----^R-_F---^F--+-----_Z---_L--
(EQ. 20)
Transhybrid Circuit
The purpose of the transhybrid circuit is to remove the
receive signal (V_RX) from the transmit signal (V_TX), thereby
preventing an echo on the transmit side. This is
accomplished by using an external op amp (usually part of
the CODEC) and by the inversion of the signal from the
4-wire receive port (RSN) to the 4-wire transmit port (V_TX).
Figure 17 shows the transhybrid circuit. The input signal will
be subtracted from the output signal if I₁ equals I₂. Node
analysis yields the following equation:
R^{-V----TT----XX-- + -V--}Z^--R--_B^{--X-- = 0}
(EQ. 21)
The value of Z_B is then
Z_B = –R_TX • ^V-_V----^RT----_XX--
(EQ. 22)
Where V_RX/V_TX equals 1/ A_4-4
Therefore
Z_B
=
R_T
X
•
Z_----R-----X-
Z_T
•
-^1-------Z0------⁰--^T------⁰------+-----²---^R-----^F-----+-----^Z---^L--
Z_L + 2R_F
(EQ. 23)
Example:
Given: R_TX = 20kΩ, Z_RX = 280kΩ, Z_T = 562kΩ (standard
value), R_F = 20Ω and Z = 600Ω
The value of Z_B = 18.7kΩ
R_FB
I₂
V_TX
R_TX
HC5523
Z_T
I₁
Z_B
RSN
Z_RX
-
+
+
V_TX
-
+
V_RX
-
CODEC/
FILTER
FIGURE 17. TRANSHYBRID CIRCUIT
Supervisory Functions
The loop current, ground key and the ring trip detector
outputs are multiplexed to a single logic output pin called
DET. See Table 1 to determine the active detector for a given
logic input. For further discussion of the logic circuitry see
section titled “Digital Logic Inputs”.
65