HC5515_06 Datasheet, PDF (11/20 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5515
I_M
TIP
R_F
Z_L
V_TR
+
-
Z_TR
+
E_G
-
+
V_TX
-
I_M
RING ^RF
+_-
1
HC5515
A = 250
A = 250
A=4
V_TX
+
V_TX
Z_T
-
RSN
I_M
1000
Z_RX
+
V_RX
-
FIGURE 15. SIMPLIFIED AC TRANSMISSION CIRCUIT
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----^T---
Z_RX
•
1_--
2
(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
•
-----------^Z---^L-----+-----²----^R----^F------------
----^Z---^T-----
1000
+
2
R_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 16 shows the transhybrid
circuit. The input signal will be subtracted from the output signal
if I₁ equals I₂. Node analysis yields the following equation:
-^V----^T----^X-- + -^V----^R----^X-- = 0
R_TX Z_B
(EQ. 21)
The value of Z_B is then:
Z_B
=
–R_TX
•
V_{-----R----X--}
V_TX
(EQ. 22)
Where V_RX/V_TX equals 1/ A_4-4.
Therefore:
Z_B
=
R_TX
•
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
I₁
HC5515
Z_T
Z_B
RSN
Z_RX
-
+
+
V_TX
-
+
V_RX
-
CODEC/
FILTER
FIGURE 16. TRANSHYBRID CIRCUIT
Supervisory Functions
The loop current 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”.
Before proceeding with an explanation of the loop current
detector and the longitudinal impedance, it is important to
understand the difference between a “metallic” and
“longitudinal” loop currents. Figure 17 illustrates 3 different
types of loop current encountered.
Case 1 illustrates the metallic loop current. The definition of
a metallic loop current is when equal currents flow out of tip
and into ring. Loop current is a metallic current.
11
FN4235.6
June 6, 2006