HC5513_03 Datasheet, PDF (11/20 Pages) Intersil Corporation – TR909 DLC/FLC SLIC with Low Power Standby

HC5513
The value of Z_B is then:
Z_B
=
–R_TX
•
V_{-----R----X--}
V_TX
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. 22)
(EQ. 23)
Example:
Given: R_TX = 20kΩ, Z_RX = 280kΩ, Z_T = 562kΩ (standard
value), R_F = 20Ω and Z_L= 600Ω,
The value of Z_B = 18.7kΩ.
R_FB
I₂
V_TX
R_TX
HC5513
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”.
Before proceeding with an explanation of the loop current
detector, ground key detector and later the longitudinal
impedance, it is important to understand the difference
between a “metallic” and “longitudinal” loop currents. Figure
18 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.
Cases 2 and 3 illustrate the longitudinal loop current. The
definition of a longitudinal loop current is a common mode
current, that flows either out of or into tip and ring
simultaneously. Longitudinal currents in the on-hook state
result in equal currents flowing through the sense resistors
R₁ and R₂ (Figure 18). And longitudinal currents in the off-
11
hook state result in unequal currents flowing through the
sense resistors R₁ and R₂. Notice that for case 2,
longitudinal currents flowing away from the SLIC, the current
through R₁ is the metallic loop current plus the longitudinal
current; whereas the current through R₂ is the metallic loop
current minus the longitudinal current. Longitudinal currents
are generated when the phone line is influenced by magnetic
fields (e.g., power lines).
Loop Current Detector
Figure 18 shows a simplified schematic of the loop current
and ground key detectors. The loop current detector works
by sensing the metallic current flowing through resistors R₁
and R₂. This results in a current (I_RD) out of the
transconductance amplifier (gm₁) that is equal to the product
of gm₁ and the metallic loop current. I_RD then flows out the
R_D pin and through resistor R_D to V_EE. The value of I_RD is
equal to:
I_RD
=
--^I--^T---^I--^P-----–-----^I--^R----^I--^N----^G----
600
=
---^I--^L----
300
(EQ. 24)
The I_RD current results in a voltage drop across R_D that is
compared to an internal 1.25V reference voltage. When the
voltage drop across R_D exceeds 1.25V, and the logic is
configured for loop current detection, the DET pin goes low.
The hysteresis resistor R_H adds an additional voltage
effectively across R_D, causing the on-hook to off-hook
threshold to be slightly higher than the off-hook to on-hook
threshold.
Taking into account the hysteresis voltage, the typical value
of R_D for the on-hook to off-hook condition is:
R_D
=
---------------------------------⁴---⁶----⁵----------------------------------
I_{ON – HOOK to OFF – HOOK}
(EQ. 25)
Taking into account the hysteresis voltage, the typical value
of R_D for the off-hook to on-hook condition is:
R_D
=
---------------------------------³---⁷----⁵----------------------------------
I_{OFF – HOOK to ON – HOOK}
(EQ. 26)