HC5523_03 Datasheet, PDF (12/22 Pages) Intersil Corporation – LSSGR/TR57 CO/Loop Carrier SLIC with Low Power Standby

HC5523
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-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
Ground Key Detector
+_-
TIP
R₁
RING
ORDERING INFORMATION
CASE 1
CASE 2
CASE 3
I_METALLIC
←
I_LONGITUDINAL
←
I_LONGITUDINAL
→
I_METALLIC
→
I_LONGITUDINAL
←
I_LONGITUDINAL
→
R₂
-
+
HC5523
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)
A filter capacitor (C_D) in parallel with R_D will improve the
accuracy of the trip point in a noisy environment. The value
of this capacitor is calculated using the following Equation:
C_D
=
---^T----
R_D
(EQ. 27)
where: T = 0.5ms
gm₁(I_METALLIC)
CURRENT
LOOP
COMPARATOR
gm₁
gm₂(I_TIP - I_RING)
gm₂
I_GK
R_H
+_-
GROUND
D₂
D₁
I₁
KEY
COMPARATOR
R_D
R_H
I_{RD RD}
C_D
+_-
-
V_REF
1.25V
V_EE
-5V
DIGITAL MULTIPLEXER
DET
FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS
12