HC5523 Datasheet, PDF (11/18 Pages) Intersil Corporation – LSSGR/TR57 CO/Loop Carrier SLIC with Low Power Standby

HC5523
+_-
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
gm₁(I_METALLIC)
CURRENT
LOOP
COMPARATOR
gm₁
gm₂(I_TIP - I_RING)
gm₂
I_GK
R_H
+_-
GROUND
KEY
D₂
D₁
I₁
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
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
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-----₆^–---₀--^I-₀-^R----^I--^N----^G---- = ₃---^I-₀-^L--₀--
(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 = _I--_O-----_N-----_–----_H----_O-----_O----_K----⁴--_t-⁶-_o---⁵--_O-----_F---_F-----_–----_H----_O----_O-----_K--
(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 = _I--_O-----_F----_F----_–----_H----_O-----_O----_K-³---⁷--_t--⁵_o-----_O-----_N-----_–----_H----_O----_O-----_K--
(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 = _R---^T--_D--
(EQ. 27)
where: T = 0.5ms
Ground Key Detector
A simplified schematic of the ground key detector is shown
in Figure 18. Ground key, is the process in which the ring
terminal is shorted to ground for the purpose of signaling an
Operator or seizing a phone line (between the Central Office
and a Private Branch Exchange). The Ground Key detector
is activated when unequal current flow through resistors R₁
66