HC5526 Datasheet, PDF (11/18 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5526
I₂
V_TX
R_TX
I₁
R_FB
-
+
+
V_TX
-
HC5526
Z_T
Z_B
RSN
Z_RX
+
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-
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 = _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--
where: T = 0.5ms.
(EQ. 27)
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 and
R₂. This results in a current (I_GK) out of the transconductance
amplifier (gm₂) that is equal to the product of gm₂ and the
differential (I_TIP -I_RING) loop current. If I_GK is less than the
internal current source (I₁), then diode D₁ is on and the output
of the ground key comparator is low. If I_GK is greater than the
internal current source (I₁), then diode D₂ is on and the output
of the ground key comparator is high. With the output of the
ground key comparator high, and the logic configured for
ground key detect, the DET pin goes low. The ground key
detector has a built in hysteresis of typically 5mA between its
trigger and reset values.
Ring Trip Detector
Ring trip detection is accomplished with the internal ring trip
comparator and the external circuitry shown in Figure 19. The
process of ring trip is initiated when the logic input pins are in the
following states: E0 = 0, E1 = 1/0, C1 = 1 and C2 = 0. This logic
condition connects the ring trip comparator to the DET output,
and causes the Ringrly pin to energize the ring relay. The ring
relay connects the tip and ring of the phone to the external
circuitry in Figure 19. When the phone is on-hook the DT pin is
more positive than the DR pin and the DET output is high. For
off-hook conditions DR is more positive than DT and DET goes
low. When DET goes low, indicating that the phone has gone
off-hook, the SLIC is commanded by the logic inputs to go into
the active state. In the active state, tip and ring are once again
connected to the phone and normal operation ensues.
67