HC5515 Datasheet, PDF (11/17 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5515
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 17). 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 17 shows a simplified schematic of the loop current
detector. 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
=
---------------------------------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 = _R---^T--_D--
(EQ. 27)
where: T = 0.5ms.
Ring Trip Detector
Ring trip detection is accomplished with the internal ring trip
comparator and the external circuitry shown in Figure 18.
The process of ring trip is initiated when the logic input pins
are in the following states: E0 = 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 18. 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.
Figure 18 illustrates battery backed unbalanced ring injected
ringing. For tip injected ringing just reverse the leads to the
phone. The ringing source could also be balanced.
NOTE: The DET output will toggle at 20Hz because the DT input is
not completely filtered by C_RT. Software can examine the duty cycle
and determine if the DET pin is low for more that half the time, if so
the off-hook condition is indicated.
TIP
RING
CASE 1 CASE 2
CASE 3
I_METAL-
LIC
←
I_LONGITUDI-
NAL
←
I_LONGITUDI-
NAL
→
+_-
R₁
R₂
-
+
gm₁(I_METALLIC)
gm₁
CURRENT
LOOP
COMPARATOR
R_D
R_H
I_{RD RD}
C_D
+_-
-
V_REF
1.25V
V_EE
-5V
DIGITAL MULTIPLEXER
DET
HC5515
FIGURE 17. LOOP CURRENT DETECTOR
65