HC5513_03 Datasheet, PDF (12/20 Pages) Intersil Corporation – TR909 DLC/FLC SLIC with Low Power Standby

HC5513
TIP
RING
CASE 1
CASE 2
CASE 3
I_METALLIC I_LONGITUDINAL I_LONGITUDINAL
¨
¨
Æ
I_METALLIC I_LONGITUDINAL I_LONGITUDINAL
Æ
¨
Æ
+_-
R₁
R₂
-
+
HC5513
gm₁(I_METALLIC)
gm₁
CURRENT
LOOP
COMPARATOR
gm₂(I_TIP - I_RING)
gm₂
I_GK
R_H
+_-
GROUND
D₂
D₁
I₁
KEY
COMPARATOR
R_D
R_H
+_-
-
V_REF
1.25V
I_RD
R_D
C_D
V_EE
-5V
DIGITAL MULTIPLEXER
DET
FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS
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
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 (IGK) out of the transconductance amplifier (gm2)
that is equal to the product of gm2 and the differential (ITIP -
IRING) loop current. If IGK is less than the internal current
source (I1), then diode D1 is on and the output of the ground
key comparator is low. If IGK 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.
Figure 19 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.
R_RT
C_RT
R₃
TIP
R₄
E_RG
R₁
DT
-
+
DR
R₂
RING TRIP
COMPARATOR
DET
RING
RING
RELAY
V_BAT
RINGRLY
HC5513
FIGURE 19. RING TRIP CIRCUIT FOR BATTERY BACKED RINGING
Longitudinal Impedance
The feedback loop described in Figure 20(A, B) realizes the
desired longitudinal impedances from tip to ground and from
ring to ground. Nominal longitudinal impedance is resistive
and in the order of 22Ω.
In the presence of longitudinal currents this circuit attenuates
the voltages that would otherwise appear at the tip and ring
terminals, to levels well within the common mode range of
the SLIC. In fact, longitudinal currents may exceed the
programmed DC loop current without disturbing the SLIC’s
VF transmission capabilities.
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