HC5503 Intersil Corporation, HC5503 Datasheet - Page 9

• Manufacturer Part Number: HC5503
• Description: Low Cost 24V SLIC For PABX/Key Systems
• Manufacturer: Intersil Corporation

The longitudinal amplifier’s principal functions is Ring Trip
Detection. The output of the amplifier after being filtered by
R
whose output inhibits the ring relay driver to remove ringing
signals from the line in an off-hook condition, reference
Figure 8.
Ringing The Line
The Ring Command (RC) input is taken low during ringing.
This activates the ring relay driver (RD) output providing the
telephone is not off-hook or the line is not in a power denial
state. The ring relay connects the ring generator to the
subscriber loop. The ring generator output is usually an
80V
150V peak. Since the telephone ringer is AC coupled only
ring current will flow. This ringing current flows directly into
V
terminal RFS is provided so that the low impedance V
node can be isolated from the hot end of the ring path in the
battery referenced ring scheme.
The AC ring current flowing in the subscriber circuit will be
sensed across R
output of the longitudinal amplifier. R
signal before it reaches the ring trip detector to prevent false
ring trip. C
When the subscriber goes off-hook, a DC path is established
between the output of the ring generator and the battery
ground or V
established since no tip feed current is flowing through the
tip feed resistors. The longitudinal amplifier output is driven
negative. Once it exceeds the ring trip threshold of the ring
trip detector, the logic circuitry is driven by GK to trip the ring
relay establishing an off-hook condition such that SHD will
become active as loop metallic current starts to flow.
In addition to its ability to be used for tip or ring injected
systems, the HC5503 can also be configured for systems
utilizing balanced ringing. The main advantage of balanced
ringing is that it tends to minimize cross coupling effects owing
to the differential nature of the ring tone across the line.
Figure 5 illustrates the sequence of events during ring trip with
ring synchronization for a tip injected ring system. Note that
owing to the 90 degree phase shift introduced by the low pass
BAT
20
RMS
and C
FIGURE 4. DC LOOP CURRENT CHARACTERISTICS
30
20
10
via a set of relay contacts. The high impedance
, 20Hz signal. The ring signal should not exceed
0
R
2
2
LOOP
BAT
is nominally set at 1.0 F.
to attenuate AC signals is fed into a detector
terminal. A DC longitudinal imbalance is
= R
B2
B1
, and will give rise to an AC voltage at the
+ R
B2
533
R
LOOP
+ Z
TF
9
HC5503 I
( )
+ Z
RF
20
+ R
LOOP
and C
LINE
SATURATION
+ R
2
attenuate this
SET
RF
HC5503
filter (R
point of the attenuated ring signal that is fed into the ring trip
detector. Hence, when DC conditions are established for
SHD, the AC component actually assists ring trip taking place.
For a ring side injected ring system, the RS pulse should
occur at the positive zero crossing of the ring signal as it
appears at RFS. If ring synchronization is not used, then the
RS pin should be held permanently to a logic high of 5V
nominally: ring trip will occur asynchronously with respect to
the ring voltage. Ring trip is guaranteed to take place within
three ring cycles after the telephone going off-hook.
It is recommended that an RC snubber network is placed
across the ring relay contacts to minimize inductive kick-
back effects from the telephone ringer. Typical values for
such a network are shown in Figure 10.
Transhybrid Circuit
The purpose of the transhybrid circuit is to remove the
receive signal (R
preventing an echo on the transmit side. This is
accomplished by using an external op amp (usually part of
the CODEC) and by the inversion of the signal from the
4-wire receive port (R
Figure 6 shows the transhybrid circuit. Because the voltage
at R
input signal will be subtracted from the output signal if I
equals I
I
The voltage at T
(A
gains, and is therefore equal to 0.633. The voltage at R
when taking into account the negative feedback through R
THRESHOLD
1
150V
+
4-2
I
X
2
V
RS
150V
V
C4
= 0.633) and 2-wire to 4-wire (A
PEAK
is 180 degrees out of phase with the voltage at T
=
RING
RING
TRIP
MAX
20
2
T
------ -
R
5V
0V
0V
. Node analysis yields the following Equation:
, C
X
4
,
+
2
R
------- -
) the RS pulse will occur at the most negative
R
FIGURE 5. RING TIP SEQUENCE
X
3
SUBSCRIBER
GOES OFF-HOOK
X
X
=
) from the transmit signal (T
is the product of the 4-wire to 2-wire
0
DC CURRENT DIFFERENCE
BETWEEN I
X
) to the 4-wire transmit port (T
DC SHIFT OWING TO
>50 s
TIP
AND I
2-4
RING
= -1.0) voltage
RING RELAY
HAS TRIPPED
X
), thereby
C
TO 0V
QUESCENT
VALUE
2
CHARGES
(EQ. 24)
X
X
).
1
X
, the
,
2
,