HC5503

FN4344 Rev 5.00

Page 10 of 17

June 2004

The longitudinal amplifier’s principal function is Ring Trip

Detection. The output of the amplifier after being filtered by R20

and C2 to attenuate AC signals is fed into a detector 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

80VRMS, 20Hz signal. The ring signal should not exceed 150V

peak. Since the telephone ringer is AC coupled only ring

current will flow. This ringing current flows directly into VBAT

via a set of relay contacts. The high impedance terminal RFS is

provided so that the low impedance VRF 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 RB2, and will give rise to an AC voltage at the

output of the longitudinal amplifier. R20 and C2 attenuate this

signal before it reaches the ring trip detector to prevent false

ring trip. C2 is nominally set at 1.0F.

When the subscriber goes off-hook, a DC path is established

between the output of the ring generator and the battery

ground or VBAT terminal. A DC longitudinal imbalance is

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

filter (R20, C2) the RS pulse will occur at the most negative 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 (RX) from the transmit signal (TX), thereby 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 (RX) to the

4-wire transmit port (TX). Figure 6 shows the transhybrid

circuit. Because the voltage at RX is 180 degrees out of phase

with the voltage at TX, the input signal will be subtracted from

the output signal if I1 equals I2. Node analysis yields the

following Equation:

The voltage at TX is the product of the 4-wire to 2-wire (A4-2 =

0.633) and 2-wire to 4-wire (A2-4 = -1.0) voltage gains, and is

therefore equal to 0.633. The voltage at RX, when taking into

account the negative feedback through R2, is the calculated

HC5503 ILOOP SATURATION

RLOOP ()

533

0

20

30

RLOOP = RB1 + RB2 + ZTF + ZRF + RLINE + RSET

10

FIGURE 4. DC LOOP CURRENT CHARACTERISTICS

FIGURE 5. RING TIP SEQUENCE

150V

VRING

150VPEAK,

MAX

>50

s

C2 CHARGES

TO 0V

VALUE

RING RELAY

HAS TRIPPED

DC SHIFT OWING TO

DC CURRENT DIFFERENCE

BETWEEN ITIP AND IRING

QUIESCENT

SUBSCRIBER

GOES OFF-HOOK

RING

TRIP

THRESHOLD

0V

0V

5V

RS

VC4

I1 I2

+

TX

R4

-------

RX

R3

--------

+

0

==

(EQ. 24)