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Número de pieza

componentes Descripción

Fabricante

HC5503

Low Cost 24V SLIC For PABX/Key Systems

Intersil 

HC5503

30

HC5503 ILOOP SATURATION

20

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

10

0

533

RLOOP (Ω)

FIGURE 4. DC LOOP CURRENT CHARACTERISTICS

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 1.0µF.

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

10

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.

150V

VRING

150VPEAK,

MAX

5V

RS 0V

>50µs

VC4

0V

RING

TRIP

THRESHOLD

SUBSCRIBER

GOES OFF-HOOK

C2 CHARGES

TO 0V

QUIESCENT

VALUE

RING RELAY

DC SHIFT OWING TO HAS TRIPPED

DC CURRENT DIFFERENCE

BETWEEN ITIP AND IRING

FIGURE 5. RING TIP SEQUENCE

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:

I1 + I2

=

-T---X-- + -R----X--

R4 R3

=

0

(EQ. 24)

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 ,

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