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HC5503

Low Cost 24V SLIC For PABX/Key Systems

Intersil 

HC5503

The Transversal Amplifier (TA)

Whereas the feed amplifiers perform the 4-wire to 2-wire

transmission function, the transversal amplifier acts as the

2-wire to 4-wire hybrid. The TA is a summing amplifier

configured to reject common mode signals. It will reject 2-

wire common mode signals. RB1 and RB2 act as loop

current sense resistors. The voice signal output of the

amplifier is a function of the differential voltages appearing

across RB1 and RB2 .

The transversal amplifier also has a DC output proportional

to the metallic current in the loop. The output voltage is

given by:

VTX = 2(ITIP + IRING) (RB1 + RB2)

This DC level is used as an input to a comparator whose

output feeds into the logic circuitry as SH. This signal is used

to gate SHD output.

Voice signals on the loop are transformed by the TA into

ground referenced signals. Since the TA output has a DC

offset it is necessary to AC couple the output to any external

circuitry. Note, that during 4-wire to 2-wire transmission, the

transversal amplifier will have an audio signal at its output

proportional to the 4-wire audio receive signal and the loop’s

equivalent AC impedance. This is called the transhybrid

return, and must be cancelled (or balanced) out to prevent

an echo effect. Reference the Transhybrid Circuit section for

more information.

Loop Current Limiting

The maximum loop length for this application is a 533Ω load

across the feed amplifiers (24VSUPPLY - 8VOVERHEAD)/

30mAMAX loop current). However, on a short loop the line

resistance often approaches zero. Thus, a need exists to

control the maximum DC loop current that can flow around

the loop to prevent an excessive current drain from the

system battery. This limit is internally set to 30mA on the

HC5503. Figure 3 depicts the feedback network that

modifies the VRF voltage as a function of metallic current.

Figure 4 illustrates the loop current characteristics as a

function of line resistance.

As indicated above, the TA has a DC voltage output

directly proportional to the loop current. This voltage level

is scaled by R19 and R18 . The scaled level forms the

‘Metallic’ input to one side of a Transconductance

Amplifier.

VTF

VRING

RB2

RB1

VTIP

VRF

TRANSVERSAL

AMP

+-

VTX = -600 ILOOP

VTX

AVCL = 2

R18

90K

KVTX

-

R19

+

1.8K

VB5

IGM > 0,

FOR KVTX < VB5

-4V

VRF

+-

RING

FEED

R21

90K

C1

VB /2

FIGURE 3. DC LOOP CURRENT CHARACTERISTICS

The reference input to this amplifier is generated in the bias

network, and is equivalent to 30mA. When the metallic input

exceeds the set reference level, the transconductance

amplifier sources current. This current will charge C1 in

positive direction causing the VRF (Ring Feed) voltage to

approach the VTF (Tip Feed), effectively reducing the battery

feed across the loop which will limit the DC loop current. C1

will continue to charge until an equilibrium level is attained at

ILOOP = ILOOPmA (Max). The time constant of this feedback

loop is set by R21 (90kΩ) and C1 which is nominally 0.33µF.

The VRF voltage level is also modified to reduce or control

loop current during ring line faults (e.g., ground or power line

crosses), and thermal overload. Figure 8 illustrates this. The

thermal and fault current circuitry works in parallel with the

transconductance amplifier.

Longitudinal Amplifier

The longitudinal amplifier is an operational amplifier

configured as a closed loop differential amplifier with a

nominal gain of 0.1. The output is a measure of any

imbalance between ITIP and IRING . The transfer function of

this amplifier is given by:

VLONG = 0.1(ITIP - IRING) 150.

The gain factor is much less than one since ring voltage (up

to 150VPEAK) can appear at the Ring or Ring Feed Sense

terminals and are attenuated to avoid exceeding the

common mode range of the longitudinal amplifier’s input.

9

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