HC5503 Просмотр технического описания (PDF) - Intersil

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HC5503

Low Cost 24V SLIC For PABX/Key Systems

Intersil 

HC5503

Design Information

Line Feed Amplifiers

The line feed amplifiers are high power operational

amplifiers and are connected to the subscriber loop through

150Ω of feed resistance as shown in Figure 1. The feed

resistors and synthesized impedance via feedback provide a

600Ω balanced load for the 2-wire to 4-wire transmission.

The tip feed amplifier is configured as a unity gain

noninverting buffer. A -4V bias (derived from the negative

battery (VBAT) in the bias network) is applied to the input of

the amplifier. Hence, the tip feed DC level is at -4V. The

principal reason for this offset is to accommodate sourcing

and sinking of longitudinal noise currents up to 15mARMS

without saturating the amplifier output and to provide

sufficient overhead for receive signals. The tip feed amplifier

also feeds the ring feed amplifier, which is configured as a

unity gain inverting amplifier as seen from the tip feed

amplifier. The noninverting input to the ring feed amp is

biased at a VBAT/2. Looking into this terminal the amplifier

has a noninverting gain of 2. Thus, the DC output at ring

feed is:

VRF(DC) = (4 + VBAT) Volts

For a -24V battery, VRF = -20V. Hence, the nominal battery

feed across the loop provided by the SLIC is 16V. When the

subscriber goes off-hook this DC feed causes current

(metallic current) to flow around the loop.

The received audio signal RX is fed into the tip feed amplifier

and appears at the tip feed terminal. It is also fed through the

ring feed amplifier and is inverted. Thus, a differential signal

of 2VRX appears between tip feed and ring feed. The RX

signal causes AC audio currents to flow around the loop

which are then AC coupled to the earpiece of the telephone

set.

2-Wire Impedance Matching

The HC5503 is optimized for operation with a -24V battery.

Impedance matching to a 600Ω load, is achieved through

the combination of the feed resistors (RB1, RB2) and

negative feedback through resistor R2 (reference Figure 1).

RB1 and RB2 are sense resistors that detect loop current

and provide negative feedback to synthesize the remaining

300Ω required to match a 600Ω line.

The impedance looking into the tip terminal is 150Ω (RB1)

plus the synthesized impedance of the tip amplifier. The

synthesized tip impedance is equal to the tip feed voltage Va

divided by ∆IL. (Note, the tip feed amplifier is a voltage

follower. Thus, the tip feed voltage is equal to the receive

input voltage VRX , both are labeled Va.) The synthesized

impedance of the ring terminal is calculated the same way

and is the ring feed voltage divided by ∆IL. (Note, the ring

feed voltage is equal in magnitude to the tip feed voltage, but

opposite in phase as a result of the ring feed amplifier gain.)

Va

TO TRANSHYBRID OP-AMP

+ ∆IL -

C3

R3

R1

INPUT

TIP

RB1

RX

TIP FEED

+-

90kΩ

FROM

VIN CODEC

150Ω

-

+

4VDC

R2

(NOTE)

ZIN

RB1 = RB2 = RS = 150Ω

+-

+- 2

TX

C4

-

+ +4RS∆IL

4RS∆IL

+

RING RB2

150Ω

- ∆IL +

RING FEED

R

R

-

+

+-

VBAT

2

HC5503

(NOTE)

NOTE: Grounded for AC analysis.

FIGURE 1. IMPEDANCE MATCHING CIRCUITRY

6

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