HC5503

FN4344 Rev 5.00

Page 11 of 17

June 2004

value of 0.633 plus the feedback which is 1/4 TX (for matching

to a 600

 load, reference Equation 8).

The voltage at Rx is calculated in Equation 25.

Substituting the values for TX and RX into Equation 24 and

setting them equal to each other, the values of R3 and R4 can

then be determined.

Setting the value of R3 to 150ksets the value of R4 to be

200k

.

Notice that the input voltage for the incoming signal (I1) is

taken at RX, instead of the conventional method at the CODEC

(point A, Figure 6). This alternative method is used because

the tolerance effects of R1 on the transhybrid balance are

eliminated.

Power Denial (PD)

Power denial limits power to the subscriber loop: it does not

power down the SLIC, i.e., the SLIC will still consume its

normal on-hook quiescent power during a power denial period.

This function is intended to “isolate” from the battery, under

processor control, selected subscriber loops during an

overload or similar fault status.

If PD is selected, the logic circuitry inhibits RC and switches in a

current source to C1. The capacitor charges up to a nominal -

3.5V at which point it is clamped. Since tip feed is always at -4V,

the battery feed across the loop is essentially zero, and

minimum loop power will be dissipated if the circuit goes off-

hook. No signaling functions are available during this mode.

After power denial is released (PD = 1), it will be several

hundred milliseconds (300ms) before the VRF output reaches

its nominal battery setting. This is due to the RC time constant

of R21 and C1.

The Logic Network

The logic network utilizes I2L logic. All external inputs and

outputs are LS TTL compatible: the relay driver is an open

collector output that can sink 60mA with a VCE of 1V.

Figure 9 is a schematic of the combination logic within the

network. The external inputs RC (Relay Control) and PD

(Power Denial) allow the switch controller to ring the line or

deny power to the loop, respectively. The Ring

Synchronization input (RS) facilitates switching of the ring relay

near a ring current zero crossing in order to minimize inductive

kickback from the telephone ringer.

Line Fault Protection

The subscriber loop can exist in a very hostile electrical

environment. It is often in close proximity to very high voltage

power lines, and can be subjected to lightning induced voltage

surges. The SLIC has to provide isolation between the

subscriber loop and the PBX/Key telephone system.

The most stringent line fault condition that the SLIC has to

withstand is that of the lightning induced surge.

The Intersil monolithic SLIC, in conjunction with a simple low

cost diode bridge, can achieve up to 450V of isolation between

the loop and switch. The level of isolation is a function of the

packaging technology and geometry together with the chip

layout geometries. One of the principal reasons for using DI

technology for fabricating the SLIC is that it lends itself most

readily to manufacturing monolithic circuits for high voltage

applications.

Figures 10 shows the application circuit for the HC5503. A

secondary protection diode bridge is indicated which protects

the feed amplifiers during a fault. Most line systems will have

primary protection networks. They often take the form of a

carbon block or arc discharge device. These limit the fault

voltage to less than 450V peak before it reaches the line cards.

Thus when a transient high voltage fault has occurred, it will be

transmitted as a wave front down the line. The primary

protection network must limit the voltage to less than

450V.

The attenuated wave front will continue down the line

towards the SLIC. The feed amplifier outputs appear to the

surge as very low impedance paths to the system battery.

Once the surge reaches the feed resistors, fault current will

flow into or out of the feed amplifier output stages until the

relevant protection diodes switch on. Once the necessary

diodes have started to conduct all the fault current will be

handled by them.

If the user wishes to characterize SLIC devices under simulated

high voltage fault conditions on the bench, he should ensure that

the negative battery power supply has sufficient current capability

to source the negative peak fault current and low series

inductance. If this is not the case, then the battery supply could

be pulled more negative and destroy the SLIC if the total (VCC +

VBAT) voltage across it exceeds 75V.

RX

0.633 1

4

--- 0.633



–

0.474

==

(EQ. 25)

0.633

R4

---------------

0.474

R3

---------------

=

(EQ. 26)

RX

TX

CODEC/

FILTER

I1

I2

V0

+

-

VIN

+

-

+

-

FIGURE 6. TRANSHYBRID CIRCUIT

R1

R3

R4

R5

R2

A

150k



200k

