62

Circuit Operation and Design Information

The HC5523 is a current feed voltage sense Subscriber Line

Interface Circuit (SLIC). This means that for short loop

applications the SLIC provides a programed constant current to

the tip and ring terminals while sensing the tip to ring voltage.

The following discussion separates the SLIC’s operation into

its DC and AC paths, then follows up with additional circuit

and design information.

Constant Loop Current (DC) Path

SLIC in the Active Mode

The DC path establishes a constant loop current that flows

out of tip and into the ring terminal. The loop current is

current circuit, to become the tip and ring loop currents.

For the purpose of the following discussion, the saturation

guard voltage is defined as the maximum tip to ring voltage

at which the SLIC can provide a constant current for a given

battery and overhead voltage.

For loop resistances that result in a tip to ring voltage less

than the saturation guard voltage the loop current is defined

as:

is determined by Equation 2:

where T = 30ms

NOTE:

Figure 14 illustrates the relationship between the tip to ring

voltage and the loop resistance. For a 0

Ω loop resistance both

so does the voltage differential between tip and ring. When

this differential voltage becomes equal to the saturation guard

voltage, the operation of the SLIC’s loop feed changes from a

constant current feed to a resistive feed. The loop current in

the resistive feed region is no longer constant but varies as a

function of the loop resistance.

Active State (C1, 2 = 0, 1)

On-Hook

-

3.7

5.8

mA

-

1.1

1.8

mA

-

2.2

3.7

mA

PSRR

(Note 30, Figure 12)

-

40

-

dB

(Note 30, Figure 12)

-

40

-

dB

(Note 30, Figure 12)

-

40

-

dB

FIGURE 12. POWER SUPPLY REJECTION RATIO

Electrical Specifications

number references in the figures refer to the 28 lead PLCC package. (Continued)

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

600k

Ω

300k

Ω

600

Ω

-48V SUPPLY

+5V SUPPLY

-5V SUPPLY

TIP

27

19

RING

28

RSN

16

I

L

2.5V

R

DC1

R

DC2

+

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

1000

×

=

(EQ. 1)

C

DC

T

1

R

DC1

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

1

R

DC2

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

+





×

=

(EQ. 2)

HC5523