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Intersil Corporation 2000
The HC-5502X/4X Telephone Subscriber Line
Interface Circuits (SLIC)
Introduction
The HC-5502X/4X family of telephone subscriber line
interface circuits (SLIC) integrate most of the BORSCHT
functions of the traditional hybrid transformer interface circuits
onto one chip. The circuits are manufactured in a 200V
dielectric isolation (DI) process and together with a secondary
protection diode bridge give 1kV of isolation from lightning
induced faults between the subscriber loop and the telephone
office.
The BORSCHT functions provided are:
Battery Feed With Loop Current Limiting
Overvoltage Protection
Ringing
Supervision/Signalling
Hybrid
The HC-5502X is intended for use in systems utilizing single
ended tip (positive side) injected ringing and limits the short
loop current to 30mA; the HC-5504X is intended for use in
ring side (negative side) injected ringing systems and will limit
the short loop current to 40mA. It should be noted that the
HC-5504X can also be configured to operate in switches
employing either of the two single ended ringing methods and
in balanced ringing systems.
This note will describe each subfunction of the SLIC and will
discuss several system design features, including balance
networks and complex impedance matching.
An Overview of the Basic Phone Loop
And Its Environment
Figure 1 illustrates a simplified telephone network. Each
subscriber is connected via a 2-wire (2W) loop to a switch
office which provides intersubsciber loop switching and
signal processing (analog and/or digital).
The SLIC is the primary interface between the 4 wire (4W)
(ground referenced) low voltage switch environment and the
2W (“floating”) high voltage loop environment.
The loop consists of a wire A (the Tip wire), the telephone
set or its equivalent, and wire B (the ring wire). A DC voltage
is applied across the Tip and Ring wires at the line card
which is housed in the telephone office: The battery is
usually a nominal -48V, and is often called the quiet or
talking battery. When the telephone is off-hook, a DC path is
established around the loop. DC loop current will flow around
the loop from tip feed to ring feed. This is called Battery
Feed.
FIGURE 1. SIMPLIFIED TELEPHONE NETWORK
Application Note
January 1997
Author: Geoff Phillips