The HC-5502X/4X Telephone Subscriber Line Interface Circuits

Category: Telephone, cellular phone and intercom
Manufacture: Intersil Corporation
Datasheet: Download this application note

The HC-5502X/4X Telephone Subscriber Line Interface Circuits (SLIC)
A ppl i c a ti on Note J a nu a r y 1997 AN549.1
Author: Geoff Phillips
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.
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The SLIC must be able to sense this DC current and flag the switch controller: This is referred to as Switch Hook Detection (SHD). It tells the switch controller that the line is busy, and is a supervisory function. The subscriber set is often located very close to the switch office. Thus, the loop resistance will be very low and the SLIC should incorporate a feedback network that will limit the loop current to a specified maximum to prevent battery power drain and minimize power dissipation at the board level. The HC5502X/4X SLICs sense the loop current and adjust the voltage on the ring side of the line to cause line current saturation. The telephone can be rung by switching a ring relay to connect a ring generator to the loop. The on-off switching of the relay (cadencing) is controlled by the Ring Command (RC) input which gates the relay driver output. When the user answers the telephone, the ring relay is automatically tripped, the ring command signal is inhibited and the 2W loop is made ready for voice transmission. Voice signals are transmitted onto the loop by directly modulating the DC feed. This AC voice signal is coupled to the users earpiece via a transformer in the telephone set. Voice transmission for the 2W to the 4W system is called the hybrid function. For 2W to 4W transmission, the subscriber talking into his set modulates the resistance of the telephone microphone. This causes AC current in the loop which is sensed by the SLIC and transmitted as a ground referenced voltage signal to the signal processing electronics within the switch. Subscriber loops are usually measured in terms of loop resistance. The nominal loop length is 1200. Owing to the length of the lines and their location near power lines, common mode or longitudinal currents are often induced. The SLIC has to distinguish between these noise signals (longitudinal) and the transversal signals, and reject the unwanted longitudinal components: this is a measure of the SLIC's longitudinal balance. The primary noise sources are 60/50Hz power lines, cable cross talk, and RF transmissions. The Intersil SLICs will accommodate 15mARMS of noise currents on each side of the loop. The line is also subjected to lightning strikes. Together with primary and secondary protection networks, the SLIC must withstand 1kV peak of lightning induced energy. In fact, the plastic encapsulated Intersil SLIC can withstand a 1kV peak strike with a small signal diode bridge providing voltage clamping and current steering. Figure 2 shows the functional schematic of the SLIC. The subfunctions to be described are: Line Feed Amplifiers Transversal Amplifiers Loop Current Limiting: Metallic, Fault and Thermal Limiting Ring Trip and Ground Key Detection Spare or Uncommitted Operational Amplifier Logic Network
The line feed amplifiers are high power op amps, and are connected to the subscriber loop through 300 of feed resistance; the configuration is shown in Figure 3. The feed resistors provide a 600 balanced load for the 2W to 4W transmission, and limit longitudinal currents; the two resistors immediately adjacent to the feed amplifiers function as sense resistors for 2W to 4W transmission and signalling purposes. The tip feed amplifier is configured as a unity gain noninver ting buffer. A -4V bias (derived from the negative battery (VB-) 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. The tip feed amplifier also feeds the ring feed amplifier, which is configured as a unity gain inverting amplifier as seen from the TF amplifier. The noninverting input to the RF amp is biased at a VB-/2. Looking into this terminal the amplifier has a noninverting gain of 2. Thus, the DC output at ring feed is: VRF(DC) = (4 + VB-) Volts For a -48V battery, VRF = -44V. Hence, the nominal battery feed across the loop provided by the SLIC is 40V. When the subscriber goes off-hook this DC feed causes current (metallic current) to flow around the loop. The received audio signal VRX from the switch is fed into the tip feed amplifier and appears at the TF terminal. It is also fed through the ring feed amplifier and is inverted. Thus, a differential signal of 2VRX appears across the line: for a 600 line this compensates the 6dB loss due to the 600 of line feed resistance. The VRX signal causes AC audio currents to flow around the loop which are then AC coupled to the earpiece of the telephone set. Figure 4 shows the single ended AC equivalent circuit of the subscriber loop for voice transmission. In the general case the signal design equation for 4W to 2W transmission is given by:
ZL I N E V L I N E = -------------------------------- 2 V R X 600 + Z L I N E
The Intersil HC-5502X/4X
The HC-5502X/4X family of SLICs are primary intended for use within Private Branch Exchanges (PBX) although they can be used in the larger switch networks found in Central Offices (CO).

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