Electronic Design
POTS: Still Alive And Useful With SLIC And Modem Chips

POTS: Still Alive And Useful With SLIC And Modem Chips

The plain old telephone system (POTS), also known as the public switched telephone network (PSTN), has been in decline for years as more subscribers drop their wireline connections in favor of a cell phone and/or Voice over Internet Protocol (VoIP) service. Yet those unshielded twisted pair (UTP) wires are still widely used and will stay active for decades to come.

In fact, DSL and some Internet Protocol television (IPTV) systems still use the PSTN. Wireline connections are also used in many places other than voice service for the home or business. The service is cheap, available, and ideal for many low-speed data connections.

The subscriber line interface circuit (SLIC) is a part of all POTS connections. It’s an interface on the line side that performs all of the functions of a central office. These functions are normally called BORSCHT for battery supply (–48 V), overvoltage protection, ringing (20 Hz, 100+ V), supervision (coding/decoding), hybrid (two- to four-line conversion), and testing. At one time, the SLIC was a relatively large module on a printed-circuit board (PCB) with discrete components capable of handling the high voltages. Today, the SLIC is an IC or chipset.

The Dual ProSLIC Si3226x family of SLICs from Silicon Labs targets products like low-channel-count VoIP customer premises equipment (CPE) such as fiber to the home (FTTH) gateways, DSL integrated access devices, and cable embedded multimedia terminal adaptors as well as high-channel-count products including multiple-dwelling unity gateways and public branch exchange (PBX) systems. All of these SLICs provide the interface to all standard wired and cordless telephones.

These foreign exchange station (FXS) solutions feature an integrated level shifter/driver that allows a direct connection to a dc-dc converter’s power transistor regardless of input voltage. This eliminates the need for the MOSFET pre-drive circuit required by most other SLIC designs, reducing the cost and footprint of a two-channel design by at least 12 external devices.

The Si3226x family minimizes the total FXS system power consumption, requiring less than 55 mW per channel when on-hook, making the devices a good solution as telecommunication service providers and governments mandate more energy-efficient electronic products. The on-chip intelligent dc-dc converters optimize energy efficiency by generating only the voltages needed in on-hook, off-hook, and ringing states.

Ultra-low power consumption makes it easier to meet green energy requirements. It also enables longer battery standby time in battery-backed designs. And, it reduces heat dissipation and eliminates the need for fans in multiline gateway designs, further reducing system cost.

The Si3226x family is designed to generate tracking batteries for lowest power consumption, as well as work with shared battery supplies (e.g., fixed-rail) for the lowest bill of materials (BOM) cost. The integrated dc-dc controllers can be configured as either single-output tracking dc-dc converters for each SLIC channel or as a single multi-rail dc-dc converter shared by two or more channels.

This flexibility allows developers to use the same SLIC device for both low-power tracking designs and low-cost fixed-rail designs. Furthermore, the tracking shared supply (TSS) capability significantly reduces the power consumption of two-channel shared-battery designs compared to competitive fixed-rail designs (see the figure).

The devices in the Si3226 family come in a 6- by 8-mm, 50-pin, quad flat no-lead (QFN) package. Pricing begins at $5.31 in 10,000-unit quantities.

Meanwhile, the Silicon Labs Si24xx ISOmodem data modem ICs are single-chip modems that are designed for use in voice, machine-to-machine (M2M), security and home automation, smart utility metering, set-top box, point-of-sale (POS) terminal, video phone, medical monitor, and any other application that connects to the PSTN.

The ISOmodems implement a full set of ITU-T V.dot modem protocols from 2400 to 56 kbits/s, including error correction and data compression, the AT command set, and security protocols. Also included is full dual-tone multi-frequency (DTMF) generation and detection.

All of these chips use the company’s patented direct access arrangement (DAA) technology to implement a globally compliant telephone line interface. DAA also eliminates the need for a separate DSP, modem controller, codes, transformer, relay, opto-isolators, and two- to four-wire hybrid. The Si24xx ISOmodems consume a lot less power, too. Using a 3.3-V supply, they only require 56 mW in normal operation.

The devices come in a 5- by 7-mm QFN package. The new versions can also use a low-cost, 32-kHz “watch” crystal instead of the more expensive crystal normally used in modems. The devices additionally offer the option of using an SPI in addition to UART and parallel interfaces. Prices range from $6.44 to $11.43 in 10,000-unit quantities.

Evaluation boards for the ISOmodems and ProSLICs are available as well.

Silicon Laboratories   

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