Slic Chip

[Santi Dubrova]

TheSilicon Labs Si3226 and Si3227 house the low-voltage portion of the SLIC while the Si3290 and Si3299 contain the high-voltage linefeed circuits. The Si3226 and Si3227 include the voice codecs, DTMF detection, and signal generation functions needed for two complete analog telephone interfaces. All the BORSCHT functions are supported as well.

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 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.

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).

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.

Lou Frenzel is a Contributing Technology Editor for Electronic Design Magazine where he writes articles and the blog Communique and other online material on the wireless, networking, and communications sectors. Lou interviews executives and engineers, attends conferences, and researches multiple areas. Lou has been writing in some capacity for ED since 2000.

With the growing interest in voice-over-IP services, many communications equipment manufacturers are looking at broadband IP technologies such as DSL and cable modems as vehicles for broadband data. The concept underlying voice-over-IP (VoIP) is to digitize the voice and then run the voice packets over the same cable or DSL network as the data packets. The rub, of course, is that users expect to utilize their traditional analog-based telephone handsets and fax machines on the packet network. This means the cable or DSL modem must interface to standard analog telephone equipment. Devices that include the cable or DSL modem together with analog telephone interfaces are known as integrated access devices (IADs).

In a VoIP application, the subscriber line interface circuit (SLIC), the interface between the telephone subscriber's copper lines and the electronic switching system, must meet a different and in some cases more stringent set of requirements than in traditional central office applications. These requirements are:

With the exception of the SLIC device, the integrated circuits found in a typical VoIP design operate from a Vcc supply of 3.3 V. But most SLIC parts today require a Vcc of 5 volts. Hence, an SLIC chip that can operate at 3.3 V, rather than the traditional 5-V supply, can save the designer the time and cost of having to integrate a separate dedicated 5-V source for the SLIC interface. ICs that operate at 3.3 V rather than 5 V also can help reduce power dissipation.

Many of the digital signal-processor-based chip sets for next-generation cable modems and DSL access devices are highly integrated circuits that include support for industry-standard voice codecs. A next-generation SLIC, one designed for VoIP, should be designed to work with these chips and to minimize external components required at all device interfaces. In some applications, a standard codec is still used in conjunction with a DSP to perform the necessary signal processing. In this type of architecture, a dual-gain SLIC is necessary. The dual gain will allow the SLIC to interface to first- or third-generation codecs. With a first-generation model, a high SLIC gain allows synthesis of any worldwide termination via external filtering at the SLIC-codec interface. With a third-generation, or programmable codec, a low-gain SLIC enables the codec to operate at high signal output levels, thereby maximizing the signal-to-noise performance (S/N ratio), while meeting Transmission Level Point requirements at T/R without the need for external resistor dividers at the SLIC-codec interface. Another way to minimize the number of external components is to choose proper levels and fix various thresholds such as loop closure and current limit on-chip.

In telephony, lifeline considerations are an important factor. It is desirable that a consumer should be able to dial 911 not only when there is no power, as may be the case during bad weather or other emergencies, but also when there has been no power for several hours. A typical requirement is eight hours of scan mode operation under power backup conditions, as might be the case when the main power grid has failed. Then, when the phone does go off-hook for the user to place a call, the device must be able to operate for an extended time: A typical requirement is one hour of talk time under power backup conditions. In the case of a VoIP device and an SLIC, this means scan power (power consumption while scanning the line while in on-hook mode to detect an off-hook condition) and quiescent on-hook active power (power consumed by the SLIC when the phone is active) should be as low as possible. Even for applications that provide remote power, such as cable modems with power over the cable, SLIC power can be critical because it is important to minimize overall system power requirements.

One power-saving design technique is to include an on-chip battery switch in the SLIC design. Phone systems typically employ two separate voltage sources-a high-voltage supply (typically, 70 V to 80 V), which is used for ring signal amplification and for scan and on-hook transmission modes-and a low-voltage (20-V) source for powering the handset itself and delivering the 20 mA of current typically required to operate the voice electronics. Using the 20-V source to provide the 20 mA can reduce the power dissipation by a factor of 3.5 times compared with using the low-voltage source. A SLIC chip should be able to detect when the handset needs to be powered (off-hook condition) and automatically switch between voltage sources. An SLIC chip may be able to operate at less than 50-mW power dissipation in the active, transmission-ready, on-hook quiescent mode before the phone goes off-hook-for example, in the case of on-hook transmission of caller ID information. With a battery switch, the SLIC off-hook power consumption is limited to about 500 mW (20-V battery, 22.5 mA delivered to the phone and 50-mW quiescent power).

In a traditional analog system the combination of an SLIC and a ring generator is used. In such systems the distances from the central office to the device are very long (measured in miles) and the SLIC cannot generate the ringing signal. Therefore, a relay device is used to switch out the SLIC and switch in the ring generator to ring the phone. However, VoIP systems look very different. The power source is local and distance from the phones to the SLIC, inside the cable modem or DSL modem or other IAD, is short, often as little as 200 feet and hardly ever over 2,000 feet. So an SLIC designed for the VoIP market should be able to power the voice signal and ring the phone. By doing both, the SLIC can save the designer the cost of integrating a ring generator and relay at the customer premises.

Hence, an SLIC intended for an IAD should have a ring mode in which a power ring signal is provided to the telephone tip-ring pair. During the ring mode, a user-supplied low-voltage ring signal (ac coupled) should be input to the device's RING input pin. This signal is then amplified to produce the power ring signal, which may be a sine wave or filtered square wave to produce a sine wave on trapezoidal output. A trapezoidal waveform will have a peak to average ratio, or crest factor, of about 1.2 to 1.3. A pure sine waveform has a crest factor of 1.414. The lower crest factor will yield a better power transfer to the ringing load, but at the same time the distortion associated with the lower crest factors may introduce some problems for certain pieces of Customer Premise Equipment (CPE). The system designer must weigh the engineering trade-offs and decide, based on his or her particular market needs, which is the better option. The important factor for an SLIC device is to give the system designer the flexibility to choose.

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