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Legerity's line card access switch struts dual-channel architecture

Posted: 13 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:le75181? le75282? legerity? agere? chartered semiconductor?

It's been a little more than two years since Legerity Inc. acquired the voice interface division of Agere, which developed the first generation of line-card access switches (LCAS). In that time, Legerity has significantly improved the product, employing a special manufacturing technique called trench isolation.

At Chartered Semiconductor, its foundry partner in Singapore, Legerity's high-voltage, high-bandwidth semiconductors are manufactured using BCD3T, 350V DMOS transistor technology.

Legerity has since improved the switches and redesigned them based on market needs. Its latest introduction is a pair of LCAS devices, the Le75181 and Le75282, which the company contends are the first non-custom parts to use foldback technology. It also says that the Le75282 is the only LCAS with a two-channel architecture.

"We've integrated two complete channels in a single devicethat's never been done before," said Michael Stibila, VP of marketing applications for Legerity worldwide. "It saves board space, but most importantly, the cost. A lot of the device's cost is in packaging and test, and having to test one device for two channels versus two devices with two channels just saves money."

Another key characteristic of the dual channel architecture is that the control interface provides a seemless bridge to other components, simplifying board design.

Replacing relays

The LCAS devices are a superior alternative to electromechanical relays in analog line interface cards, Stibila said. The high-voltage semiconductor devices feature 350-Volt capability for switching high-voltage rings at 105Vrms to 110Vrms. "When a person goes off the hook, we detect what happens and stop ringing the line," Stibila said.

When telephone companies test the line and the line circuit, they put high-voltage signals of about 150V Vcc on the line and look at the current that flows. The access devices also have contacts to connect to the line and test the voice circuit. Many applications implement electromechanical relays to check for leakage resistance and capacitance to determine the condition of the line.

"But as boards get more dense and as people put more and more ports or lines on a card, space and power become important criteria," Stibila said. "Integrating these functions on semiconductors really does a good job of lowering the power and space requirements to implement the functions."

Indeed, as the trend towards putting additional channels on a line card advances, LCAS technology offers significantly more density. In the past, it was common to have 16 or 24-channels per card. Legerity's customers are developing cards with 48 and 64 channels. It even has one customer developing a 96-line channel card.

"The reason this is important is because not only is the board, connectors and baseplate, all the mechanics, common, but so is the processor, the interface and backplane. Instead of dividing that and sharing that cost over say 24 lines, when you divide it by 48 or 96 lines, you have the costs associated with that," Stibila said.

For a three-relay access network, the LCAS takes up about 25% of the board space. Three relays per line would take up 1.5-square inches, and the LCAS would house 25% of that area, Stibila said, adding that the designers gain quite a bit of space using semiconductor technology.

Foldback technology

By employing foldback technology, the Le75181 and Le75282 LCAS devices become self-protecting. When switches are off, they may see voltages in the circuit that are higher than the normal from lightening or power cross that comes on the line. Foldback technology allows the switch to fold back from an off-stay to a low-resistance on-stay so that the surge is passed over the switch into the protector, Stibila explained.

"What that does is it protects the device itself due to damage from foreign voltages," he said. "In the past, the circuitry would have to be separate, or different configurations would have to be used to protect the switches."

When the high frequency DSL signal carries the ADSL data payloads in integrated voice and data line cards, the peak signal reaches 40V, added Bert Baker, marketing manager of Legerity. "We had to not only improve the LCAS for this protection, but we also had to make it work with higher voltage protection to pass the ringing plus data signal," he said.

Both surface-mount devices are available in volume production. The single-channel Le75181 is pin compatible to older Agere-style LCAS devices. It is packaged in a 16-pin SOIC. The Le75282, the dual-channel device, is packaged in a 44-pin TQSP.

In 10,000-quantities, pricing for the Le75181 and Le75282 start at $2.15 and $5.30 per line, respectively.

- Ismini Scouras


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