Analog Devices revealed to technologists attending ISSCC a breakthrough new amplifier architecture that dramatically improves power efficiency in ADSL (asymmetrical digital subscriber line) line drivers. The Adaptive Linear Powerý (ALP) architecture anticipates the signal peaks and instantaneously raises the internal supply voltage when peak power is needed. The first implementation of this important advancement in high performance analog technology is the AD8393, the industry's first 575 mW, single-supply ADSL line driver. Using the ALP architecture, the AD8393 can achieve over 18 volts of signal on a single 12-volt supply.

Line driver power consumption currently accounts for over 60% of the total power per port in ADSL CO applications, limiting the overall port density of CO systems and contributing significantly to the cost of system thermal management. Higher port density helps ADSL providers to minimize equipment costs and optimize subscriber service, for significant cost savings. Compared to competing class AB line drivers, the ALP architecture can increase CO power efficiency by more than 50%, enabling increased port density. Furthermore, the single-supply capability of the ALP amplifier architecture enables system designers to eliminate as many as three analog supply voltages as compared to competing solutions, such as class G line drivers, thus reducing system cost and design complexity.

"Power consumption is the primary concern of ADSL providers looking to reduce costs by maximizing lines per board," said Mike Wolf, Director, Enterprise and Residential Communications at Cahner's In-Stat Group. "With its new Adaptive Linear Power architecture for line drivers, Analog Devices is offering direct savings to the ADSL provider's bottom line."

According to Lewis Counts, Vice President, Linear Products, Analog Semiconductor Components Division, Analog Devices, "Line drivers-and the power per port they consume-have long limited high port densities in ADSL central office applications. By applying our years of expertise in high performance analog design and DSL applications know-how garnered from our market-leading position, ADI has shown how the limitations of class AB ADSL-CO line drivers can be overcome without the need for additional supplies."

About the AD8393

The AD8393 ADSL CO line driver-the first in a family of products based on the ALP amplifier architecture-is designed for driving DMT (discrete multi-tone) signals onto a twisted pair line with a crest factor (peak voltage:rms voltage) range of 3.3 to 6.4, while offering the industry's lowest power dissipation and low distortion, while operating from only a single +12 V supply.

In the AD8393, Analog Devices' ALP technology permits +/-12 V performance on a single +12 V supply or +/-6 V supplies. The full power dissipation of the AD8393 for full rate ADSL is only 575 mW for non-overlapped applications (19.8 dBm line power) and only 624 mW for overlapped applications (20.4 dBm line power). The AD8393 is optimized for driving a 1:1.2 transformer; however, it has sufficient output current to drive up to a 1:2 transformer.

Analog Devices, Inc 804 Woburn Street, Wilmington, MA 01887. Tel: 800-ANALOGD (800-283-5643), Fax: 781-937-1021. http://www.analog.com/

It may take a few minutes to realize why it's worth looking at the AD8393 for your ADSL designs, but the closer you look the more interesting it gets. At first blush, you may notice that the design provides somewhat better efficiency than Class AB and Class G amplifiers. Okay, it's not bad at 22% improved efficiency but hardly something that will make you drop your current design for a new one. Looking a little closer, you'll notice that it uses only a single supply, instead of the four used in Class G (typically ý5V and ý15 supplies) or Class AB (typically two ý12V supplies). Analog Devices distinguishes its Adaptive Linear Power (ALP) chip from a Class AB with its power efficiency, and from the Class G by the number of power supplies. The result of dropping supplies is reduced system cost and reduced system complexity, which translates to cost savings.

Granted, there exist some Class G designs with two supplies. There also is a single supply Class AB design but it uses a 15V supply and has a turns ratio of nearly 2:1. To improve the efficiency, the turns ratio must be reduced. And, that's just what Analog Devices did, they reduced the turns ratio to 1:1.2.

The company expects the 8393 to be an important product for the ADSL market because it offers lower power dissipation and better efficiency than what's available now and it provides a pathway to even further reductions in power dissipation, possibly below 1/2W, as well as improved efficiency. And improved efficiency is something that Class AB probably will not be able to accomplish. Lower power dissipation of course, leads to better thermal management of the board, which enables companies to pack more channels on one board.

The reduced number of power supplies also simplifies the system design. For example, in an ADSL (Asymmetric Digital Subscriber Line) type system a DMT (Discrete Multitone Technology) modulation scheme is used, which presents significant power challenges to the system designer. The system designer is always trying to cram as many ports on the board as possible. In addition, designers are charged with reducing system cost and reducing system complexity while maintaining compliant levels of performance.

All Analog Devices line drivers have positive feedback - a necessity to squeeze every last bit of power out of the system. It samples the signal at the input, which is different from the Class G which triggers based on the output. You can vary your supply and base it on the output signal, which is like waiting for it to happen before varying the supply. Another way to do it is to look at it from the input so you will vary the supply before it reaches the output stage. The AD8393 chip pumps the signal supply up before it reaches the output stage.

When the crest appears, it goes through the 8393 circuitry and varies the internal supply modulated above Vcc and below ground, to provide the voltage clearance needed by the system. These crest factor signals are statistical in nature and a typical crest factor of 5.3 will appear about every 4 seconds, which isn't very often in relation to the RMS voltage. So, the Class AB is powering up to handle that crest all the time. The 8393 chip anticipates when the peak occurs and internally varies a supply based on some charting, to provide the headroom needed when a signal occurs.

The ALP type of circuit works very well on a DSL system - however, it's not geared toward systems that take sinusoids. The circuits can't respond to more than three or four peaks. But for a DSL system with a noise like a DMT wave form, this circuit works very well.

The AD8393 is available in 28-lead TSSOP (thin-shrink small outline packaging) or 32-lead 5x5 mm CSP (chip-scale packaging), vital for space efficiency in ADSL CO systems. The product is sampling now, and production is planned for second quarter 2002.

Preliminary data sheet and other information about the AD8393 DSL line driver.