Electronic Design

Direct Digital Amplification Improves Efficiency And Eliminates Heatsinks

Direct Digital Amplification (DDX) boosts digital audio signals without converting them to analog form. This new technique also improves power utilization, eliminating the need for large heatsinks. Employed in the DDX-2000 Controller and DDX-2060 Power Device ICs from Apogee Technology Inc., Norwood, Mass., the technology is ideal for digital audio applications such as CD-ROM, DVD, and MP3 players (see the figure).

Typical analog designs have poor power efficiency because of the difference in voltage between the amplifier's output and power supply. As a result, the active components consume a lot of power. By continuously controlling the supply voltage, power efficiency peaks at about 50%. De-vices that use this design re-quire large power supplies and heat-sinks.

Class D switching amplifiers overcome this problem by discretely connecting the load to the power supply, yielding a higher efficiency. But Class D is still considered an analog solution because it uses an analog interface and control. Apogee's new Class D amplifier uses digital control, though.

This digital approach eliminates the need for a high-quality digital-to-analog converter (DAC) while increasing efficiency by a factor of three. Users can take audio information directly from a digital source such as a CD-ROM or disk file and deliver it to the DDX amplifier. Digital format conversion may be required, but this can be a lossless process.

DDX technology can be utilized in standalone amplifiers and in digital connections such as SPDIF or IEEE-1394 interfaces. Since Apogee's reference designs are compact, designers can incorporate them directly into devices like CD-ROM players. Other applications include digitally powered speakers, PC sound cards, automobile audio, surroundsound systems, and digital audio components.

The technology has other advantages over analog designs, too. Conventional Class D amplifiers use binary operation, which consists of positive power and negative power switching states. But DDX uses damped ternary—or three-state—modulation, which includes positive, negative, and a zero power switching state. These states are implemented via signal processing, enabling improvements and adjustments via software. DDX also reduces power requirements by 20% compared to conventional two-state digital amplifiers, and by 300% versus Class D analog amplifiers.

The DDX-2000/2060 combo provides two channels up to 30 W into an 8-Ω load, or a single channel with 60 W into a 4-Ω load. Specifically, the DDX-2000 Controller is a 3.3-V, 44-pin QFP digital IC that implements the DDX processing. It includes digital volume control, automatic mute, and anticlipping functionality to simplify product design. The DDX-2060 includes a power-down mode and short-circuit, overvoltage, and thermal protection circuitry with automatic recovery.

Commercially available, the DDX-2000/2060 chip set costs $6.98 per set in 1000-piece quantities. Reference designs are available from Apogee Technology as well. For more information, visit the company's Web site at www.apogeeddx.com.

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