Fully Digital PWM Controller Streamlines Class-D Audio Amps

Leveraging digital pulse-width modulation (PWM) technology obtained by acquiring AudioLogic last year, Cirrus Logic has crafted a fully digital PWM controller for audio power amplification. Designed to overcome the limitations of traditional class-D audio amplifiers, this digital PWM controller raises power efficiency to new heights. It also significantly curbs EMI and RFI normally encountered with existing class-D and hybrid analog/digital PWM technologies.

Based on a 32-bit fixed-point DSP, this controller promises to outperform existing class-D audio amplifiers in signal-to-noise ratio (SNR), total harmonic distortion (THD), and frequency response. Its unprecedented power-amplifier efficiency exceeds 90%, with a dynamic range of 120 dB. With this latest technology in its arsenal, Cirrus intends to serve all audio market segments, from headphone amplifiers to high-end professional speakers. Toward that goal, Cirrus' technology roadmap shows digital PWM amplifiers ranging from 1 W all the way up to 500 W.

The device was manufactured on a 0.35µm CMOS process. Cirrus initially plans to release low-power versions for headphone amplifiers and portable stereos in the first quarter of 2001. These include the 5-V, 1-W CS44L10 and the 16-V, 15-W CS44L11. Devices with higher power are expected to follow soon. Plans also call for migrating to 0.18µm CMOS technology before the end of 2001. Though the pinouts have been defined, the company is still nailing down the package for these devices.

"We have not only broken down the barriers in performance that are inherent in conventional PWM technology, we can surpass the performance of the best linear approaches as well," asserts Cirrus vice president Skip Taylor. "By providing ten-to-one savings in power dissipation, the size of the power amplifier can be reduced by a factor of four."

Yet Cirrus isn't alone. In March, Texas Instruments acquired Toccata Technology of Copenhagen, Denmark, to quickly migrate to an all-digital PWM solution for audio power amplifiers targeting next-generation digital speakers. Since then, TI has replaced the Toccata DSP used in the design with one of its own. The first fruit of this acquisition was revealed and demonstrated at last month's 109th Audio Engineering Society Convention in Los Angeles.

With this launch, Cirrus is developing complete reference designs using International Rectifier's PWM optimized power driver ICs and discrete power MOSFETs. The partners also have inked a joint marketing agreement, letting each supplier market a complete digital PWM audio power-amplifier system, including software, based on the new technology.

Reference Designs On The Way Reference designs for three power levels are in the works. Expected in the first quarter of next year, the first reference design and evaluation board is tailored for applications requiring less than 15 W. This will be followed by a 150-W design in the summer. A 500-W reference design and evaluation module is slated for next fall. Meanwhile, investigation is under way to combine the digital PWM controller, power driver, and discrete MOSFETs in a single package using chip-on-board assembly technologies.

A 32-bit 100-MIPS fixed-point DSP that controls the power-amplifier circuitry and handles audio-processing tasks lies at the heart of Cirrus' digital PWM technology (see the figure). The embedded DSP "sync locks" the switching power supply to eliminate the frequency fluctuations common to other amplifier designs. At the same time, the DSP controls the audio signal, providing such functions as equalization, gain control, and crossover separation.

The DSP also is adaptable and automated, continuously monitoring the amplifier environment and automatically adjusting parameters in real time to optimize performance and efficiency. For instance, it constantly monitors the amplifier's output load and controls both the signal and current simultaneously. This enables the amplifier to adjust to load changes in real time, providing a stable platform under widely varying load conditions typically encountered in speaker applications. The operating frequency is selectable from 350 to 500 kHz. To handle multiple audio sources, the built-in sample-rate converter allows varying sampling rates at the input and standard update rates at the output.

For more information, go to Cirrus Logic's web site at www.cirrus.com.