Low-power audio codec incorporates charge pump for Class G headphone amplifier
By Paul Rako, Technical Editor -- EDN, June 17, 2008
recently announced the low-power WM8903 audio codec for handheld consumer electronics. Wolfson varies the power-supply voltage to the headphone amplifier for Class G operation for a 30% increase in battery life. Furthermore, the DAC employs a capacitor-switching architecture that allows system designers to trade off audio performance for power consumption. Wolfson has also provided an output-clamp circuit that minimizes clicks and pops. The chip integrates a dc servo to control offset voltages through the signal path. The device features a high PSRR (power-supply-rejection ratio) so that you can operate the audio system from a switching power supply. The unit provides a dynamic-range controller that improves the quality and intelligibility of recorded sound. Applications include portable media players, multimedia phones, and digital videocameras.
You cannot use headphone amplifiers in a bridge configuration because not all four stereo-speaker leads are available. The stereo plugs tie two of the speaker leads to a common, meaning that single nonbridged amplifiers must drive consumer headphones, thus requiring either an output-coupling capacitor, which allows a single-supply voltage, or a charge pump to create a negative voltage rail. Wolfson uses a charge pump but also modulates the output voltage to lower the power amplifier’s supply voltage when the audio is at low volume. The industry refers to this approach as Class G operation, but Wolfson dubs it Class W. To properly evaluate the power savings the chip offers, Wolfson points out, simplistic testing using sine waves does not fully demonstrate the device’s real power-saving potential. JEITA (Japan Electronics and Information Technology Industries)tests use a sine wave, but the more sophisticated IEC (International Electrotechnical Commission)-60268-5 test uses filtered pink noise, which better replicates an audio signal and allows system engineers to better explore the true power-saving potential of the chip.
The WM8903 comes in a 40-pin, 5×5×0.55-mm QFN package. The operating-temperature range is −25 to +85°C, and price is $1.80 (10,000). The part is available for sampling now, with production slated for September 2008.
