Shrinking form factors and expanded feature sets are driving the integration of mixed-signal audio functions in mobile phones and other portables. The risk inherent in this trend is increased power consumption or the need to add external components such as regulators or passives. The introduction of audio hub devices simplifies this design puzzle.
The audio hub interfaces analogue signals of varying magnitudes, source impedances, DC offsets and bandwidths, such as FM receivers, microphones, send/receive voice data, ring tones or Hi-Fi line input, plus digital data in diverse formats. Flexible I/O and mixing configurations can support these diverse signal characteristics in different system architectures while minimising pin count, saving space and reducing costs. Of all the options to choose in designing the hub, the technology used in the speaker driver has the largest impact on overall efficiency.
Class AB speaker drivers typically waste more power in the device than they transmit to the speaker. A stereo Class AB speaker driver delivering 1W per channel at 40 per cent efficiency drains at least 5W from the battery, 3W dissipated as heat in the device. The total of all other audio-related power can be two orders of magnitude lower than this in some applications.
Class D speaker drivers maintain the audio quality of Class AB while increasing efficiency, extending battery life and simplifying thermal-management issues that can constrain device functionality and increase costs. In portable applications supporting multimedia functions, the loudspeakers can be active for a significant period, and Class D technology is highly effective at extending battery life.
--Robert Hatfield--
Robert Hatfield (robert.hatfield@wolfsonmicro.com) is an audio engineer for Wolfson Microelectronics plc. He earlier held engineering positions at Systolix, Zetex and Cypress Semiconductor U.K.
