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Audio codec extends battery life in media portables

Wolfson Microelectronics designed its WM8903 ultra-low power CODEC for portable audio applications, such as portable media players, multimedia-enables cellular phone handsets and portable game terminals. Many of the performance upgrades result, according to head of new product architecture Nick Roche, from the transition to a new fabrication process. This is the first such device the company has built in 180-nm silicon, on what Roche describes as a fullycharacterised mixed-signal platform in standard CMOS. Noting that the processor power that’s necessary to decode a compressed audio file has decreased significantly in recent times, Wolfson has reduced the quiescent current of the audio codec, and also the power it requires to deliver typical headphone drive levels in the range 0.1 – 1.0 mW. The 8903 uses Wolfson’s SmartDAC switched-capacitor-architecture D/A converter that allows designers to tailor performance/power profiles for different use-models, such as docked or mobile operation. In mobile media players you could see a 30% increase in battery life on playback. Mobile phone specifications most often quote battery life in terms of their calling function – in that platform, using the new codec could increase the playback battery life by as much as 400%, Wolfson claims. Features of the chip include a headphone amplifier design that the company calls “class W”. This is an evolution of class G/H (which uses charge pumps that adapt the amplifier power rails to the music signal level, to optimise the efficiency of the output stage), extended with a balanced configuration that eliminates output capacitors. With very high power-supply noise rejection, the chip can operate from 1.8V on both analogue and digital rails, reducing the need for power-line filtering. The charge pumps employ variable clock rates, to maintain efficiency at low signal levels. Clamping and sequencing on the signal and power rails implements SilentSwitch clickand pop-suppression during power-up and signal-source switching: dc offset on the output is as low as 2 mV. All blocks on the chip are under control of a sequencer that you can run in a default mode, or that you program for your own needs. The chip has inputs for both traditional and the newer silicon microphones, in analogue or digital (PDM, pulse-density-modulated) form.

Particularly for use in record mode, the chip has dynamic range control that product architect Eric Haber describes as a superset of automatic level control, limiter, and compressor; it acts, he says, as a signal range compressor but with variable and programmable attack and decay time-constants. With this, you can maintain the intelligibility of recorded sound in the presence of loud noise transients–passing the so-called hand-clap test. The complete signal chain through the device is differential, yielding a 98 dB signal/noise ratio. DACto- headphone power consumption is 4.4 mW. The chip comes in a 40-pin ultra-thin QFN package measuring 5X5X0.55 mm and costs $1.80 (10,000).

Wolfson Microelectronics, www.wolfsonmicro.com