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Power Struggles: Reducing power consumption in mobile audio devices

Portable Media Players and multimedia handsets are becoming ever more capable, both in terms of the range and the complexity of functions offered to consumers. Notwithstanding the power demands placed on batteries by the widening array of functions on offer, consumers still expect continuous improvements in battery life from the next generation of devices, in addition to the increased diversity of functions and copious amounts of free memory.

Between 2002 and 2008, successive generations of leading Portable Media Players (PMPs) have delivered dramatic reductions in power usage. Average battery life has improved from less than 16 hours (assuming a 300mAh battery capacity) in 2002 to a massive 40 hours of continuous playback in 2008, when using the latest technology. Whilst this achievement is obviously attractive from a consumer perspective, allowing fewer recharges and the prospect of using smaller, cheaper batteries, the good news for manufacturers is that the best is yet to come in terms of reducing power consumption in audio codecs.

Overall, power consumption in the mixed-signal ICs used in portable devices have seen significant reductions being achieved roughly every eighteen months. This trend is similar to that of microprocessors following Moore's law. The result of this trend is that the last four generations of audio codecs in PMPs have seen an overall reduction in quiescent power consumption from around 27mW in 2002 to less than 5mW in 2008 with the launch of Wolfson's WM8903.

These reductions have been achieved by applying different techniques to the digital and analog domains. The core challenge for engineers seeking to reduce power consumption in audio codecs is that reductions in digital power consumption are mostly achieved by changes in process geometry whilst analog reductions usually occur as a result of design changes. However, any changes to reduce power consumption should be made in the context of maintaining the high signal-to-noise ratio (SNR) that consumers have come to expect.

In the digital arena, reductions in power consumption were achieved between 2002 and 2006 without any process changes. Power consumption of the digital core was reduced by clock gating and optimised Digital Signal Processing (DSP). Digital Input/Output power was reduced by reducing supply voltage, resulting in a 4x reduction.

In 2006, 0.18¼m processes became economically viable for mass-market audio codecs, paving the way for further improvements. This enabled digital gates and connections to be drawn smaller, which saves power by reducing capacitance. More significantly, the move to 0.18¼m technology enabled the digital supply voltage to be reduced to from 1.8v to 0.9v to deliver a further 4x reduction in power consumption. Figure 1 compares power consumption on 0.35¼m and 0.18¼m devices at different supply voltages. Overall, power consumption in the digital core was reduced by 90% (from 5mW in 2002 to 0.5mW in 2008) through a combination of the design, process and supply voltage changes described above.

The story in the analog domain is a little different. For analog signals, there is a clear trade-off between designing reductions in power consumption and maintaining high Signal-to-Noise Ratio. Analog solutions aren't scalable – reduce the size of the MOS device in the codec, and flicker noise increases; resistance increases, and thermal noise increases. And even reducing the supply voltage in the analog environment isn't necessarily the answer, as this has the effect of reducing signal swing, which has a negative effect on SNR. Figure 2 shows how achieving the same SNR from a lower supply voltage can increase power consumption. This is due to the need to increase bias currents to reduce thermal noise to compensate for the loss of signal swing.

In successive generations of audio codec since 2002, Wolfson has achieved a 77% reduction in analog power consumption. All major blocks are powered down when not in use, and we have minimised the number of amplifiers in each signal path, reducing the amount of power required and the amount of noise generated. Signal swing and noise are carefully selected to optimise the SNR vs. power consumption trade-off. Fine grain software control can be used to “gearshift” silently between high quality and ultra-low power modes. Figure 3 shows how analog power consumption of Wolfson codecs has been reduced from 11mW in 2002 to 2.5mW today.

Further reductions in the power requirement for codecs have been realised at the output stage. Most PMPs need to drive a 16 or 32 headphone load at a typical listening level of 0.1mW. As the chart shows, the extra power required to move from quiescent to 0.1mW has been reduced by around 50% since 2002.

Typically, class AB amplifiers have been used in PMP devices, but these deliver maximum efficiency only at maximum volume. At more typical listening levels efficiency is dramatically reduced. By using Class D and Class G amplifiers where appropriate, efficiency at typical listening levels can be greatly improved.

Whilst these are not new ideas (most recently, this class G has been applied to ADSL line drivers), it is by no means a simple solution. Although Class G amplifiers save power by reducing the supply voltage of a Class AB at low volume, there is still a requirement to switch power rails, which is technically complex and could cause audible artefacts. The latest Wolfson Codecs use new, proprietary class W technology, an evolution of class G and H topologies, to silently reconfigure the output amplifier to deliver optimal efficiency for the selected listening level.

Wolfson's WM8903 audio codec, available from June 2008, is the first product from their next generation of ultra low-power; high performance audio devices that utilise class W technology. The WM8903 also features SmartDAC and SilentSwitch. When combined, these innovations deliver world class, high performance audio and market leading power consumption, enabling portable device designers to take battery life to new levels. The proprietary new technology provides the foundation for Wolfson's next generation of ultra low power audio devices.

For more information on the WM8903, visit Wolfson’s website

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