Pump up the volume! Enhance audio quality and power in mobile phones and portable media players

Intro
When it comes to delivering high-power and high-quality audio in portable devices, some inherent problems exist such as weak bass, distortion, flat-sounding music, etc. This article discusses a power architecture (BriteSound) that uses a thin, prismatic supercapacitor combined with a Class-D audio amplifier with an integrated boost converter to deliver five-watt power bursts to offload peak-power functions from the battery. The integrated boost converter effectively manages supercapacitor inrush current at power on. This combination of components " a prismatic supercapacitor and a Class-D audio amplifier with an integrated boost converter - increases peak audio power while saving the designer both board space and component cost. It can also multi-task to enable high-power LED flash photography without compromising a mobile handset's thin profile.

This paper builds on a previously published article that also describes a BriteSound power architecture solution, but in this case a prismatic supercapacitor powered a generic Class-D audio amplifier, Supercapacitors enhance audio quality and power in mobile phones. [1]

Audio quality problems in portable audio devices
Consumers are demanding ever more high performance from battery-powered portable audio devices. Mobile phones, portable media players (PMP) and personal navigations devices are typical applications where the audio output needs to be loud, clear and high quality. These devices, powered by a Lithium-Ion (Li-Ion) battery, typically power a Class-D audio amplifier directly across the battery, usually at ~3.7V. Figure 1 shows this typical case which limits peak power for an 8 ohm speaker to 3.7V²/8 ohm = 1.7W, or 3.4W for a stereo pair. The battery current for peak stereo audio power = 3.4W/3.7V = 0.92A. This arrangement results in an audio playback capability which can suffer from power limitations, distortion and interference.

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Figure 1: Typical configuration for Class-D amplifier

An alternative arrangement is for a PMP to use one or more AA or AAA batteries in series to power a boost or buck-boost converter, which supplies the audio amp at 5V as shown in Figure 2.

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Figure 2: Class-D amplifier with boost converter

In this case, peak power is limited by the impedance of the alkaline batteries and the current they can deliver, as well as by the peak inductor current the boost converter can handle. As an example, if a PMP is powered by two AA batteries, then the internal impedance of these two batteries in series is approximately 500m ohm. Therefore, if the PMP attempts to deliver 3.4W of power as shown in Figure 2, the unloaded battery voltage equals 3V. Assuming a boost converter efficiency of 90 percent, the battery current can be calculated as:

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where Vbatt(unloaded) = 3V, LoadPower = 3.4W, efficiency = 90%, Rbatt = 0.5 ohm.

The battery current equals 1.8A. With 30% ripple, the peak inductor current will be 2.1A. However, this is not practical to implement. The inductor is too large, and battery voltage will drop to 2.1V.