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Here come supercapacitors

Posted: 03 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:supercapacitor? cellphone power audio? Class D audio amplifiers?

As multimedia and music phones become more popular, consumers want an iPod-quality audio experience without the buzzing and distortion associated with wireless transmissions. This article describes the problems in delivering high power and high-quality audio in music-enabled mobile phones, and how a supercapacitor can overcome them.

This same supercapacitor can also enable high-power LED flash photography without compromising the handset's thin profile.

Supercapacitors bridge the power gap between batteries and conventional capacitors, delivering higher power bursts than batteries and storing more energy than capacitors. Supercapacitors provide the power bursts needed for peak power events (GSM/GPRS RF transmission bursts, GPS readings, music, flash photos and video) and then recharge from a battery.

Benefits include improved talk time, longer battery life, a brighter flash and better audio. Designers also save space and cost because they can size the battery and power circuitry to cover average power consumption rather than peak loads.

Today's mobile phones typically use Class D audio amplifiers (Figure 1). These use two pairs of FETs in an H-bridge to control the speaker coil. Q1 and Q4 on with Q2 and Q3 off, drives the speaker coil in one direction, while Q1 and Q4 off with Q2 and Q3 on, drives the coil in the opposite direction. The power supply for this arrangement is typically the battery, which is ~3.6V. A mobile phone with stereo audio will have a pair of amplifiers and speakers. For an 8speaker, the maximum audio power = 3.6V?/8= 1.6W or 3.2W for a stereo pair. The battery current for peak stereo audio power = 3.2W/3.6V = 0.9A. This arrangement results in an audio playback capability that can suffer from power limitations, distortion and interference.

The battery is unable to supply the simultaneous peak power demands of wireless data transmissions and the audio amplifier, resulting in distortion.In a GSM/GPRS/Edge phone, the battery will not be able to supply both the peak audio current and the peak RF transmit power for a response to a network poll while the user is listening to music. Networks periodically poll a mobile phone to keep track of which cell it's in, and to determine the transmit power that the phone should use. During such a network poll, the audio amp supply may droop as the phone responds, which sounds as a "click" to the user. The battery, however, is easily capable of supplying the average audio current which is approximately 100-200mA.

Shown is the typical configuration for Class D amplifier.

Audio noise/buzzing results from peak battery currents in excess of 1A, which cause significant ripple in the audio amplifier supply voltage.If the battery pack + connector + PCB trace total impedance = 150m, a 1A peak will result in a 150mV ripple in the supply voltage, while a 1.8A peak will cause a 270mV ripple. The user hears this ripple in the supply voltage as noise in the audio. GSM/GPRS/Edge transmissions with peak currents of 1.8A will also cause audio noise, which the user may hear as a 217Hz buzz during a phone call.

There is limited audio power and poor bass response in CDMA, GSM and 3G phones.The audio capability and quality of all mobile phonesirrespective of their typedepends on the power output of the audio amplifiers and the impedance of the speakers. In a typical setup where two Class D amplifiers operate off a 3.6V power supply from the battery to drive a pair of 8 speakers, the maximum audio power is 3.2W and peak battery current is 0.9A. The result is thin, low-power audio performance, with a very limited bass response, whether delivered through the phone's internal speaker or through externally attached speakers/headphones.

Problem solver
Figure 2 shows an alternative Class D amp configuration using a supercapacitor, which solves all issues outlined and quadruples the peak audio power. A 0.55F, 85mdual-cell supercapacitor such as the CAP-XX HS206 provides the peak power, while the battery supplies the average power. The boost converter charges the supercapacitor to 5V. Here are the results:

  • Peak power for a stereo phone increases to 2 x 5V?/8= 6.25W or approximately double the power of the previous case. Moreover, because the supercapacitor can supply very high peak currents, designers can use 4 speakers, increasing peak audio power to 12.5W or 4x the original power.

  • A 0.55F, 85m supercapacitor will have only a 200mV ripple after supplying peak power of 12.5W for 10ms with a peak battery contribution of 1.8W (0.5A at 3.6V).

  • The battery, which now only supplies an average audio current of 150-300mA to recharge the supercapacitor, can also supply peak RF power for a network poll response without compromising audio power, so there are no "clicks" while responding to network polls.

Figure 2: Shown is a Class D amplifier configuration with a supercapacitor. Shown is a Class D amplifier configuration with a supercapacitor.

  • Furthermore, any ripple at the battery voltage due to RF transmission is not reflected at the audio amp. This ripple is filtered by the line regulation of the boost converter and the supercapacitor, eliminating any 217Hz buzz.

- Pierre Mars
VP of Applications Engineering

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