This article describes the problems 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, as described in a previous article, http://www.powermanagementdesignline.com/showArticle.jhtml?printableArticle=true&articleId=188100789
Before outlining the problems, I'll describe a supercapacitor and its overall role in power management. 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 " 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.
Audio quality and power issues in current music-phone designs
Today's mobile phones typically use class D audio amplifiers. These use 2 pairs of FETs in an H-bridge to control the speaker coil. The configuration is shown in Fig 1. Q1 & Q4 ON with Q2 & Q3 OFF drive the speaker coil in one direction, while Q1 & Q4 OFF with Q2 & Q3 ON drive 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 8ω speaker the maximum audio power = 3.6V2/8ω = 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 which can suffer from power limitations, distortion and interference.
Problem 1: 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 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 100mA " 200mA.
Problem 2: 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 = 150mΩ, then 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.
Problem 3: Limited audio power and poor bass response in CDMA, GSM & 3G phones.
The audio capability and quality of all mobile phones, irrespective of their type, depends on the power output of the audio amplifiers and the impedance of the speakers. In a typical set-up where two class D amplifiers operate off a 3.6V power supply from the battery to drive a pair of 8Ω speakers, the maximum audio power is 3.2W, and peak battery current is 0.9A as outlined above. 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.
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