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An energy-efficient consumer device audio power output stage provides improved battery life and reduced power dissipation. A power supply having a selectable operating mode supplies the power supply rails to the power amplified output stage. The operating mode is controlled in conformity with the au

An energy-efficient consumer device audio power output stage provides improved battery life and reduced power dissipation. A power supply having a selectable operating mode supplies the power supply rails to the power amplified output stage. The operating mode is controlled in conformity with the audio signal level, which may be determined from a volume control setting of the device and/or from a signal level detector that determines the amplitude of the signal being amplified. The power supply may be a charge pump in which the operating mode uses a capacitive divider to provide for selection of a power supply output voltage that is a rational fraction of the power supply output voltage in a full-voltage operating mode.

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1. An audio amplifier circuit for providing an output signal to an audio transducer, said audio amplifier circuit comprising: a power amplifier having an audio input for receiving an audio input signal, an audio output for providing said output signal, and a pair of power supply rail connections sub

1. An audio amplifier circuit for providing an output signal to an audio transducer, said audio amplifier circuit comprising: a power amplifier having an audio input for receiving an audio input signal, an audio output for providing said output signal, and a pair of power supply rail connections substantially symmetric about a reference potential, forming a bi-polar power supply input; anda charge pump power supply for providing a bi-polar power supply voltage across said pair of power supply rail connections of said power amplifier, wherein said charge pump power supply has a select input for selecting an operating mode of said power supply, wherein in a first operating mode, said bi-polar power supply voltage is equal to a first voltage, and wherein in a second operating mode said bi-polar power supply voltage is substantially equal to a rational fraction of said first voltage, wherein in both of the first operating mode and the second operating mode, the bipolar power supply voltage is substantially symmetric about the reference potential, and wherein in the second operating mode a first flyback capacitor and a second flyback capacitor are coupled in series during a first charging phase of the charge pump power supply, and separately provide charge to corresponding ones of the pair of power supply rail connections during a second discharging phase of the charge pump power supply. 2. The audio amplifier circuit of claim 1, wherein in said second operating mode said power supply voltage is substantially equal to half of said first voltage. 3. The audio amplifier circuit of claim 1, wherein said charge pump comprises a plurality of capacitors and a switching circuit for switching said capacitors alternatively from a series to a parallel combination, and wherein in response to said select input, a pattern of switching of said switching circuit is altered, whereby a rational relationship between said power supply voltage in said first operating mode and said power supply voltage in second operating mode is provided. 4. The audio amplifier circuit of claim 1, wherein said charge pump power supply includes a switched-capacitor voltage divider formed by the first flyback capacitor and the second flyback capacitor that uses the series-coupling of the first and second flyback capacitors in the first charging phase of said second operating mode to produce said rational fraction of said first voltage. 5. The audio amplifier circuit of claim 1, further comprising a first amplifier stage circuit having an output coupled to said audio input of said power amplifier, wherein said first amplifier stage has a pair of first stage power supply rail connections connected to a fixed voltage power supply, wherein a fixed voltage of said fixed voltage power supply does not change substantially with selection of said operating mode of said power supply. 6. The audio amplifier circuit of claim 1, further comprising a control circuit for selecting said operating mode in conformity with a volume setting of said audio amplifier circuit. 7. The audio amplifier circuit of claim 6, wherein said audio amplifier circuit further comprises a potentiometer coupled to said audio input for controlling a magnitude of said output signal, and wherein said control circuit selects said magnitude in conformity with a control level of said potentiometer. 8. The audio amplifier of claim 7, wherein said potentiometer is a digitally controlled potentiometer, and wherein said control level is determined from a digital control value supplied to said potentiometer. 9. The audio amplifier circuit of claim 1, further comprising a control circuit for providing a selection signal to said select input in conformity with an amplitude of said audio input, whereby clipping of said power amplifier is prevented when said second operating mode of said charge pump power supply is selected by instead selecting said first operating mode. 10. The audio amplifier circuit of claim 9, wherein said control circuit generates an indication of an amplitude of said audio input, and wherein said control circuit provides said selection signal to said select input in conformity with said indication of said amplitude of said audio input, whereby said first operating mode is selected when said indication of amplitude indicates that a higher power level is required from said audio output and said second operating mode is selected when said indication of amplitude indicates that a higher power level is not required from said audio output. 11. The audio amplifier circuit of claim 10, wherein said control circuit comprises a signal level detector for providing said indication of said amplitude by detecting an amplitude of a signal provided to said audio amplifier. 12. The audio amplifier circuit of claim 11, wherein said audio input is provided from a digital-to-analog converter, and wherein said signal level detector is a digital circuit that compares a digital signal representation coupled to an input of said digital-to-analog converter to a threshold value. 13. The audio amplifier circuit of claim 1, wherein said charge pump power supply comprises a first switching circuit for alternatively coupling said second flyback capacitor in said first operating mode between a positive one of said power supply input terminals and a negative one of said pair of power supply rail connections in first phase and between said pair of power supply input terminals in a second phase, and wherein when said charge pump power supply is in said second operating mode, said first switching circuit couples said positive one of said power supply input terminals to said positive one of said pair of power supply rail connections. 14. The audio amplifier circuit of claim 13, wherein said charge pump power supply further comprises a second switching circuit coupled to said first flyback capacitor for, in said second operating mode, alternatively coupling a first terminal of said first flyback capacitor to positive one of said pair of power supply rail connections and a second terminal of said first flyback capacitor to a second terminal of said second flyback capacitor in said first phase, and coupling said first terminal of said second flyback capacitor to said negative one of said pair of power supply input terminals in said second phase. 15. The audio amplifier circuit of claim 14, wherein said first terminal of said second flyback capacitor is connected to a first terminal of said first flyback capacitor. 16. A method of improving efficiency of an amplifier for providing a power output to an audio transducer, said method comprising: receiving an indication of a magnitude of a signal to be amplified by said amplifier;selecting an operating mode of a charge pump power supply that provides a bi-polar power supply output symmetric about a reference potential; andsupplying a bi-polar power supply voltage to said amplifier from said charge pump in conformity with said selected operating mode, wherein in a first operating mode of said charge pump power supply, said power supply voltage is equal to a first voltage and in a second operating mode of said charge pump power supply, said power supply voltage is substantially equal to a rational fraction of said first voltage, wherein in both of the first operating mode and the second operating mode, the bipolar power supply voltage is substantially symmetric about the reference potential, and wherein in the second operating mode a first flyback capacitor and a second flyback capacitor are coupled in series during a first charging phase of the charge pump power supply, and separately provide charge to corresponding ones of a pair of power supply rail connections providing the bipolar power supply output, during a second discharging phase of the charge pump power supply. 17. The method of claim 16, wherein said receiving receives a volume control level corresponding to a gain of a signal path of said signal and wherein said selecting is performed in conformity with a magnitude of said volume control level. 18. The method of claim 16, wherein said receiving receives an indication of amplitude of said signal, and wherein said selecting is performed in conformity with said indication of magnitude. 19. A charge-pump power supply circuit, comprising: a pair of power supply input terminals;a pair of power supply output terminals forming a bi-polar output of said charge-pump power supply circuit that is substantially symmetric about a reference potential;a first flyback capacitor;a second flyback capacitor; anda switching circuit for alternatively coupling said second flyback capacitor in a first operating mode between a positive one of said power supply input terminals and a negative one of said power supply input terminals connections in first charging phase and between said negative only of the power supply input terminals and a negative one of said power supply output terminals in a second discharging phase, and wherein when said charge pump power supply is in said first operating mode, said switching circuit couples said positive one of said power supply input terminals to said positive one of said pair of power supply output terminals, and wherein in a second operating mode the switching circuit couples the first flyback capacitor and the second flyback capacitor in series during the first charging phase of the charge pump power supply, and separately provide charge to corresponding ones of the pair of power supply output terminals during the second discharging phase of the charge pump power supply. 20. The power supply circuit of claim 19, wherein said first terminal of said second flyback capacitor is connected to a first terminal of said first flyback capacitor. 21. The power supply circuit of claim 19, wherein said switching circuit comprises: a first switch responsive to selection of said first operating mode, wherein said first switch is connected between said positive one of said power supply output terminals and said positive one of said power supply input terminals;a second switch responsive in the first charging phase when said first operating mode is selected, wherein said second switch connects said positive one of said power supply input terminals to a first terminal of said second flyback capacitor;a third switch responsive in said first charging phase when said first operating mode is selected, wherein said third switch connects said positive one of said power supply input terminals to a second terminal of said second flyback capacitor;a fourth switch responsive in the second discharging phase when said first operating mode is selected, wherein said fourth switch connects said negative one of said power supply input terminals to a first terminal of said second flyback capacitor; anda fifth switch responsive in said second phase when said second operating mode is selected, wherein said fifth switch connects said negative one of said power supply output terminals to a second terminal of said second flyback capacitor. 22. The power supply circuit of claim 21, further comprising: a sixth switch responsive in said second operating mode in said first charging phase, wherein said sixth switch connects a positive one of said power supply input terminals to a second terminal of said first flyback capacitor; anda seventh switch responsive in the second operating mode to the second discharging phase, wherein said seventh switch connects said negative one of said power supply input terminals to said second terminal of said second flyback capacitor. 23. The power supply circuit of claim 21, further comprising an eighth switch responsive in said first operating mode to the first charging phase, wherein said eighth switch connects said positive one of said power supply output terminals to a second terminal of said first flyback capacitor.