Amplifier circuit

1. An audio amplifier circuit, comprising:

a first input, for receiving an audio input signal to be amplified;

an amplifier, for amplifying the audio input signal;

a clock generator for generating a clock signal having a frequency that varies in accordance with a received volume control signal such that a first value of the volume control signal results in a first non-zero clock frequency and a second, different value of volume control signal results in a second, different non-zero clock frequency; and

a charge pump comprising a second input for receiving the clock signal, said charge pump supplying at least one supply voltage to the amplifier, wherein the charge pump comprises a plurality of switches and is configured such that the clock signal is used for clocking said switches.

2. An audio amplifier circuit as claimed in claim 1 , wherein the circuit amplifies the audio input signal by a variable gain, said variable gain varying in accordance with the volume signal.

3. An audio amplifier circuit as claimed in claim 2 , wherein said clock signal frequency varies in accordance with the variable gain.

4. An audio amplifier circuit as claimed in claim 2 , wherein said charge pump comprises:

an input terminal for receiving an input voltage;

first and second output terminals;

first and second flying capacitor terminals;

a switch network for interconnecting said input terminal, said first and second flying capacitor terminals and said first and second output terminals to generate either a first positive output voltage and a first negative output voltage that are each substantially equal in magnitude to the magnitude of the input voltage or a second positive output voltage and a second negative output voltage that are each substantially equal in magnitude to half the magnitude of said input voltage.

5. An audio amplifier circuit as claimed in claim 1 , wherein said charge pump comprises:

an input terminal for receiving an input voltage;

first and second output terminals;

first and second flying capacitor terminals;

a switch network for interconnecting said input terminal, said first and second flying capacitor terminals and said first and second output terminals to generate a positive and negative output voltage that are each substantially equal in magnitude to half the magnitude of said input voltage.

6. An audio system, comprising an circuit as claimed in claim 1 , wherein the audio system is at least one of an MP3 player, a mobile phone, a camera, a satellite navigation system, a portable device, a mains-powered device, an in-car, in-train or in-plane entertainment system.

7. A method of amplifying an audio signal, comprising:

receiving an input signal;

supplying at least one supply voltage from a charge pump to an amplifier, wherein said charge pump comprises a plurality of switches;

amplifying the audio input signal in the amplifier,

generating a clock signal with a frequency that varies in accordance with a volume control signal such that a first value of the volume control signal results in a first non-zero clock frequency and a second, different value of volume control signal results in a second, different non-zero clock frequency; and

using said clock signal to control clocking of said switches of the charge pump.

8. An audio amplifier circuit, comprising:

a first input, for receiving an audio input signal to be amplified;

an amplifier for amplifying the audio input signal;

a clock generator for generating a clock signal having a frequency that varies in accordance with the envelope of the audio input signal such that a first envelope value of the audio input signal results in a first non-zero clock frequency and a second, different envelope value of the audio input signal results in a second, different non-zero clock frequently; and

a charge pump having a second input for receiving said clock signal, said charge pump supplying at least one supply voltage to the power amplifier, wherein the charge pump comprises a plurality of switches and is configured such that the clock signal is used for clocking said switches.

9. An audio amplifier circuit as claimed in claim 8 , further comprising:

an envelope detector for detecting the envelope of the audio input signal.

10. An audio amplifier circuit as claimed in claim 9 , wherein the envelope detector is further adapted to output a control signal in accordance with the detected audio input signal envelope, and wherein the clock generator receives said control signal.

11. An audio amplifier circuit as claimed in claim 9 wherein the clock signal frequency further varies in accordance with the value of a volume control signal.

12. An audio amplifier circuit as claimed in claim 11 , wherein the input signal is modified based on the volume signal prior to the envelope detector detecting the envelope of the input signal.

13. An audio amplifier circuit as claimed in claim 11 , wherein the envelope detector is adapted to adjust said control signal based on the volume control signal.

14. An audio amplifier circuit as claimed in claim 11 , wherein the control signal is modified based on the volume control signal after the control signal is output from the envelope detector.

15. An audio amplifier circuit as claimed in claim 11 , comprising an amplifier configured to amplify the input signal by a variable gain, said variable gain varying in accordance with the volume control signal.

16. An audio amplifier circuit as claimed in claim 15 , wherein said clock signal frequency varies in accordance with the variable gain.

17. An audio amplifier circuit as claimed in claim 8 , wherein said charge pump comprises:

an input terminal for receiving an input voltage;

first and second output terminals;

first and second flying capacitor terminals;

a switch network for interconnecting said input terminal, said first and second flying capacitor terminals and said first and second output terminals to generate a positive and negative output voltage that are each substantially equal in magnitude to half the magnitude of said input voltage.

18. An audio amplifier circuit as claimed in claim 8 , wherein said charge pump comprises:

an input terminal for receiving an input voltage;

first and second output terminals;

first and second flying capacitor terminals;

a switch network for interconnecting said input terminal, said first and second flying capacitor terminals and said first and second output terminals to generate either a first positive output voltage and a first negative output voltage that are each substantially equal in magnitude to the magnitude of the input voltage or a second positive output voltage and a second negative output voltage that are each substantially equal in magnitude to half the magnitude of said input voltage.

19. An audio amplifier circuit as claimed in claim 8 , wherein said at least one supply voltage is supplied to the power amplifier via a respective capacitor.

20. An audio system, comprising an audio amplifier as claimed in claim 8 , wherein the audio system is at least one of an MP3 player, a mobile phone, a camera, a satellite navigation system, a portable device, a mains-powered device, an in-car, in-train or in-plane entertainment system.

21. A method of amplifying an audio signal, comprising:

receiving an audio input signal;

supplying at least one supply voltage from a charge pump to an amplifier, wherein said charge pump comprises a plurality of switches;

amplifying the audio input signal in the amplifier,

generating a clock signal with a frequency that varies in accordance with the envelope of the audio input signal such that a first envelope value of the audio input signal results in a first non-zero clock frequency and a second, different envelope value of the audio input signal results in a second, different non-zero clock frequency; and

using said clock signal to control clocking of said switches of the charge pump.

22. A method as claimed in claim 21 , further comprising:

amplifying the audio input signal based on a volume control signal; wherein the frequency of said clock signal further varies with said volume control signal.

23. A method as claimed in claim 21 , further comprising:

detecting the envelope of the audio input signal;

wherein the frequency of said clock signal varies with the detected envelope of the audio input signal.

24. A method as claimed in claim 23 , further comprising:

outputting a control signal indicative of the detected audio input signal envelope.

25. A method as claimed in claim 24 , wherein the audio input signal is modified based on a volume control signal prior to detecting the envelope of the audio input signal.

26. A method as claimed in claim 24 , wherein the control signal is modified based on a volume control signal after the control signal is output from the envelope detector.

27. An audio amplifier circuit comprising a charge pump comprising a plurality of switches, wherein the clocking of said switches is controlled by a variable frequency clock signal such that the switching frequency of the charge pump can vary in use between at least a first non-zero switching frequency and a second, different non-zero switching frequency, wherein the frequency of said variable frequency clock signal reduces when at least one of an audio input single level and a volume control reduces in order to reduce switching power losses.

28. An audio amplifier circuit as claimed in claim 27 , wherein the frequency of said variable frequency clock signal varies in accordance with an audio input signal.

29. An audio amplifier circuit as claimed in claim 27 , wherein the frequency of said variable frequency clock signal varies in accordance with a volume control signal.