Audio amplifier having multiple sigma-delta modulators to drive an output load

1. An audio amplifier comprising:

a first clock generator configured to generate a first clock signal defining a first sampling rate;

a second clock generator configured to generate a second clock signal defining a second sampling rate greater than the first sampling rate;

a first sigma-delta modulator configured to receive a digital audio signal and the first clock signal, and configured to generate a first multi-level output signal according to the first clock signal; and

a second sigma-delta modulator configured to receive the first multi-level output signal from the first sigma-delta modulator and the second clock signal, and configured to generate a second multi-level output signal according to the second clock signal, the second multi-level output signal having a number of levels less than a number of levels of the first multi-level output signal, the number of levels of the second multi-level output signal being two or three.

2. The audio amplifier of claim 1 , wherein the number of levels of the first multi-level output is at least three times more than the number of levels of the second multi-level output.

3. The audio amplifier of claim 1 , wherein the number of levels of the first multi-level output is greater than four.

4. The audio amplifier of claim 1 , wherein the second sampling rate is at least four times greater than the first sampling rate.

5. The audio amplifier of claim 1 , further comprising:

a third sigma-delta modulator coupled between the first sigma-delta modulator and the second sigma-delta modulator.

6. The audio amplifier of claim 1 , further comprising:

an H-bridge circuit configured to be driven by the second multi-level output signal, wherein the H-bridge circuit is configured to drive an impedance load of less than fifty ohms.

7. The audio amplifier of claim 6 , wherein the H-bridge circuit is configured to be coupled to one or more speakers having an impedance load of less than ten ohms.

8. The audio amplifier of claim 1 , wherein the first sigma-delta modulator is an N-order sigma-delta modulator, and N is greater or equal to two.

9. The audio amplifier of claim 8 , wherein the second sigma-delta modulator is a first order sigma-delta modulator.

10. The audio amplifier of claim 1 , wherein the audio amplifier is a class-D amplifier.

11. A device comprising:

an audio amplifier configured to be coupled to one or more speakers and a battery, the audio amplifier including:

a first sigma-delta modulator configured to receive a digital audio signal and generate a first multi-level output signal based on the audio signal according to a first sampling rate, the first sigma-delta modulator having an order greater or equal to two;

a second sigma-delta modulator configured to receive the first multi-level output signal from the first sigma-delta modulator and generate a second multi-level output signal according to a second sampling rate, the second sampling rate being greater than the first sampling rate, the second multi-level output signal having a number of levels less than a number of levels of the first multi-level output signal, the second sigma-delta modulator being a first order sigma-delta modulator, the number of levels of the second multi-level output signal being two or three; and

an H-bridge circuit configured to be driven by the second multi-level output signal and generate an analog output signal to the one or more speakers,

wherein a time in which the audio amplifier applies a voltage of the battery to the one or more speakers is less than one percent of a total battery time.

12. The device of claim 11 , wherein the one or more speakers are coupled to the H-bridge circuit, the one or more speakers having an impedance equal to or less than ten ohms.

13. The device of claim 11 , further comprising:

an application specific integrated circuit (ASIC), the ASIC including the audio amplifier.

14. The device of claim 13 , wherein the ASIC further includes one or more voltage regulators, one or more clock generators, and one or more analog to digital converters.

15. A method for modulating an audio signal to drive one or more speakers, the method comprising:

receiving, by a first sigma-delta modulator of an audio amplifier, a digital audio signal;

generating, by the first sigma-delta modulator, a first multi-level output signal based on the digital audio signal; and

generating, by a second sigma-delta modulator of the audio amplifier, a second multi-level output signal from the first multi-level output signal, the second multi-level output signal having a number of levels less than a number of levels of the first multi-level output signal, the number of levels of the second multi-level output signal being two or three; and

switching a polarity of a voltage applied to one or more speakers according to the second multi-level output signal.

16. The method of claim 15 , further comprising:

generating, by a first clock generator, a first clock signal defining a first sampling rate;

generating, by a second clock generator, a second clock signal defining a second sampling rate;

receiving, by the first sigma-delta modulator, the first clock signal from the first clock generator; and

receiving, by the second sigma-delta modulator, the second clock signal from the second clock generator.

17. The method of claim 15 , further comprising:

applying, by the audio amplifier, a voltage of a battery to the one or more speakers during a period of time that is less one percent of a total battery time.

18. The method of claim 16 , wherein the first sigma-delta modulator is an N-order sigma-delta modulator, and N is greater or equal to two, wherein the second sigma-delta modulator is a first order sigma-delta modulator, wherein the second sampling rate is greater than the first sampling rate, wherein the one or more speakers has an impedance less than fifty ohms.