Class D switching amplifier and method of controlling a loudspeaker

1. A Class-D amplifier comprising:

a first fully-differential amplifier having differential inputs configured to receive an input signal;

a PWM control stage coupled to the first fully-differential amplifier and having a common-mode input, wherein the PWM control stage comprises a low-pass filter coupled between the common-mode input of the PWM control stage and a first output of the PWM control stage;

a first half-bridge coupled between the PWM control stage and a first output node; and

a second half-bridge coupled between the PWM control stage and a second output node, wherein

the PWM control stage is configured to control the first half-bridge and the second half-bridge,

an input of the first half-bridge is coupled to the common-mode input of the PWM control stage via a first impedance,

an input of the second half-bridge is coupled to the common-mode input of the PWM control stage via a second impedance,

the first output node is coupled to the common-mode input of the PWM control stage via a third impedance, and

the second output node is coupled to the common-mode input of the PWM control stage via a fourth impedance.

2. The Class-D amplifier of claim 1 , further comprising a second fully-differential amplifier coupled between the first fully-differential amplifier and the PWM control stage.

3. The Class-D amplifier of claim 1 , further comprising:

a first output filter coupled to the first output node;

a second output filter coupled to the second output node; and

a load coupled between the first output filter and the second output filter.

4. The Class-D amplifier of claim 3 , wherein

a first terminal of the load is coupled to the common-mode input of the PWM control stage via a fifth impedance; and

a second terminal of the load is coupled to the common-mode input of the PWM control stage via a sixth impedance.

5. The Class-D amplifier of claim 1 , wherein the PWM control stage further comprises:

a first control amplifier coupled to the first output of the PWM control stage;

a second control amplifier coupled to a second output of the PWM control stage; and

an error amplifier coupled between the low-pass filter and the first and second control amplifiers.

6. The Class-D amplifier of claim 1 , further comprising:

a first comparator coupled between the PWM control stage and the first half-bridge;

a second comparator coupled between the PWM control stage and the second half-bridge; and

a reference generator coupled to the first and second comparator.

7. The Class-D amplifier of claim 6 , wherein the reference generator is configured to generate a triangular signal.

8. The Class-D amplifier of claim 1 , further comprising:

a first switch coupled between the input of the first half-bridge and the first impedance;

a second switch coupled between the input of the second half-bridge and the second impedance; and

a clipping detector coupled to the first switch and the second switch.

9. The Class-D amplifier of claim 8 , wherein the clipping detector is configured to turn on the first switch and the second switch when a clipping event occurs.

10. A method of controlling a Class-D amplifier, the method comprising:

receiving an input signal with a first fully-differential amplifier;

generating a first and second PWM signals with a PWM control stage based on differential outputs of the fully differential amplifier;

controlling a first half-bridge with the first PWM signal;

controlling a second half-bridge with the second PWM signal; and

setting a common-mode of the PWM control stage based on a feedback node, wherein the setting the common-mode of the PWM control stage comprises

receiving the first PWM signal at the feedback node via a first impedance,

receiving the second PWM signal at the feedback node via a second impedance,

receiving a first signal from an output of the first half-bridge at the feedback node via a third impedance,

receiving a second signal from an output of the second half-bridge at the feedback node via a fourth impedance, and

low-pass filtering a signal from the feedback node with a low-pass filter.

11. The method of claim 10 , wherein the setting the common-mode of the PWM control stage further comprises:

receiving a third signal from a first output filter coupled to the first half-bridge at the feedback node via a fifth impedance; and

receiving a fourth signal from a second output filter coupled to the second half-bridge at the feedback node via a sixth impedance.

12. The method of claim 10 , wherein the first PWM signal and the second PWM signal correspond to an in-phase modulation scheme.

13. The method of claim 10 , wherein the first PWM signal and the second PWM signal correspond to an out-of-phase modulation scheme.

14. The method of claim 10 , wherein the low-pass filter has a cut-off frequency lower than a switching frequency of the first PWM signal.

15. The method of claim 10 , further comprising:

detecting a clipping event;

closing a first switch coupled between a first output of the PWM control stage and the first impedance after detecting the clipping event; and

closing a second switch coupled between a second output of the PWM control stage and the second impedance after detecting the clipping event.

16. The method of claim 15 , wherein detecting the clipping event comprises:

determining that the output of the first half-bridge has a 100% duty cycle; or

determining that the output of the second half-bridge has a 100% duty cycle.

17. The method of claim 15 , further comprising opening the first and second switches during an absence of the clipping event.

18. A Class-D audio amplifier comprising:

a first fully-differential amplifier stage configured to receive an audio signal;

a fully-differential PWM control stage comprising

differential inputs coupled to differential outputs of the first fully-differential amplifier, and

a low-pass filter coupled between a common-mode input of the fully-differential PWM control stage and a first and second differential outputs of the fully-differential PWM control stage;

a first half-bridge coupled between the first differential output of the fully-differential PWM control stage and a first output node; and

a second half-bridge coupled between the second differential output of the fully-differential PWM control stage and a second output node, wherein

the first differential output of the fully-differential PWM control stage is coupled to the common-mode input of the fully-differential PWM control stage via a first impedance,

the second differential output of the fully-differential PWM control stage is coupled to the common-mode input of the fully-differential PWM control stage via a second impedance,

the first output node is coupled to the common-mode input of the fully-differential PWM control stage via a third impedance, and

the second output node is coupled to the common-mode input of the fully-differential PWM control stage via a fourth impedance.

19. The Class-D audio amplifier of claim 18 , further comprising:

a first output filter coupled to the first output node;

a second output filter coupled to the second output node; and

a loudspeaker coupled between the first output filter and the second output filter.

20. The Class-D audio amplifier of claim 19 , wherein

a first terminal of the loudspeaker is coupled to the common-mode input of the fully-differential PWM control stage via a fifth impedance; and

a second terminal of the loudspeaker is coupled to the common-mode input of the fully-differential PWM control stage via a sixth impedance.