Feedback controller for PWM amplifier

1. A feedback controller for a PWM amplifier, the controller comprising:

a signal input for receiving a pulse width modulated (PWM) input signal whose duty cycle represents a desired analogue output signal;

a feedback input for receiving a feedback signal representing an actual PWM output signal of the amplifier;

means for generating a filtered error signal comprising a filtered representation of differences between the input signal and the feedback signal;

means for comparing the filtered error signal with a reference to generate a provisional PWM switching control signal for controlling the PWM amplifier; and

a pulse conditioner comprising a sequential logic circuit arranged to receive both the provisional PWM switching control signal and the PWM input signal for outputting to the amplifier a conditioned PWM switching control signal comprising the provisional PWM switching control signal modified so as to inhibit a transition in the switching control signal under conditions defined by reference to past transitions in the PWM input signal.

2. A feedback controller as claimed in claim 1 wherein, depending on the further evolution of the PWM input signal and the provisional switching control signal, the transition in said provisional switching control signal which is inhibited by said pulse conditioner will be either eliminated altogether in said conditioned control signal or reproduced subsequently in response to a transition in the input signal.

3. A feedback controller as claimed in claim 1 wherein the pulse conditioner implements a “single pulse” function whereby it is ensured that for every input pulse, there is no more than one output pulse.

4. A feedback controller as claimed in claim 1 wherein the logic circuit is constructed such that, subject to constraints based on the PWM input signal, the conditioned control signal responds asynchronously to transitions in the provisional control signal.

5. A feedback controller as claimed in claim 1 , wherein the logic circuit further comprises a clock input for a clock signal having a frequency many times the pulse repetition frequency of said PWM input signal.

6. A feedback controller as claimed in claim 3 wherein the pulse conditioner is formed such that the conditioned PWM switching signal will be identical to the provisional PWM switching control signal for as long as supply voltages to the amplifier and modulation depth in the PWM input signal remain within predetermined operating limits.

7. A feedback controller as claimed in claim 3 wherein the pulse conditioner further comprises a “missing pulse” function for generating additional pulses in the conditioned control signal in the event that an interval between pulses in the conditioned control signal would exceed a predetermined value.

8. A feedback controller as claimed in claim 7 wherein the missing pulse function is applied after the single pulse function.

9. A feedback controller as claimed in claim 3 wherein the pulse conditioner further includes a “minimum pulse” function for imposing a minimum duration on pulses in the conditioned control signal.

10. A feedback controller as claimed in claim 9 wherein the minimum pulse function is applied after the single pulse function and after the missing pulse function, is present.

11. A feedback controller for a PWM amplifier, the controller comprising:

a signal input for receiving a pulse width modulated (PWM) input signal whose duty cycle represents a desired analogue output signal;

a feedback input for receiving a feedback signal representing an actual PWM output signal of the amplifier;

a feedback loop filter for generating a filtered error signal comprising a filtered representation of differences between the input signal and the feedback signal;

a comparator for comparing the filtered error signal with a reference to generate a provisional PWM switching control signal for controlling the PWM amplifier; and

a pulse conditioner comprising a sequential logic circuit arranged to receive both the provisional PWM switching control signal and the PWM input signal for outputting to the amplifier a conditioned PWM switching control signal comprising the provisional PWM switching control signal modified to inhibit a transition in the switching control signal under conditions defined by reference to past transitions in the PWM input signal.

12. A feedback controller as claimed in claim 11 wherein, depending on the further evolution of the PWM input signal and the provisional switching control signal, the transition in said provisional switching control signal which is inhibited by said pulse conditioner will be either eliminated altogether in said conditioned control signal or reproduced subsequently in response to a transition in the input signal.

13. A feedback controller as claimed in claim 11 , wherein the pulse conditioner implements a “single pulse” function whereby it is ensured that for every input pulse, there is no more than one output pulse.

14. A feedback controller as claimed in claim 11 wherein the logic circuit is constructed such that, subject to constraints based on the PWM input signal, the conditioned control signal responds asynchronously to transitions in the provisional control signal.

15. An amplifier comprising at least one output switch device for reproducing in PWM form an amplified version of a PWM input signal, the amplifier including a feedback controller according to claim 1 , the output switch device having a control input coupled to receive said conditioned switching control signal.

16. An amplifier as claimed in claim 15 wherein the output switch device comprises a pair of transistors in push-pull configuration.

17. An amplifier as claimed in claim 16 , further comprising a second output switch device and a second feedback controller, the output switch devices may being connected so that their four transistors are connectable to a load in an H-bridge configuration.

18. An amplifier as claimed in claim 15 wherein the controller comprises an integrated circuit separate from the output switch device.

19. An amplifier as claimed in claim 15 wherein the controller comprises an integrated circuit integrated with the output switch device.

20. An amplifier as claimed in claim 18 wherein said integrated circuit additionally includes circuitry for generating said PWM input signal from coded digital data.

21. A method of controlling a PWM amplifier, the method comprising:

receiving a pulse width modulated (PWM) input signal whose duty cycle represents a desired analogue output signal;

receiving a feedback signal representing an actual PWM output signal of the amplifier;

generating a filtered error signal comprising a filtered representation of differences between the input signal and the feedback signal;

comparing the filtered error signal with a reference to generate a provisional PWM switching control signal for controlling the PWM amplifier; and

outputting to the amplifier a conditioned PWM switching control signal comprising the provisional PWM switching signal modified to inhibit a transition in the switching control signal under conditions defined by reference to past transitions in the PWM input signal.

22. A method as claimed in claim 21 wherein, depending on the further evolution of the PWM input signal and the provisional switching control signal, the transition in said provisional switching control signal which is inhibited by said pulse conditioner will be either eliminated altogether in said conditioned control signal or reproduced subsequently in response to a transition in the input signal.

23. A method as claimed in claim 21 , wherein said constraint includes a “single pulse” constraint whereby it is ensured that for every input pulse, there is no more than one output pulse.

24. A method as claimed in claim 23 wherein a “missing pulse” constraint is further applied, the method generating additional pulses in the conditioned control signal in the event that an interval between pulses in the conditioned control signal would exceed a predetermined value.

25. A method as claimed in claim 24 wherein the missing pulse function is applied after the single pulse function.

26. A method as claimed in claim 23 , wherein a “minimum pulse” constraint is further applied, imposing a minimum duration on pulses in the conditioned control signal.

27. A method as claimed in claim 26 wherein the minimum pulse constraint is applied after the single pulse function, and after the missing pulse function if present.

28. An amplifying method using at least one output switch device to reproduce in PWM form an amplified version of a PWM input signal, the method including a control method according to claim 22 , the output switch device having a control input driven in accordance with said conditioned switching control signal.

29. A method as claimed in claim 28 wherein the output switch device comprises a pair of transistors in push-pull configuration.

30. A method as claimed in claim 29 , further comprising using a second output switch device and generating a second conditional switching control signal by a method as claimed in claim 21 , the output switch devices being connected to drive a load in an H-bridge configuration.