Transition smoothing apparatus for reducing spurious input to a system under feedback control

1. A transition smoothing apparatus for reducing spurious input to a system under feedback control connected to a control loop of the transition smoothing apparatus comprising:

a loop filter to integrate an error between an input signal applied to the loop filter and an output signal of the system under feedback control;

an analog-to-digital converter to digitize the integrated error every loop cycle of the control loop to provide a digitized integrated error value;

a controller to generate in a normal operation mode output values supplied to the system under feedback control in response to the digitized integrated error values and adapted in a start-up sequence to control a feedback digital-to-analog converter of the control loop according to the digitized integrated error values to supply a first control signal to the loop filter of the control loop and to control the system under feedback control to generate an independent second control signal; and

an alignment detector to detect a phase alignment between the first control signal provided by the feedback digital-to-analog converter of the control loop and the second control signal provided by the system under feedback control to control a smooth transition into a closed loop operation of the control loop in the normal operation mode during the start-up sequence with reduced spurious input to the system under feedback control.

2. The transition smoothing apparatus according to claim 1 wherein the alignment detector is integrated in the controller of the control loop of the transition smoothing apparatus.

3. The transition smoothing apparatus according to claim 1 wherein the loop filter includes an analog low pass filter.

4. The transition smoothing apparatus according to claim 1 wherein the analog-to-digital converter of the control loop is adapted to supply the digitized integrated error value every sample period to a digital input of the controller of the control loop.

5. The transition smoothing apparatus according to claim 4 wherein the digitized integrated error value supplied by the analog-to-digital converter to the digital input of the controller comprises an ADC code with an offset binary format.

6. The transition smoothing apparatus according to claim 1 wherein the controller includes a supply voltage monitoring and prediction circuit adapted to maintain a target voltage.

7. The transition smoothing apparatus according to claim 1 wherein the controller includes a common mode control circuit adapted to maintain a set-point common mode voltage.

8. The transition smoothing apparatus according to claim 1 wherein the system under feedback control is an H-bridge driver of a class D-amplifier including a positive H-bridge driver circuit having a positive output terminal and a negative H-bridge circuit having a negative output terminal.

9. The transition smoothing apparatus according to claim 8 wherein a loudspeaker is connected to the positive output terminal and to the negative output terminal of the H-bridge driver of the class D-amplifier.

10. The transition smoothing apparatus according to claim 8 wherein the controller is adapted to supply a first gray coded output value to the positive H-bridge driver circuit of the H-bridge driver and a second gray coded output value to the negative H-bridge driver circuit of the H-bridge driver of said class D-amplifier.

11. The transition smoothing apparatus according to claim 1 wherein the system under feedback control includes a Phase Locked Loop circuit.

12. The transition smoothing apparatus according to claim 1 wherein the analog-to-digital converter comprises a SAR analog-to-digital converter.

13. The transition smoothing noise reduction apparatus according to claim 1 wherein a control block of the transition smoothing apparatus is adapted to generate at least one start-up control signal applied to the controller to initiate during the start-up sequence a transition from an idle operation mode to the normal operation mode.

14. The transition smoothing apparatus according to claim 1 wherein the alignment detector is adapted to detect a phase alignment between a second fixed pattern control signal provided by the system under feedback control during the start-up sequence and the first control signal provided by the feedback digital-to-analog converter during the start-up sequence used to control during the start-up sequence a smooth transition from an idle operation mode to the normal operation mode with a reduced spurious input to the system under feedback control.

15. The transition smoothing apparatus according to claim 1 wherein a control block of the transition smoothing apparatus is adapted to generate a shutdown control signal applied to the controller to initiate in a shutdown sequence a transition from the normal operation mode to an idle operation mode.

16. The transition smoothing apparatus according to claim 1 wherein the controller is adapted to generate in the normal operation mode in response to the digitized integrated error values gray-coded output values supplied to the system under feedback control.

17. The transition smoothing apparatus according to claim 1 wherein the control loop of the transition smoothing apparatus comprises a PWM control loop including the loop filter, the analog-to-digital converter, the controller and the feedback digital-to-analog converter.

18. The transition smoothing apparatus according to claim 1 wherein the transition smoothing apparatus comprises a noise reduction apparatus adapted to reduce acoustic start-up noise forming spurious input to the system under feedback control.

19. An alignment detector adapted to detect a phase alignment between a first control signal provided by a feedback digital-to-analog converter of a control loop and a second control signal provided by a system under feedback control to control a smooth transition into a closed loop operation of the control loop in a normal operation mode during a start-up sequence with reduced spurious input to the system under feedback control.

20. The alignment detector according to claim 19 wherein the system under feedback control comprises an H-bridge driver of a class D-amplifier including a positive H-bridge driver circuit having a positive output terminal and a negative H-bridge circuit having a negative output terminal, wherein a loudspeaker is connected to the positive output terminal and to the negative output terminal of the H-bridge driver of said class D-amplifier.

21. The alignment detector according to claim 19 wherein the system under feedback control comprises a Phase Locked Loop circuit.

22. A class D-amplifier including a transition smoothing apparatus having a system under feedback control connected to a control loop, the transition smoothing apparatus comprising:

a loop filter to integrate an error between an input signal applied to the loop filter and an output signal of the system under feedback control;

an analog-to-digital converter adapted to digitize the integrated error every loop cycle of the control loop to provide a digitized integrated error value;

a controller to generate in a normal operation mode in response to the digitized integrated error values output values supplied to the system under feedback control and in a start-up sequence to control a feedback digital-to-analog converter of the control loop according to the digitized integrated error values to supply a first control signal to the loop filter of the control loop and to control the system under feedback control to generate an independent second control signal; and

an alignment detector to detect a phase alignment between the first control signal provided by the feedback digital-to-analog converter of the control loop and the second control signal provided by the system under feedback control and to control a smooth transition into a closed loop operation of the control loop in the normal operation mode during the start-up sequence with reduced spurious input to the system under feedback control.