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

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.