Abstract:
A circuit and a method are provided for suppressing the pop and click noise during the power on and power off of Class D amplifiers. The technique also suppresses pops and clicks when the Class D amplifier enters or exits standby mode. A duplicate feedback network is used to establish the stable operating points, including offsets in the Class D circuit without turning on the outputs. The technique works by gradually propagating or dissipating the offset through the signal path of a Class D amplifier by swapping the differential outputs using switches to suppress pops and clicks when starting up and shutting down the amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to Class D amplifiers. More particularly, this invention relates to a circuit and a method for suppressing pop and click noise during power-on and power-off of the amplifier. 
     2. Description of Related Art 
     The major components of a basic class D amplifier are shown in  FIG. 1 . The raw input  1  is connected to an adder circuit  4 , which subtracts from the input  1  a feedback signal  5 . The basic amplifier uses feedback from the output of a half-bridge output stage  19  to help compensate for variations in the bus voltages and to correct distortion. The half-bridge output  19  is connected to a feedback path or network  2 , whose output  5  is then combined with the raw input  1  at the adder  4 . The adder output  6  then is connected to a filter  3 , which generates a modified audio input  14 . The comparator, C,  11  compares a high frequency triangular wave  15  and the modified audio input  14  and generates pulses whose width corresponds to the amplitude and frequency of the audio input signal  14 . The pulse width modulated (PWM) output  18  is fed into the speaker  16 . The output  17  of the comparator drives the switching controller  12 . The switching controller  12  drives a high power switch  19 , whose output  18  is a higher power replica of the comparator&#39;s output  17 . 
     When audio circuits are turned on, pop and click noise can occur on the audio speaker output as quiescent voltages are established. Such effects are undesirable and measures must be taken to suppress them. In the case of Class D amplifiers, there is a feedback loop which includes the nodes which are connected directly to the audio speaker output. There is the problem of establishing the operating point of the feedback loop without causing a step on the output which would lead to unwanted audio artifacts. 
     The following patents represent the prior related art.
     U.S. Pat. No. 7,528,653 B2—Class-D Amplifier with Noise Immunity Feedback (May) describes a class-D amplifier which includes a switching transistor section, a filter, an analog to digital converter, and a feedback module. This amplifier structure provides a method and an apparatus for dynamically minimizing and/or compensating for total harmonic distortion (THD), transient noise, and/or power supply noise of class-D amplifiers.   U.S. Pat. No. 7,432,759 B2—Elimination of Pop-Up Noise in Class D Amplifier (Cho et al.) describes a class D amplifier which includes a driver circuit and a reset circuit. The driver circuit is configured to amplify a PWM (pulse width modulation) signal to generate an amplified PWM signal. The reset circuit applies a predetermined voltage at an input of the driver circuit for a time period after a power supply voltage is applied or before the power supply voltage is deactivated, for eliminating pop-up noise.   U.S. Pat. No. 7,417,504 B2—Startup and Shutdown Click Noise Elimination for Class D Amplifier (Strydom et al.) describes a method of minimizing an audible click noise from a speaker in a Class D audio power amplifier upon shutdown or startup of a switching stage, having two switches series connected at a node, the speaker connected to the node via an output filter comprising an inductor and a capacitor, the switches being controlled by a controller.   U.S. Pat. No. 7,394,314 B2—Class-D Amplifier with Noise-Immunity Feedback (May) describes a class-D amplifier which includes a switching transistor section, a filter, an analog to digital converter, and a feedback module. This amplifier structure provides a method and an apparatus for dynamically minimizing and/or compensating for total harmonic distortion (THD), transient noise, and/or power supply noise of class-D amplifiers.   

     BRIEF SUMMARY OF THE INVENTION 
     It is the objective of this invention to provide a circuit and a method for suppressing the pop and click noise during the power on and power off of Class D amplifiers. 
     The objects of this invention are further achieved by a duplicate feedback resistor network in Class D amplifiers. The objects of this invention are further achieved by an input section with input switches which takes audio input and drives a first input to a control circuit, which drives output pre-drivers. These output pre-drivers drive an H-bridge switch, which drives differential audio outputs and a regular feedback network. The regular feedback network which is in communication with the differential audio outputs is connected to the input section and drives a second input of said control circuit. The duplicate feedback network which is in communication with the control circuit output is connected to the input section, through said input switches back into said control circuit. The duplicate feedback network is used to complete a control loop of said Class D amplifier in order to suppress pops and clicks noise when starting up and exiting a standby mode and shutting down or entering said standby mode in said Class D amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a basic prior art Class D amplifier, which is single-ended with no H-bridge. 
         FIG. 2  shows the main embodiment of the duplicate feedback network of the invention, which is a differential design with an H-bridge. 
         FIG. 3  shows a method flowchart for the elimination of clicking and popping noise for Class D amplifiers during Power-up or during exiting of Standby. 
         FIG. 4  shows a method flowchart for the elimination of clicking and popping noise for Class D amplifiers during Power-down or during entry of Standby. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows the main embodiment of the invention.  FIG. 2  shows a Class D amplifier, which consists of a control circuit  21 , an input section  22  made of resistors  20  and switches  32 , 33 , and an output section which includes pre-drivers  50  and an H-bridge switch  51 . The H-bridge switches are connected to Out+( 43 ) and Out−( 45 ). The audio input  24  and the H-bridge output  25  are fed into the control circuit  21 . The output of the control circuit  26  is a pulse-width modulated (PWM) signal which is buffered and used to drive an H-bridge switching stage. When the loop is running, the feedback is taken from the H-bridge output  43 . The issue is that any offsets in the signal path need to be propagated to the output  43  without causing any pops and clicks on the audio output  43 . 
     In this invention, a duplicate feedback network  41 , with its input connected to the common mode voltage (vcm)  34  and its output connected to the pulse-width modulated output  27  prior to the buffers  35 , is included. This network is used to establish the operating voltages, including any offsets in the signal path, when powering up or exiting standby. Switches B  32  and C  29  are closed and switches A  28  and D  33  are opened (see  FIG. 2 ). The key point is that the circuit in  FIG. 2  operates differentially around a common mode bias point. The common mode voltage is fed to the inputs of the duplicate feedback network to establish the bias point. The control circuit is then powered up. After power up, the control circuit may initially be in a reset state from which it is released after the bias currents are established. The H-bridge  51  output  43  is grounded while this takes place. 
     The next step is to gradually introduce the offset on the H-bridge output  43 . The H-bridge output  43  is enabled and then the signal is transferred from the control circuit output to the H-bridge output  43  using switches E  31  and F  30 . By alternating which pulses are sent through switches E  31  and which pulses are sent through switches F  30 , the output  43  rise time can be controlled thereby avoiding any audible signals on the output. The duty cycle of selecting F switches  30  over E switches  31  is gradually increased from 50% to 100%. The rate of increase in duty cycle on the F switches  30  is slow enough to be inaudible. Finally, the duplicate feedback network is deselected by opening switches C  29 , and the real feedback network  42  selected by closing switches A  28 . In  FIG. 2 , we see the regular feedback paths coming from Out+43 and Out−45 going through switch A  28 . 
     The process to assert powerdown or standby requires muting the output by removing the audio input and then gradually reducing the offset before powering down the signal path. Firstly, the signal is removed by selecting the duplicate feedback path. This is selected by opening switches D  33  and A  28 , and closing switches C  29  and B  32 . The offset is then removed from the H-bridge  51  by beginning to alternate pulses between the F  30  and E  31  switches until the duty cycle is reduced from 100% to 50%. This is carried out at a rate which is slow enough to be inaudible. When the duty cycle has been at 50% for long enough for the offset to have been dissipated from the output, the H-bridge  51  is powered down. Thereafter the control circuit is powered down. 
       FIG. 3  shows the flowchart of a method for eliminating popping and clicking noise in Class D amplifiers during power-up or during exiting standby. The method assumes a duplicate feedback network which is used to establish operating voltages and offsets in a signal path during power-up and during exiting standby and a set of switches A, B, C, D, E and F which enable said duplicate feedback network. The first block  301  in the flowchart shows the grounding of an H-bridge output circuit during powering up of the control circuit. The next block  302  is the powering up of a control circuit which will then be in a reset state. Next is the  303  block which is the releasing of a control circuit from the reset state after bias currents are established. The  304  block shows the closing of switches B and C and the opening of switches A and D during power-up or while exiting standby. Next, the H-bridge needs to be enabled  305  and  306  before beginning the gradual introduction of offsets in blocks  307  and  308 . The next block  305  shows the enabling of the H-bridge output. The next block  306  shows transferring control circuit output to the H-bridge output using switches E and F with a 50% duty cycle. The next block  307  and  308  shows the alternating of pulses which are sent through switches E and F so that the Class D amplifier output rise time can be controlled so as to avoid audible signals on the amplifier output by gradually changing the duty cycle. The  309  block shows the deselecting of the duplicate feedback network by opening switches B and C, and the  310  block shows the selecting of a real feedback network by closing switches A and D. 
       FIG. 4  shows the flowchart of a method for eliminating popping and clicking noise in Class D amplifiers during power-down or during entry of standby. The method assumes a duplicate feedback network which is used to remove operating voltages and offsets in a signal path during power-down and when exiting standby using a set of switches A, B, C, D, E and F which enable said duplicate feedback network. The first block  401  in the flowchart shows the removal of signal from the Class D amplifier by selecting the duplicate feedback network path by opening switches D and A, then closing switches B and C. The next block  402  is the removing of the offset from the H-bridge by using switches F and E, gradually changing the duty cycle from 100% to 50%. In addition, block  402  also shows the alternating of pulses which are sent through switches F and E until the duty cycle reaches 50% and the offsets are removed the Class D amplifier output. The  403  block shows the powering down of the H-bridge. Finally, the  404  block shows the powering down of the control circuit. 
     The key advantages of this device and method are as follows. 
     1) The invention allows the elimination of pop and click noise from the output of Class D amplifiers during the transitions of power-on, power-off, entering standby and exiting standby. 
     2) The method prevents pop and click noise for all operating conditions, since the control circuit  21  controls the opening and closing of switches A, B, C, D, E and F as a function of operating conditions such as temperature, humidity, input waveform, internal noise, and external noise. Since the technique adapts to the operating points of the circuit, it is immune to operating conditions. 
     While this invention has been particularly shown and described with Reference to the preferred embodiments thereof, it will be understood by those Skilled in the art that various changes in form and details may be made without Departing from the spirit and scope of this invention.