Systems and methods for over-current protection

Abstract

Systems and methods for over-current protection in all-digital amplifiers using low-cost current sensing mechanisms. An over-current hard clipping unit receives a digital audio signal, clips the signal according to a clip level, and provides the signal to a modulator. The modulator modulates the signal to produce, e.g., a PWM signal and provides the modulated signal to an output stage which generates an output current to drive a speaker. An over-current sensing unit is compares the output current to a threshold value and generates a binary signal indicating whether the output current exceeds the threshold value. The hard clipping unit receives the binary signal and ramps down the clip level during time periods in which the binary signal indicates that the output current exceeds the threshold. When the binary signal indicates that the output current does not exceed the threshold value, the hard clipping unit ramps up the clip level.

What is claimed is: 
     
       1. A method implemented in a digital amplifier comprising:
 detecting an over-current condition;   while the over-current condition exists, ramping down a clipping level at which an audio signal is clipped;   clipping the audio signal according to the clipping level.   
     
     
       2. The method of  claim 1 , A method implemented in a digital amplifier that includes an output stage that is driven in dependence on an audio signal, the method comprising:
 detecting an over-current condition in the output stage that is driven in dependence on the audio signal; 
 while the over-current condition exists in the output stage that is driven in dependence on the audio signal, ramping down a clipping level at which the audio signal is clipped; 
 clipping the audio signal according to the clipping level; 
 wherein ramping down the clipping level comprises:
 upon detecting the over-current condition, determining an audio input signal value; 
 setting the clipping level to an initial clip value that is less than or equal to the audio input signal value; and 
 for each successive time period in which the over-current condition persists, decreasing the clipping level. 
 
 
     
     
       3. The method of  claim 2 , further comprising, when the over-current condition does not exist, increasing the clipping level in successive time periods until a maximum clipping level is reached. 
     
     
       4. The method of  claim 2 , wherein the digital amplifier is a pulse width modulation (PWM) amplifier and wherein the successive time periods comprise successive PWM switch periods. 
     
     
       5. The method of  claim 2 , wherein detecting the over-current condition comprises monitoring a current level at an in the output stage of the amplifier, detecting when the current level exceeds a threshold value, and generating a binary signal which indicates whether the current level at in the output stage of the amplifier exceeds the threshold value. 
     
     
       6. The method of  claim 5 , wherein detecting the over-current condition is performed and the binary signal is updated for each successive time period. 
     
     
       7. The method of  claim 5 , wherein detecting the over-current condition further comprises low-pass filtering the binary signal. 
     
     
       8. The method of  claim 2 , wherein clipping the audio input signal comprises clipping the audio input signal that is input to a noise shaper. 
     
     
       9. The method of  claim 2 , wherein clipping the audio input signal comprises clipping the audio input signal that is input to a modulator. 
     
     
       10. A digital amplifier comprising:
 an over-current hard clipping unit configured to receive a digital audio signal and clip the digital audio signal according to a clip level; 
 a modulator configured to receive the digital audio signal from the over-current hard clipping unit and to modulate the digital audio signal to produce a modulated audio signal; 
 an output stage configured to receive the modulated audio signal and to generate an output current to drive a load; and 
 an over-current sensing unit configured to compare the output current to a threshold value and to generate an over-current signal which indicates whether an over-current condition exists; 
 wherein the over-current hard clipping unit is configured to receive the over-current signal and to ramp down the clip level while the over-current condition exists; 
 wherein in order to ramp down the clip level while the over-current condition exists, the over-current hard clipping unit is configured to
 set the clip level to a value of the audio signal in the immediately preceding time period minus a decrement value, if the audio signal was not clipped in the immediately preceding time period; and 
 decrease the clip level by the decrement value, if the audio signal was clipped in the immediately preceding time period. 
 
 
     
     
       11. The digital amplifier of  claim 10 , wherein
 the over-current sensing unit is configured to compare the output current to a threshold value and to generate the over-current signal as a binary signal which indicates whether the output current exceeds the threshold value. 
 
     
     
       12. The amplifier of  claim 11 , wherein the over-current hard clipping unit is configured to receive the binary signal and to decrease the clip level for each successive time period in which the binary signal indicates that the output current exceeds the threshold value. 
     
     
       13. The amplifier of  claim 12 , wherein the over-current hard clipping unit is further configured to increase the clip level for each successive time period in which the binary signal indicates that the output current does not exceed the threshold value. 
     
     
       14. The amplifier of  claim 13 , wherein the over-current hard clipping unit is configured to increase the clip level by a programmable amount for each successive time period. 
     
     
       15. The amplifier of  claim 12 , wherein the over-current hard clipping unit is configured to decrease the clip level by a programmable amount for each successive time period. 
     
     
       16. The amplifier of  claim 12 , wherein the threshold value to which the over-current sensing unit compares the output current is programmable. 
     
     
       17. The amplifier of  claim 12 , wherein the digital amplifier is a pulse width modulation (PWM) amplifier, the modulator is configured to modulate the digital audio signal to produce a PWM signal, and the successive time periods comprise successive PWM switch periods. 
     
     
       18. The amplifier of  claim 17 , wherein the over-current sensing unit is configured to compare the output current to the threshold value and to update the binary signal for each successive time period. 
     
     
       19. The amplifier of  claim 12 , A digital amplifier comprising:
 an over-current hard clipping unit configured to receive a digital audio signal and clip the digital audio signal according to a clip level; 
 a modulator configured to receive the digital audio signal from the over-current hard clipping unit and to modulate the digital audio signal to produce a modulated audio signal; 
 an output stage configured to receive the modulated audio signal and to generate an output current to drive a load; and 
 an over-current sensing unit configured to compare the output current to a threshold value and to generate an over-current signal which indicates whether an over-current condition exists; 
 wherein the over-current hard clipping unit is configured to receive the over-current signal and to ramp down the clip level while the over-current condition exists; 
 wherein the over-current hard clipping unit is configured to receive the binary signal and to decrease the clip level for each successive time period in which the binary signal indicates that the output current exceeds the threshold value; and  
 further comprising a low-pass filter coupled between the over-current sensing unit and the over-current hard clipping unit and configured to low-pass filter the binary signal and provide the filtered binary signal to the over-current hard clipping unit. 
 
     
     
       20. The amplifier of  claim 12 , A digital amplifier comprising:
 an over-current hard clipping unit configured to receive a digital audio signal and clip the digital audio signal according to a clip level; 
 a modulator configured to receive the digital audio signal from the over-current hard clipping unit and to modulate the digital audio signal to produce a modulated audio signal; 
 an output stage configured to receive the modulated audio signal and to generate an output current to drive a load; and 
 an over-current sensing unit configured to compare the output current to a threshold value and to generate an over-current signal which indicates whether an over-current condition exists; 
 wherein the over-current hard clipping unit is configured to receive the over-current signal and to ramp down the clip level while the over-current condition exists; 
 wherein the over-current hard clipping unit is configured to receive the binary signal and to decrease the clip level for each successive time period in which the binary signal indicates that the output current exceeds the threshold value; and  
 further comprising a noise shaper coupled between the over-current hard clipping unit and the modulator and configured to quantize and filter the digital audio signal received from the over-current hard clipping unit and to provide the quantized and filtered digital audio signal to the modulator. 
 
     
     
       21. A method implemented in a digital amplifier that includes an output stage that is driven in dependence on an audio signal, the method comprising:
 detecting an over-current condition in the output stage that is driven in dependence on the audio signal;   while the over-current condition exists in the output stage that is driven in dependence on the audio signal, ramping down a clipping level at which the audio signal is clipped;   clipping the audio signal according to the clipping level;   wherein ramping down the clipping level comprises   if the audio signal was not clipped in the immediately preceding time period, then setting the clipping level to a value of the audio signal in the immediately preceding time period minus a decrement value; and   if the audio signal was clipped in the immediately preceding time period, then decreasing the clipping level by the decrement value.   
     
     
       22. The method of claim 21, further comprising, when the over-current condition does not exist, increasing the clipping level in successive time periods until a maximum clipping level is reached. 
     
     
       23. The method of claim 21, wherein the digital amplifier is a pulse width modulation (PWM) amplifier and wherein the successive time periods comprise successive PWM switch periods. 
     
     
       24. The method of claim 21, wherein detecting the over-current condition comprises monitoring a current level in the output stage of the amplifier, detecting when the current level exceeds a threshold value, and generating a binary signal which indicates whether the current level in the output stage of the amplifier exceeds the threshold value. 
     
     
       25. The method of claim 21, wherein clipping the audio input signal comprises clipping the audio input signal that is input to one of a noise shaper and a modulator. 
     
     
       26. A digital amplifier comprising:
 an over-current hard clipping unit configured to receive a digital audio signal and clip the digital audio signal according to a clip level;   a modulator configured to receive the digital audio signal from the over-current hard clipping unit and to modulate the digital audio signal to produce a modulated audio signal; an output stage configured to receive the modulated audio signal and to generate an output current to drive a load; and   an over-current sensing unit configured to compare the output current to a threshold value and to generate an over-current signal which indicates whether an over-current condition exists;   wherein the over-current hard clipping unit is configured to receive the over-current signal and to ramp down the clip level while the over-current condition exists;   wherein in order to ramp down the clip level while the over-current condition exists, the over-current hard clipping unit is configured to
 determine an audio input signal value upon receiving the over-current signal which indicates that the over-current condition exists; 
 set the clipping level to an initial clip value that is less than or equal to the audio input signal value; and 
 decrease the clip level, for each successive time period in which the over-current condition persists.