Abstract:
Circuitry is provided to compensate for distortion introduced into the output signal of a delta sigma digital to analog converter (DAC) by the power output stage of the amplifier. Such distortion is not consistent for a given output data value or short series of data values, but must be either measured and corrected in real time or must be corrected in real time based upon a sophisticated model of the system that predicts the distortion. Correction is applied to one or more feedback loops in the delta sigma converter. Distortion caused by fluctuations in the power supply voltage may also be modeled and corrected in real time.

Description:
BACKGROUND OF THE INVENTION 
     This application claims the benefit of U.S. Provisional Application No. 60/121,205, filed Feb. 23, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power output stage compensation for digital output amplifiers. 
     DESCRIPTION OF THE PRIOR ART 
     A typical circuit for the creation of a digital data stream to drive a power output stage is a delta sigma modulator, with or without a pulse width modulator conversion stage at its output, feeding a digital output amplifier, which drives a speaker. FIG. 1 (prior art) is a block diagram of a digital to analog converter (DAC)  100  comprising a delta sigma modulator  104  feeding a power output stage  114  into a speaker  120 . In the circuit shown in FIG. 1, a pulse width modulator (PWM)  108  converts the format of the data  106  from delta sigma modulator  104 . PWM  108  generates control signals  110 ,  112  which feed into class D power output stage  114 . Analog power output signals  116 ,  118  from class D output  114  drive speaker  120 . It is known in the art to measure the voltage of the driver output, and to use that value  122  as feedback to the delta sigma modulator  104 . This can approximately correct the loop, but some errors remain. If the distortion is due to the rise time of data pulses being smaller than the fall time, not only the area under the waveform curve is affected, but also the time delay (or higher moments) of the waveform. 
     The conversion to PWM is a desirable, but not necessary step. The use of PWM is preferred in many power applications, but the same comments will also apply to the direct conversion to analog of a 1 bit or multi bit delta sigma data stream. 
     U.S. patent application Ser. No. 09/163,235 by the present inventor (incorporated herein by reference) describes correction of systematic nonlinear output distortion that occurs in delta sigma digital to analog converters (or DACs). Feedback here is compensated feedback, as the feedback value includes the “normal” feedback term as well as the correction. U.S. patent application Ser. No. 09/457,014 by the present inventor (incorporated herein by reference) describes compensating delta sigma converters by correlating an output signal with the same signal passed through a selected nonlinear function, and correcting the output based upon the correlation. 
     FIG. 2 (prior art) is a block diagram of a second order one bit delta sigma converter  104  which includes separate feedback correction  242 ,  244  to each of the accumulators  40 ,  44 , in order to compensate for errors relating to both the area of output wave forms and delay in the waveforms. A second order delta sigma converter  104  is shown and described for simplicity, but those skilled in the art will appreciate that the same considerations apply to higher order convertors as well. Quantizer  50  is typically a multi bit quantizer in a system where it will be combined with a PWM stage. In the non-compensated case,  242  and  244  will implement unity gain functions. In second order delta sigma quantizer  104 , feedback to each accumulator is compensated for nonlinear variations in output data. If there are time effects, such as pattern generation or slew rate issues, correction block  242  and correction block  244  must implement non-trivially different functions, to account for first and second order variations. For example, one loop may be corrected, but not the other. Or, each loop may be corrected in a different manner. A non-trivial difference is where one feedback function is not a simple linear function of the other. Obviously higher moment errors can be corrected in higher order delta sigma converters with more feedback compensation. There exist many structures for the implementation of such delta sigma modulators. “Delta-Sigma Data Converters” by Norsworthy et al., IEEE Press, 1997, is a good review of the state of the art. 
     The inventions taught in U.S. patent application Ser. No. 09/163,235 relate specifically to correcting systematic output errors that are predictable for a given data value or series of data values. In order to compensate for unpredictable output distortion, compensation blocks  242 ,  244  have to be modified in real time to properly correct for the distortion. 
     It is well understood that a weak link in the creation of a digital output amplifier such as class D output  114  is the imperfections of the output switches. These switches have non-zero impedance, and have rise and fall times that are significant. In addition, it is necessary to have a small dead time between the high side switch and the low side switch in order to guarantee reliable operation. Simultaneous closure of both switches will damage the system, because of the extreme current flow. FIG. 3 (prior art) is a block diagram of a conventional class D power output stage  114 . Inputs  110 ,  112  feed into dead time controls  302 ,  322 , which prevent high side driver  304  from operating at the same time as low side driver  306  and prevent high side driver  324  from operating at the same time as low side driver  326 . In typical operating mode, points A and C are driven to opposite extremes, and switch  310  will be operated during the same time as switch  334 , while  314  will be operated during the same time as  330 . If the duty cycle is 50%, i.e. the two times are identical, the effective voltage delivered to the load (connected between  166  and  118 ) is 0. As the duty cycle changes, the effective output voltage can be driven positive or negative. 
     Low side drivers  306  and  326  provide the current necessary to charge and discharge the gates of  314  and  334 . Similarly, high side drivers  310  and  330  provide drive for switching FETs  310  and  330 . The diode-capacitor network of  308  and  312  provides a bootstrapped power source for  304 . Similarly,  328  and  332  provide a power source for  324 . 
     The high and low drivers may have significant delays and can only charge or discharge the FET gates at a finite rate. Because of these real world limitations, the dead time control must guarantee that there is no cross-conduction. These effects give rise to the distortion that must be corrected. 
     FIG. 4 (prior art) is a timing diagram of signals from class D output  114 . In the example of FIG. 4, Signal A is high one third of the time and signal C is high two thirds of the time, resulting in filter signal B at one third level and D at two thirds level. In this case, signal A and C together average 50% on and 50% off. In some cases, it may be of value to use a modulation scheme in which the A and C signals are not mirror images of each other. 
     One prior art solution to the sensitivity of such a system to imperfections in the output stage switches is to carefully control the output waveforms using circuits to control the drive to the switch devices. This technique can produce acceptable results, but requires expensive analog circuitry. 
     A need remains in the art for apparatus and methods to correct for output distortion that is not predictable for a given value or short series of values, by either measuring the distortion and correcting it in real time or by building a model of the distortion for the system and correcting the distortion according to the model. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to compensate for distortion introduced into the output signal of a delta sigma digital to analog converter (DAC) by the power output stage of the amplifier. Such distortion is not consistent for a given output data value or short series of data values, but must be either measured and corrected in real time or must be corrected in real time based upon a sophisticated model of the system that predicts the distortion. 
     Apparatus for compensating for output distortion in a class D power output stage driven by a delta sigma modulator of at least second order includes means for determining distortion within the class D power output stage, means for generating a distortion signal based upon the distortion within the class D power output stage, and means for modifying at least two feedback signals in the delta sigma modulator, in non-trivially different manners, in real time, responsive to the distortion signal. 
     The means for determining distortion measures a signal in the class D power output stage in real time, such as output voltage or current. The measurement might take place during dead time, and the waveform might be monitored. In this case, the distortion signal would include two signals representing two moments of the waveform. Preferably 
     The means for determining distortion might also compare two output signals of the class D power output stage. Signal timing between the two signals may also be measured. 
     Apparatus for compensating for output distortion in a class D power output stage driven by a delta sigma modulator could instead include means for modelling distortion within the class D power output stage, means for generating a distortion signal based upon the model, and means for modifying a feedback signal in the delta sigma modulator in real time, responsive to the distortion signal. 
     The means for modelling could numerically compute expected distortion values based upon circuit elements in the output stage. Or, the means for modelling could includes means for monitoring two output signals of the class D power output stage, means for computing distortion in the class D power output stage for various signal patterns based on the two monitored output signals, and means for estimating distortion in the class D power output stage for a given signal pattern out of the delta sigma modulator. 
     The means for modelling distortion could also learn. Then the modelling means would include means for monitoring the signal pattern out of the delta sigma modulator in real time, and means for utilizing the modelling means to estimate the distortion for the current signal pattern out of the delta sigma modulator. 
     Two feedback signals in the delta sigma modulator could modified in response to the distortion signal, each feedback signal being modified in a non-trivially different manner from the other feedback signal. The modelling means could be slowly modified in real time based upon the two output signals of the class D power output stage. 
     The means for modelling could include means for correlating noise from the output stage with noise into the output stage passed through a nonlinear element. 
     The apparatus could include a pulse wave modulator (PWM) between the delta sigma modulator and the class D power output stage, where the PWM is fed by the delta sigma modulator and drives the class D power output stage. The apparatus could also include means for monitoring power supply voltage, so the distortion signal is further based upon the power supply voltage. 
     Apparatus for compensating for output distortion in a class D power output stage driven by a delta sigma modulator, could include means for modelling distortion within the class D power output stage, means for generating a distortion signal based upon the model, means for modifying a feedback signal in the delta sigma modulator in real time, responsive to the distortion signal, and means for updating the model based upon an output signal of the class D power output stage. 
     The means for updating preferably measures more than one moment of the output signal of the class D power output stage, and the means for modelling preferably generates two distortion signals, to modify two feedback signals in the delta sigma modulator in non-trivially different manners. 
     The means for updating could update the modelling means once on apparatus power up, or continuously. 
     The means for modelling could further comprise means for monitoring two output signals of the class D power output stage, means for computing distortion in the class D power output stage for various signal patterns based on the two monitored output signals, and means for estimating distortion in the class D power output stage for a given signal pattern out of the delta sigma modulator. 
     Apparatus for compensating for output distortion in a class D power output stage driven by a delta sigma modulator could include means for determining distortion within the class D power output stage by measuring two signals in the class D power output stage, and measuring signal timing between the two signals, in real time, means for generating a distortion signal based upon the determined distortion, and means for modifying a feedback signal in the delta sigma modulator in real time, responsive to the distortion signal. 
     The means for measuring could measure two output voltage signals during dead time. Or, the means for measuring could measure two output current signals during dead time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (prior art) is a block diagram of a second order delta sigma modulator with separate correction of the two feedback paths to compensate for predictable systematic errors in the output path. 
     FIG. 2 (prior art) is a block diagram of a digital to analog converter (DAC) comprising a delta sigma modulator feeding a power output stage into a speaker. 
     FIG. 3 (prior art) is a block diagram of a class D power output stage. 
     FIG. 4 (prior art) is a timing diagram of signals from the block diagram of FIG.  3 . 
     FIG. 5 is a block diagram of circuitry added to the class D power output stage of FIG. 3 according to the present invention for measuring signals for feedback to the delta sigma modulator of FIG.  2 . 
     FIG. 6 is a block diagram of a power output stage according to the present invention including a feedback signal from the class D power output stage. 
     FIG. 7 is a block diagram of circuitry added to the class D power output stage of FIG. 3 according to the present invention for measuring signals for modeling the power output stage. 
     FIG. 8 is a block diagram of circuitry according to the present invention for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal to the delta sigma modulator. 
     FIG. 9 is a block diagram of a second embodiment of circuitry according to the present invention for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal to the delta sigma modulator. 
     FIG. 10 is a block diagram of a third embodiment of circuitry according to the present invention for modeling the power output stage of the DAC of FIG.  3  and the power supply of the system to provide a correction signal to the delta sigma modulator. 
     FIG. 11 is a block diagram of a fourth embodiment of circuitry according to the present invention for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal to the delta sigma modulator. 
     FIG. 12 is a block diagram of an embodiment of an output distortion monitoring block of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 is a block diagram of circuitry added to class D power output stage  114   a  for measuring signals for feedback to delta sigma modulator  104 . Dead time monitoring block  502  takes digital signals A and C (shown in FIG. 4) and measures the output voltage during the dead time. One measure may be made, such as the average voltage during the dead time, or more sophisticated measures that relate to both the time and voltage. Signal  504  represents the results of that measurement, and is fed back to delta sigma modulator  104   a  as shown in FIG.  6 . 
     FIG. 6 shows feedback signal(s)  504  from class D power output stage  114   a  to delta sigma modulator  104   a.  In the particular embodiment of FIG. 6, two different feedback correction signals  504   a  and  504   b  are used as inputs to compensation blocks  242 ,  244 . For example,  504   b  might represent the area under a waveform and  504   a  might represent the timing (center of gravity) of the waveform. However it would also be possible to feedback the same signal  504  to both compensation blocks  242 ,  244 . For example, signal  504 , representing the area under the curve, could be fed back to both blocks  242  and  244 , but block  244  would compensate the feedback directly according to signal  504 , while block  242  would use signal  504  as an input to a lookup table, and estimate the waveform timing based on signal  504 . It would also be possible to supply signal  504  to only one of the two compensation blocks  242 ,  244 . For example, signal  504  might represent the rise time of the waveform, and it might be used by compensation block  244  to estimate the area under the waveform curve. 
     FIG. 7 is a block diagram showing circuitry added to class D power output stage  114   b  for measuring signals for modeling the power output stage. Output distortion monitoring block  702  takes analog signals B and D (shown in FIG. 4) and measures the difference between the output and the ideal signal. Signal  704  represents the output error and is input to modeling circuitry as shown in FIGS. 8 and 9. FIG. 12 shows one embodiment of output distortion monitoring block  702 , comprising a perfect one bit DAC  1202 , an analog subtraction circuit  1204 , and finally an output capable of driving an ADC  1208 . The output is a measure of the distortion in the switching circuit. At the low signal power levels required, the one bit DAC can be very near to perfect. The one bit DAC is driven by a delta sigma modulator that matches the time delay of the PWM stage. Low pass filter  1206  removes signals above the audio band, which would not be considered distortion. 
     FIG. 8 is a block diagram of circuitry for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal or signals to delta sigma modulator  104   b.  Signal  704   a  is converted to a digital signal by ADC  802  (preferably a delta sigma ADC). Learn block  804  monitors signals B and D as Class D stage  114   b  operates and determines the characteristics of the voltage waveforms for various signal patterns, and conditions, and modifies model  806 . As an example of the type of behavior learned, note that the voltage during the dead time is very dependent on the direction of current through the output filter inductor. Estimation of this current by a model would allow for a good estimation of the waveform during the dead time. This modification of the model may be done in a test mode or on an ongoing basis. Key to this design is that the model update is much slower than real time (i. e. the model is not updated for each data point), while the model is used in real time. 
     In use, model  806  has as its input signal  106  from delta sigma converter  104   b.  Model  806  computes the expected output voltage of class D output  114   b  for that data point, and provides a correction signal to one or more of the compensation blocks  242 ,  244  in delta sigma converter  104   a.    
     FIG. 9 is a block diagram showing a second embodiment of circuitry according to the present invention for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal to delta sigma modulator  104   c.  This circuitry correlates noise from output stage  114   c  with a noise signal generated by a nonlinear block whose purpose is to isolate the imperfection being measured. This takes place in a test mode, so that a variety of different imperfections in Class D stage  114   c  may be measured, such as different rise and fall times for the switches, different results depending upon whether the current in the speaker is positive or negative, dead time between the high side switch and the low side switch, etc. As an example,  902  could output a signal only during the rise time of one of the output switches. The correlation between the two signals would then be the error associated with that isolated imperfection (in this case rise time error). Correlator  904  feeds this error to model  906 , which builds its model to incorporate the error. 
     In use, model  906  has, as its input, signal  106  from delta sigma converter  104   c.  Model  906  computes the expected output voltage of class D output  114   b  for that data point by adding the errors from the class D stage  114   c  imperfections, and provides a correction signal to one or more of the compensation blocks  242 ,  244  in delta sigma converter  104   c.    
     FIG. 10 is a block diagram showing a third embodiment of circuitry according to the present invention for modeling both the power output stage voltage and the power supply voltage to provide a correction signal to delta sigma modulator  104   d.  The model estimates the current being drawn out of the power supply, and also the current line voltage in to the power supply filter cap. A simple model of the power supply current allows for the estimation of the current voltage, which can be used to correct the feedback in the delta sigma modulator. 
     FIG. 11 is a block diagram showing a fourth embodiment of circuitry according to the present invention for modeling the power output stage of the DAC of FIG. 3 to provide a correction signal to delta sigma modulator  104   e.  Unlike the embodiments in FIGS. 8,  9 , and  10 , model  1102  is not derived from measures of actual system performance, but rather is numerically computed based upon the driver, filters, and speaker elements of the output circuitry. Model  1102  has as its input signal  106  from delta sigma converter  104   e.  Model  1102  computes an estimate of the voltage waveform on the output of the drivers of class D stage  114   e,  and provides a correction signal to one or more of the compensation blocks  242 ,  244  in delta sigma converter  104   e.  It is here assumed that the model is developed in the laboratory, and that it is very similar between units of the same design. 
     While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit and scope of this invention. 
     Although the discussion is centered around a PWM based power amplifier, one versed in the art would be able to apply the same concepts to any DAC that has errors due to imperfections in the analog circuitry.