Abstract:
Described herein are several configurations of Class-D audio amplifiers, including a single-ended and a bridge-tied load (BTL) configuration, in which voltage-mode control and average current-mode control circuitry in feedback loops can be included to control the outputs of the Class-D amplifier to reduce open-loop errors and maintain a relatively high loop gain over an expected audio frequency range. The average current-mode control circuitry monitors current through a resistor common to both a current flow into a positive terminal of a loudspeaker associated with the amplifier and a current flow into a negative terminal of the loudspeaker. The voltage-mode control circuitry works with the average current-mode control circuitry in controlling the output of the Class-D audio amplifier.

Description:
PRIORITY INFORMATION 
     The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/181,370, filed 18 Jun. 2016, the entire contents of which are expressly incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     Aspects of the embodiments relate to use of current-mode feedback control of a multi-channel Class-D audio amplifier. 
     Background Art 
     Typically one of the last components in an audio distribution chain, audio amplifiers amplify a low power audio signal to a level suitable for driving one or more loudspeakers. Multi-channel audio amplifiers are employed throughout structures to amplify more than one channel of audio. 
     As known to those of skill in the art, typical Class-D audio amplifiers are a class of audio amplifier in which the audio signal modulates a pulse width modulated carrier signal to drive the output. Referring now to  FIG. 1 , a block diagram of class-D audio amplifier  100  is illustrated showing the different stages of operation of the audio amplifier. Input audio signal (input signal)  102  is input to pulse width modulator (PWM) comparator  104 , which is part of modulator block  109 , along with triangle wave  105  that has been generated by triangle wave generator  103 . In PWM comparator  104 , input signal  102  is compared to triangle wave  105 . Triangle wave (or sawtooth wave)  105  is typically a much higher frequency signal than input signal  102 —usually ten or more times the highest expected frequency of input signal  102 . Thus, if an audio signal of about 20 Hz to about 20 kHz is expected as input signal  102 , then triangle wave  105  should be at least about 200 kHz. As its name implies, PWM comparator  104  typically includes a comparator, and compares the amplitudes of input signal  102  to that of triangle wave  105  to form a pulse width modulated output. That is, the width of each pulse will be dependent upon the amplitude of both input signal  102  and the amplitude of triangle wave  105 , but at a frequency equal to that of triangle wave  105 . The duty cycle is proportional to the amplitude of input signal  102 . If both input signal  102  and triangle wave  105  are centered about 0 volts, then for a 0 volt input signal  102 , the output duty cycle is about 50%, and if input signal  102  is about V tmax , the maximum of triangle waveform  105 , the duty cycle is about 100%. If input signal  102  is about V tmin , the lowest voltage value of triangle waveform  105 , then the duty cycle is about 0%.  FIG. 2  illustrates an example of a typical pulse width waveform generated in the above-described manner. 
     Referring now to  FIG. 2 , input signal  102  is shown as a roughly sinusoidal shaped signal with a frequency of about 1/20 th   of that of triangle waveform  105 . Comparator stage output  106  is shown below input signal  102  that is super-imposed over triangle waveform  105 ; the logic level “high” represents the times in which the amplitude of input signal  102  exceeds that of triangle waveform  105 . It can be seen that at points A and B, the duty cycle of comparator stage output  106  (the pulse width modulated signal) is about 50%, and as input signal  102  swings closer to V tmax  (at point C) and V tmin  (at point D), the duty cycle of comparator stage output  106  changes from about 100% to about 0%, respectively. 
     As known by those of skill in the art, typical Class-D audio amplifiers with multiple channels typically comprise a global triangle ramp generator for use in all of the channels. A global analog buffer and a local analog buffer can be inserted between each channel and the triangle ramp generator. 
     Following PWM comparator  104 , comparator stage output  106  is input to switching output stage (or power stage)  108 , which is also part of modulation block  109 . This device is typically comprised of an arrangement of switching transistors configured as a “half-bridge” or “full-bridge” and it amplifies the signal input to it, to create switching output signal  110 . As can be seen in  FIG. 1 , switching output signal  110  is an amplified version of comparator stage output  106 , and switches between only two states, the positive and negative power supply rail voltages (in the case of the “half-bridge” implementation), and the positive power supply rail voltage and ground (in the case of the “full-bridge” implementation). As known to those of skill in the art, the gain of the typical Class-D audio amplifier modulator stage is set by the ratio of the power supply voltage, (or at least the power supply voltage that is available at switching output stage  108 ) and the peak-to-peak triangular wave voltage (triangle waveform  105 ). For example, the gain of the modulator stage, of a half-bridge implementation with +/−50V DC power supply rails, and a triangular wave voltage of 10 Vpp is 100V/10V=10. 
     Following switching output stage  108  is filter stage  112 . In filtering stage  112 , the amplified PWM signal is passed through an (ideally) lossless low pass filter (LPF) prior to the output device, speaker  116 . The LPF removes the high frequency components of the PWM signal (switching output signal  110 ) and recovers the original audio signal, but in an amplified form, now referred to as amplified output signal  114 . 
     Having briefly reviewed operation of a Class-D audio amplifier in a fairly general sense, attention can now be directed to specific design issues with regard to Class-D audio amplifiers. For example, it is known to those of skill in the art of power supplies to use current-mode control in buck-derived switch-mode power supplies. The principal benefits of current-mode control are the conversion of the double-pole passive inductor-capacitor (L-C) output filter of the Class-D audio amplifier topology into a single-pole output, greatly simplifying the task of feedback loop design, and achieving a stable control loop with adequate gain and phase margins. This feedback loop design task is especially difficult when the loop is closed at the output of the audio amplifier channel, after the L-C filter. However, it is after the L-C filter where the feedback loop must be closed, if state-of-the-art audio performance is desired. The goals of a state-of-the-art Class-D audio amplifier design involve achieving extremely low total harmonic distortion (THD) and noise levels, while maximizing dynamic range (signal-to-noise ratio (SNR)), frequency response, and flatness (at least over the expected frequency response). Additionally, for a Multi-Channel design, there is the requirement of minimizing channel-to-channel crosstalk. All of these design goals are aided by a feedback loop design utilizing negative feedback to reduce open loop errors. This feedback design needs to maintain a loop gain as high as possible over the audio frequency range (from about 20 Hz to about 20 KHz), and with as high a bandwidth as possible. This is made very difficult by the presence of the double-pole characteristic of the typical L-C output filter that is a part of most high-fidelity Class-D audio amplifier topologies. Current-mode control makes the design of this feedback loop more manageable, as described in greater detail below, as it simplifies the design of the feedback voltage amplifier. That is, a larger amount of passive components (such as resistors and capacitors) would be needed in the design of the feedback voltage amplifier. An additional characteristic, which is desirable for a multi-channel audio amplifier design, is the capability for two independent channels to be connected in a bridge-tied-load (BTL) configuration to drive a single speaker at up to four times the output power of a single channel. 
     As mentioned herein, the use of current-mode control is known to those of skill in the art of both switching power supplies and Class-D audio amplifiers. In the case of the latter, it is also known to those of skill in the art that current-mode control can be used as part of a double feedback loop system. However, many questions about such use remain unanswered, specifically, exactly how the current-mode control is implemented, whether peak current-mode control or average current-mode control is used, or how, and where, the current sensing signal is detected and processed. Further, there is no known knowledge or evidence from known usages of current-mode control in Class-D audio amplifiers about the suitability of the stated method of double-loop control in regard to multi-channel audio amplifier designs, in which it is desired to connect two independent channels in a BTL configuration. 
     Thus, there is a need for current-mode control of bridge tied load configurations of Class-D audio amplifiers in order to provide the benefits of both configurations in amplifying audio signals in a cost effective and efficient manner. 
     SUMMARY 
     It is to be understood that both the general and detailed descriptions that follow are explanatory only and are not restrictive of the embodiments. 
     The aspects of the embodiments described herein seek to overcome or at least ameliorate one or more of several problems, including but not limited to finding the ideal circuit location to sense current. 
     It is an object of the embodiments to substantially solve at least the problems and/or disadvantages discussed above, and to provide at least one or more of the advantages described below. 
     It is therefore a general aspect of the embodiments to provide systems, methods, and modes for current-mode feedback control of a multi-channel Class-D audio amplifier that will obviate or minimize problems of the type previously described. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     Further features and advantages of the aspects of the embodiments, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the aspects of the embodiments are not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
     According to a first aspect of the embodiments, a class-D audio amplifier is provided, comprising: current-mode control circuitry in a feedback loop adapted to control an output voltage of the audio amplifier, wherein the current-mode control circuitry monitors current though a common resistor common to both a current flow into a positive terminal of a loudspeaker and a current flow into a negative terminal of the loudspeaker to control the output. 
     According to the first aspect of the embodiments, the current-mode control circuitry reduces open-loop errors, and maintains relatively high loop gain over an expected audio frequency range, and wherein the expected audio frequency range ranges from about 20 Hz to about 20,000 Hz. 
     According to the first aspect of the embodiments, the current-mode control circuitry comprises: a current averaging current-mode control circuitry, and further wherein the current-mode control circuitry comprises: an inner current loop, the inner current loop comprising: a current path that flows from a power converter transformer, through an inductor, through the loudspeaker, through the common resistor, and returns to a center tap of the power converter transformer. 
     According to the first aspect of the embodiments, the audio amplifier further comprises voltage-mode control circuitry in the feedback look adapted to control an output of the amplifier, and wherein the voltage-mode control circuitry reduces open-loop errors, and maintains relatively high loop gain over an expected audio frequency range, and further wherein the expected audio frequency range ranges from about 20 Hz to about 20,000 Hz. 
     According to the first aspect of the embodiments, the voltage-mode control circuitry comprises: a voltage amplifier adapted to compare a divided output voltage at a first input to an input voltage at a second input, and output an error signal, based on a difference between the divided output voltage and the input voltage. 
     According to the first aspect of the embodiments, the error signal comprises: a positive slewing voltage when the divided output voltage is less than the input voltage, and wherein the amplifier further comprises: a current amplifier with a first input connected to the output of the voltage amplifier, and a second input connected to a voltage generated by the inner current loop at the common resistor, and wherein when the output voltage of the voltage amplifier slews positive, the current amplifier is further adapted to cause its output to slew positive. 
     According to the first aspect of the embodiments, the audio amplifier further comprises: a pulse width modulator (PWM), with a first input connected to a triangle wave generator output, and a second input connected to the output of the current amplifier, and wherein when the voltage output of the current amplifier at the second input is greater than the triangle wave at the first input, the PWM is adapted to output a high voltage level signal, thereby increasing a duty cycle of the output of the PWM, which causes the output voltage of the audio amplifier to slew positive. 
     According to the first aspect of the embodiments, when the output voltage of the audio amplifier increases, the inner current loop through the common resistor increases, and the voltage generated at the common resistor and which is input to the current amplifier at the second input slews negative, which compensates and balances the positive drive from the output of the voltage amplifier, which is input to the first input of the current amplifier. 
     According to the first aspect of the embodiments, the class-D amplifier is a dual channel class-D amplifier, and wherein there is a first and a second inner current loop, a first and second feedback loop, a first and second voltage-mode control circuitry, a first and a second common resistor, a first and a second voltage amplifier, a first and a second current amplifier, and a first and a second pulse width modulator. 
     According to the first aspect of the embodiments, both the first and second voltage-mode control circuitry in respective feedback looks are adapted to control an output voltage of the amplifier, and wherein each of the first and second voltage amplifiers [ 340 ] are adapted to compare a divided output voltage at a first input to an input voltage at a second input, and output an error signal, based on a difference between the divided output voltage and the input voltage, and wherein the error signal comprises a positive slewing voltage when the divided output voltage is less than the input voltage. 
     According to the first aspect of the embodiments, the first and second current amplifiers each include a first input connected to the output of their respective voltage amplifiers, and a second input connected to a voltage generated by their respective inner current loops at their respective common resistors, and wherein when the output voltage of respective voltage amplifiers slews positive, the respective current amplifiers are further adapted to cause its output to slew positive. 
     According to the first aspect of the embodiments, the first and second pulse width modulators (PWMs) each include a first input connected to a respective triangle wave generator output, and a second input connected to the output of respective current amplifiers, and wherein when the voltage output of each of the first and second current amplifiers at the second input is greater than the triangle wave at the first input, the respective PWM is adapted to output a high voltage level signal, thereby increasing a duty cycle of the output of the respective PWM, which causes the output voltage of the audio amplifier to slew positive. 
     According to the first aspect of the embodiments, when the output voltage of the audio amplifier increases, the inner current loop through the first and second common resistor increases, and the voltage generated at the first and second common resistors and which is input to respective current amplifiers at the second input slews negative, which compensates and balances the positive drive from the output of the respective voltage amplifiers, which is input to the first input of the respective current amplifiers. 
     According to the first aspect of the embodiments, the class-D amplifier is a bridge-tied load class-D amplifier adapted from a combination of two independent single ended half-bridge amplifier circuits with a common input, and wherein each channel of a multi-channel amplifier are driven by galvanically isolated power supply positive and negative rails. 
     According to the first aspect of the embodiments, the galvanically isolated power supply positive and negative rails comprises: a single power converter transformer having multiple, substantially identical, secondary windings, each of which includes respective separate center taps. 
     According to the first aspect of the embodiments, the audio amplifier further comprises a single common connection formed at a junction of a first and second common resistor to a ground plane of a circuit board that contains the audio amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the embodiments will become apparent and more readily appreciated from the following description of the embodiments with reference to the following figures. Different aspects of the embodiments are illustrated in reference figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered to be illustrative rather than limiting. The components in the drawings are not necessarily drawn to scale, emphasis instead being placed upon clearly illustrating the principles of the aspects of the embodiments. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram of a Class-D audio amplifier. 
         FIG. 2  illustrates a timing diagram of a triangle wave with an audio wave superimposed over it as inputs to a Class-D audio amplifier modulator, and a resultant output therefrom. 
         FIG. 3  is a circuit schematic that illustrates a portion of a Class-D audio amplifier with a current-mode feedback control circuit to monitor and control a loudspeaker according to aspects of the embodiments. 
         FIG. 4  is a circuit schematic that illustrates a plurality of current flow paths in a single channel of a dual channel Class-D audio amplifier with a current-mode feedback control circuit according to aspects of the embodiments. 
         FIG. 5  is a circuit schematic that illustrates a plurality of current flow paths in a bridge-tied load Class-D audio amplifier adapted from two independent single-ended half-bridge circuits with a common input and a current-mode feedback control circuit according to aspects of the embodiments. 
         FIG. 6  illustrates the circuit of  FIG. 3  with values for the discrete components according to aspects of the embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments are described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the inventive concept are shown. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like numbers refer to like elements throughout. The embodiments may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. The scope of the embodiments is therefore defined by the appended claims. The detailed description that follows is written from the point of view of a control systems company, so it is to be understood that generally the concepts discussed herein are applicable to various subsystems and not limited to only a particular controlled device or class of devices, such as media presentation products, which include such audio devices as Class-D amplifiers. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the embodiments. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular feature, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     LIST OF REFERENCE NUMBERS FOR THE ELEMENTS IN THE DRAWINGS IN NUMERICAL ORDER 
     The following is a list of the major elements in the drawings in numerical order.
           100  Typical Class-D audio amplifier     102  Input Signal     103  Triangle Wave Generator     104  Pulse Width Modulator (PWM) Comparator     105  Triangle Wave Generator Output     106  Comparator Stage Output     108  Switching Output Stage     109  Modulation Block     110  Switching Output Signal     112  Filter Stage     114  Amplified Output Signal     116  Speaker     300  Class-D Amplifier Circuit with Current-Mode Feedback Control (Class-D Amplifier)     302  Transformer     304  Diode D 1       306  Diode D 2       308  Capacitor C 1       310  Capacitor C 2       312  Capacitor C 3       314  Capacitor C 4       316  Capacitor C 5       318  Resistor/Impedance R Load        319  Switch S 1       320  Switch S 2       322  Resistor R 1       324  Resistor R 2       326  Resistor R 3       328  Resistor R 4       330  Resistor R 5       332  Resistor R 6       334  Resistor R 7       336  Inductor L 1       338  Gate Drive     340  Voltage Amplifier (VA)     342  Current Amplifier (CA)     344  Pulse Width Modulator (PWM)     346  Sawtooth Voltage Waveform Generator     400  Dual Channel Class-D Amplifier with Current-Mode Feedback Control (Dual Channel Audio Amplifier)     404  Diode D 3       406  Diode D 4       408  Capacitor C 6       410  Capacitor C 7       412  Capacitor C 8       414  Capacitor C 9       416  Capacitor C 10       418  Resistor/Impedance R Load        419  Switch S 3       420  Switch S 4       422  Resistor R 8       424  Resistor R 9       426  Resistor R 10       428  Resistor R 11       430  Resistor R 12       432  Resistor R 13       434  Resistor R 14       436  Inductor L 2       438  Gate Drive Channel  2       440  Voltage Amplifier (VA) Channel  2       442  Current Amplifier (CA) Channel  2       444  Pulse Width Modulator (PWM) Channel  2       446  Sawtooth Voltage Waveform Generator Channel  2       500  Bridge-Tied Load Class-D Amplifier Adapted from Two Independent Single Ended Half-Bridge Circuits (BTL Audio Amplifier)     502  Inverter       

     LIST OF ACRONYMS USED IN THE SPECIFICATION IN ALPHABETICAL ORDER 
     The following is a list of the acronyms used in the specification in alphabetical order.
         BTL Bridge-Tied Load   C Capacitor   CA Current Amplifier   D Duty Cycle   ICL Inner Current Loop   kHz Kilohertz   L Inductor   LPF Low Pass Filter   PWM Pulse Width Modulator/Modulation   SE Single Ended   SNR Signal-to-Noise Ratio   THD Total Harmonic Distortion   VA Voltage Amplifier   V I  Input Voltage   V O  Output Voltage   V PP  Voltage Peak-to-Peak       

     MODE(S) FOR CARRYING OUT THE INVENTION 
     The different aspects of the embodiments described herein pertain to the context of audio amplifiers, but is not limited thereto, except as may be set forth expressly in the appended claims. 
     Aspects of the embodiments implement current-mode feedback control in such a manner as to allow use of the bridge-tied-load (BTL) configuration in multi-channel Class-D audio amplifiers. At least one advantage in the use of current-mode feedback over voltage feedback is that it simplifies the design of the feedback voltage amplifier. That is, a higher amount of passive components (such as resistors and capacitors) would be needed in the design of the feedback voltage amplifier, as opposed to that of the current-mode feedback control system, described herein according to aspects of the embodiments. 
     According to an aspect of the embodiments, current-mode feedback control is added to a Class-D audio amplifier that uses a half-bridge topology. The single-ended (SE) half-bridge topology has the benefit of allowing two independent outputs (i.e., channels) of a multi-channel audio amplifier to be connected (externally, by the user) as a single, bridge-tied-load (BTL) output. This has the advantage of quadrupling the available output power to a given load impedance per speaker. According to the aspects of the embodiments,  FIGS. 3-5  illustrate how to add current sensing in a circuit location such that it does not interfere with this BTL feature, yet still provides all the benefits of current-mode control. According to aspects of the embodiments, use of average current-mode feedback control provides the greatest design flexibility and the ability to work well with fixed-frequency designs. 
     The use of average current-mode control provides more design flexibility, according to aspects of the embodiments, because the inner current loop parameters, such as gain and bandwidth, are better controlled, which gives better audio performance in terms of THD and noise. In addition, average current-mode control further allows for a fixed frequency design, which while possible to do with peak current mode control, is substantially more complicated to implement in the latter case. 
       FIG. 3  is a circuit schematic that illustrates a portion of a Class-D audio amplifier with a current-mode feedback control circuit to monitor and control a loudspeaker output according to aspects of the embodiments.  FIG. 3  illustrates portions of Class-D audio amplifier  300  with a current-mode control feedback circuit to monitor and control the loudspeaker output, V out .  FIG. 3  also illustrates the inner current loop (ICL) and outer voltage loop, comprising the total feedback loop, capable of delivering the performance goals of a state-of-the-art Class-D audio amplifier as described above. The inner current loop, shown in  FIG. 3  as ICL as a dashed line, is formed by current flowing out of L 1   336 , through C 3   312  and the load, which is loudspeaker RL  318  (note that although a current does flow through R 6   332  and R 7   334 , because of the high input impedance of VA  340 , this current is negligible in view of the current that flows through capacitor C 3   312  and R load    318 ). The inner current returns to point Z through R 1   322 , and point Z is the center tap of transformer  302 . A voltage drop forms across R 1   322  and this is input to the inverting input of current amplifier (CA)  342  through R 2   324 . 
     The outer voltage loop of the circuit  300  of  FIG. 3  comprises voltage amplifier (VA)  340 ; VA  340  senses the divided output voltage, V Out-Divided , via voltage divider R 6   332  and R 7   334 , and compares it to the input signal, V in . A difference between these signals results in an error signal which then gets amplified by VA  340 . A brief discussion of the principles of operation of the voltage and current control loops will now be provided. 
     According to a first condition, if V out  is not positive enough (V out  is input to the inverting input of VA  340  through resistors R 6   332  and R 7   334 ), i.e., is less positive than that of V in  (the non-inverting input of VA  340 ), the output of VA  340  will slew positive. The output of VA  340  is input to CA  342 , and the positive slewing input of CA  342  at the non-inverting input will cause the output of CA  342  to slew positive. The positive slewing output of CA  342  is then applied to the non-inverting input of PWM  344 , which comprises, in part, a comparator. When the voltage at the non-inverting terminal of PWM  344  is greater than V ref  (sawtooth voltage waveform  346 ), at the inverting terminal, V out  of PWM  344  is high. When this occurs, the duty cycle of the output of PWM  344  increases. This causes V out  to the speaker (R Load    318 ) to slew positive (see,  FIG. 2 ). Note, also, that this example begins with V out  being smaller than V in . Now, through operation of the voltage and current control circuits, it is being forced in an opposite manner. A positive slewing V out  causes a greater current flow through R 1   322 . As the current increases through R 1   322 , this causes the voltage at point Z to slew negative, and the voltage at the non-inverting input of CA  342  to slew negative, compensating and balancing the positive drive from the outputs of VA  340 , thus compensating for the original error that drove the loop. Since CA  342  is an integrating or averaging amplifier (because of the capacitors in the feedback loop), it will have very high gain at low frequencies, and the accuracy of the feedback loop will be extremely good according to aspects of the embodiments (i.e., high open-loop gain). 
       FIG. 4  is a circuit schematic that illustrates a plurality of current flow paths in a first channel of a dual channel Class-D audio amplifier with a current-mode feedback control circuit according to aspects of the embodiments.  FIG. 4  illustrates a plurality of current flow paths in a single channel of a dual channel Class-D audio amplifier constructed according to aspects of the embodiments. In  FIG. 4 , the current direction entering the “+” terminal of the speaker and exiting the “−” terminal of the speaker is represented by the solid arrows (denoted “positive terminal speaker current flow”), and the current direction entering the negative-terminal of the speaker and exiting the positive terminal of the speaker is represented by the dashed arrows (denoted “negative-terminal speaker current flow”). Note that both of the positive-terminal speaker current flow and the negative-terminal speaker current flow go through resistor R 1   322 . According to aspects of the embodiments, current sensing resistor R 1   322  is at an optimal location to develop a current signal suitable for the control of the feedback loop as it is included in both of the current flow directions, as can be seen in  FIG. 4 . To reduce power losses in current sensing resistor R 1   322 , which would degrade power conversion efficiency, the resistor&#39;s voltage can be boosted by a suitable high frequency voltage operational amplifier (not shown in  FIG. 4 ). One such VA is an LMH6639, manufactured by Texas Instruments, although others can also be used. Note that the configuration of each channel of dual channel amplifier  400  is substantially similar to that of amplifier  300  as shown in  FIG. 3 , and therefore operates in a substantially similar manner with respect to current-mode and voltage-mode control. 
       FIG. 5  is a circuit schematic that illustrates a plurality of current flow paths in a bridge-tied load (BTL) Class-D audio amplifier configured from two independent single-ended half-bridge circuits with a common input and a current-mode feedback control circuit (BTL audio amplifier  500 ) according to aspects of the embodiments.  FIG. 5  illustrates a BTL Class-D audio amplifier configured from two independent single-ended half-bridge circuits with a common input according to an aspect of the embodiments. According to aspects of the embodiments, the configuration of  FIG. 5  illustrates a dual channel Class-D amplifier, wherein the two channels are configured by a user in a BTL mode to drive a single speaker with two channels, thus driving the speaker at or about four times the power of a single channel driving a single speaker. Shown in  FIG. 5  are a plurality of positive-terminal and negative-terminal current paths, similar to that as shown in  FIG. 4 . As with the embodiment illustrated in  FIGS. 3 and 4 , it can be seen that current sensing resistors R 1   322  and R 8   422  provide an optimal location for producing the desired current sensing signal. According to aspects of the embodiments, the optimal location is determined by the point in the current path that encompasses both a positive-terminal current flow and a negative-terminal current flow. In this manner, therefore, the feedback control loop can use average current-mode control to deliver the desired state-of-the-art characteristics of a Class-D audio amplifier, even when configured in BTL mode of operation. Note that the configuration of each channel of BTL audio amplifier  500  is substantially similar to that of amplifier  300  as shown in  FIG. 3 , and therefore operates in a substantially similar manner with respect to current-mode and voltage-mode control. 
     According to further aspects of the embodiments, one condition for successful implementation of the BTL mode of operation, as shown in  FIG. 5 , is that all channels of the multi-channel design be supplied by separate (e.g., galvanically isolated) power supply positive and negative rails. This can be readily accomplished by the use of a single power converter transformer (T 1   302 ) having multiple, identical, secondary windings. Referring to both  FIGS. 4 and 5 , there is a single primary winding P 0 , and multiple (in this case two) secondary windings, S 1  and S 2 , which are separate not only from the primary winding P 0 , but also from each other. Further, in each of the secondary windings S 1 , S 2 , there are respective center-taps, CT 1 , CT 2 . The sole common connection of the center-tap of said secondary windings is through the current sensing resistors of each channel (R 1   322  and R 8   422 ), to the common ground plane of the printed circuit board. All channels of the multi-channel audio amplifier share this same reference ground plane (GND; note, however, that this is not the same as the chassis or earth ground). Those skilled in the art of analog design, however, will recognize that having a common ground plane among multiple channels eases signal interfacing between those channels. 
     As those of skill in the art can appreciate, the values of the discrete components have been omitted from the drawing Figures; however, as those of skill in the art can no doubt appreciate, the exact values of the discrete components are not needed in order to understand and appreciate the aspects of the embodiments; thus, in fulfillment of the dual purposes of clarity and brevity, such values have been omitted from herein. 
     INDUSTRIAL APPLICABILITY 
     To solve the aforementioned problems, certain aspects of the embodiments are directed towards a Class-D audio amplifier arranged in a BTL configuration that uses an average current-mode feedback control circuit to control the output of the audio amplifier. 
       FIG. 6  illustrates the circuit of  FIG. 3  with values for the discrete components according to aspects of the embodiments. As those of skill in the art can now appreciate, based on the discussion herein, the circuits of  FIGS. 4 and 5  are substantially similar to the circuit of  FIG. 3 , albeit in different modes and configurations, as described above, and therefore the values and tolerances of similar discrete components in  FIGS. 4 and 5  would also be substantially similar to the values and tolerances of the discrete components of  FIG. 3 . As such, the values and tolerances of similar discrete components in  FIGS. 4 and 5  have not been repeated herein in fulfillment of the dual purposes of clarity and brevity. 
     The disclosed embodiments provide a system and method for controlling the output of a Class-D BTL audio amplifier, as well as single-ended Class-D audio amplifier configurations. It should be understood that this description is not intended to limit the embodiments. On the contrary, the embodiments are intended to cover alternatives, modifications, and equivalents, which are included in the spirit and scope of the embodiments as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth to provide a comprehensive understanding of the claimed embodiments. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of aspects of the embodiments are described being in particular combinations, each feature or element can be used alone, without the other features and elements of the embodiments, or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. 
     The above-described embodiments are intended to be illustrative in all respects, rather than restrictive, of the embodiments. Thus the embodiments are capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the embodiments unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. 
     All United States patents and applications, foreign patents, and publications discussed above are hereby incorporated herein by reference in their entireties. 
     Alternate Embodiments 
     Alternate embodiments may be devised without departing from the spirit or the scope of the different aspects of the embodiments.