Patent Publication Number: US-11041890-B2

Title: Current measurement at a switching amplifier output

Description:
TECHNICAL FIELD 
     The present disclosure, in accordance with one or more embodiments, relates generally to signal processing and, more particularly for example, to sensing a current at an output of a switching amplifier. 
     BACKGROUND 
     Many modern devices such as laptop computers, computer tablets, MP3 players, and smart phones use miniature speakers. In many applications, these devices utilize switching amplifiers to efficiently provide for amplification of an audio signal. In one example, a switching amplifier may provide twenty watts of power to amplify an audio signal and drive a speaker. Due to limitations of miniature speakers used in such devices, the current to the speakers may be measured to aid in the prevention of distortion, physical damage to the speaker and other unwanted effects. Thus, there is a continued need to improve the measurement of current provided to the speaker by the switching amplifier in order to protect the speaker from distortion or damage. 
     SUMMARY 
     The present disclosure provides systems and methods that address a need in the art for accurate sensing of a current provided to a load by a switching amplifier. The scope of the present disclosure is defined by the claims, which are incorporated into this section by reference. A more complete understanding of the present disclosure will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the disclosure and their advantages can be better understood with reference to the following drawings and the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures, where showings therein are for purposes of illustrating embodiments of the present disclosure and not for purposes of limiting the same. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. 
         FIG. 1  illustrates an exemplary audio codec in accordance with one or more embodiments of the disclosure. 
         FIG. 2  illustrates a schematic diagram of an exemplary audio amplifier output driver in accordance with one or more embodiments of the disclosure. 
         FIG. 3  illustrates exemplary plots of control voltages and sensed current of an audio amplifier output driver in accordance with an embodiment of the disclosure. 
         FIG. 4  illustrates a schematic diagram of an exemplary audio amplifier output driver including a sample and hold circuit in accordance with one or more embodiments of the disclosure. 
         FIG. 5  illustrates an exemplary process flow for an audio amplifier output driver speaker protection system in accordance with one or more embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes systems and methods that address a need for accurate measurement of a current provided by a switching amplifier such as a switching regulator or a class-D switching amplifier. In one embodiment, an audio system of the present disclosure includes a switching amplifier H-bridge output stage, and one or more output current measurement circuits. Each output current measurement circuit includes a current sensing component, such as a current mirror circuit, and a shielding switch arranged to provide for accurate measurement of the current traveling through a load during all phases of amplifier switching including switching amplifier output state transitions. 
     Embodiments of the present disclosure may be contrasted to pre-existing solutions for measuring a current at an output of a switching regulator or class-D switching amplifier. For example, conventional switching amplifier current sensing circuits may use a sense resistor installed in series with a load to sense a current traveling through the load. A sensing amplifier connected to the sensing resistor may require a relatively large common mode rejection ratio to process large switching amplifier output voltage variations at the sense resistor. Many passive current sense circuits reduce efficiencies in the operation of an H-bridge amplifier output stage through additional power dissipation losses within the system. Moreover, added power dissipation may cause thermal issues for applications that include switching amplifier circuits formed within an integrated circuit die. 
     Some conventional audio systems use a current mirror circuit for load current sensing measurements. However, performance of current mirror circuits may be affected by similar common mode limitations as discussed herein. For example, a conventional current mirror circuit used with switching amplifier applications may be subject to large voltage swings, requiring a current mirror circuit with a relatively high common mode rejection ratio. Moreover, the current mirror circuit does not provide an accurate current measurement when switching transistors are in transition between “on” and “off” states. Various embodiments of the present disclosure address these issues to accurately and effectively measure the current provided to the load by a switching amplifier in order to protect the load, such as a speaker, from distortion or damage. 
       FIG. 1  illustrates a block diagram of an exemplary audio codec circuit  100  in accordance with one or more embodiments of the disclosure. Audio codec circuit  100  provides analog and digital circuitry for signal processing of audio inputs. Audio codec circuit  100  includes circuitry to process input digital signals and provide amplified output signals to a speaker, for example speaker  121 . In some embodiments, audio codec circuit  100  receives digital signals at input ports  105 A-B. Digital signals may be provided, for example, by any electronic device such as a laptop computer, a computer tablet, a smart phone, or a sensor such as a microphone. 
     Digital-to-analog converter (DAC)  107  may be configured to receive digital signals and convert the digital signals to analog signals for further processing. Control circuit  109  receives analog audio signals from DAC  107  and processes the analog audio signals. In some embodiments, control circuit  109  provides pulse width modulated signals to audio amplifier  108 . In some embodiments, audio amplifier  108  is implemented as a class-D switching amplifier and pulse width modulated signals control a switching duty cycle of audio amplifier  108 . Audio amplifier  108  amplifies the received analog audio signals and provides amplified audio signals  131 A-B to drive an output device  121  at output jacks  119 A-B. Output device  121  may be a loudspeaker, headphones or another electronic device for receiving the amplified audio signals  131 A-B. 
     Audio amplifier  108  is electrically coupled to current measurement circuit  110 . Current measurement circuit  110  is configured to sense the current signal traveling to output device  121  at a low side output switch of audio amplifier  108 . In some embodiments, current measurement circuit  110  provides an approximation of the current of the current signal traveling through output device  121 . In some embodiments, a current mirror circuit within current measurement circuit  110  provides for and measures the equivalent current. As illustrated, a measured current signal  120  is provided to an overcurrent protection circuit  117 . In some embodiments, overcurrent protection circuit  117  adjusts a frequency of the pulse width modulated signals to reduce a magnitude of the current traveling to output device  121 , if measured current signal  120  exceeds an upper current threshold. The upper current threshold may be the maximum current an output device  121  can withstand without distortion or physical damage and may be dependent on the materials and processes used to manufacture output device  121 . For example, a miniature speaker used in modern electronic devices may be capable to withstand approximately five hundred milliampere steady state. In other embodiments, overcurrent protection circuit  117  provides an overcurrent control signal  118  to turn off audio amplifier  108  if the measured current signal  120  exceeds the upper current threshold. 
     Current measurement circuit  110  is operable to provide an equivalent current of the instantaneous current traveling to output device  121  while the low side output switch is active. Current measurement circuit  110  is also operable to provide an equivalent current of the load current when the low side output switch and the high side output switch are transitioning between an “on” and “off” state. In this regard, current measurement circuit  110  and overcurrent protection circuit  117  robustly protect output device  121  from instantaneous distortion or physical damage by comparing the measured current signal  120  to the upper current threshold and acting to adjust a current in response to the threshold being exceeded. 
     Current measurement circuit  110  may also provide an analog voltage equivalent of measured current signal  120  to a speaker protection circuit  111 . In the illustrated embodiment, an analog-to-digital converter (ADC)  113  converts the analog voltage to a digital voltage signal  122  that represents the measured current signal  120 . ADC  113  provides the digital voltage signal  122  to the speaker protection circuit  111  which further processes the digital voltage signal  122 . In some embodiments, speaker protection circuit  111  provides DAC  107  with signals  114  to adjust DAC  107  signal processing based on measured current feedback to protect the speaker  121 . 
       FIG. 2  illustrates a schematic diagram of an exemplary audio amplifier output driver  200  in accordance with an embodiment of the disclosure. In some embodiments, audio amplifier output driver  200  forms part of audio amplifier  108  that is implemented in audio codec circuit  100 . Audio amplifier output driver  200  provides an audio output to drive a speaker load  235 , which may be implemented in a mobile phone, laptop computer, tablet, audio/video system, or other similar device. In various embodiments, audio amplifier output driver  200  is implemented as a class-D amplifier H-bridge output stage  201 . Audio amplifier output driver  200  is coupled to one or more current measurement circuits  210 . 
     As shown in  FIG. 2 , in some embodiments, H-bridge output stage  201  includes four n-channel laterally diffused metal oxide semiconductor field-effect transistors (MOSFET) M 1 , M 2 , M 3 , and M 4 . The respective drains of the first two high side transistors M 3 , M 4  are connected to a supply voltage Pvdd. In some embodiments, supply voltage Pvdd provides twelve volts DC power to transistors M 3 , M 4 . However, other power supply voltages may be provides in other embodiments. The respective sources are connected to drains of two low side transistors M 1 , M 2  whose sources are connected to ground signal  221 . A speaker load  235  is connected between transistor switch pairs M 3 , M 1  and M 4 , M 2 . Control circuit  109  of  FIG. 1  may provide pulse width modulated control signals  202  to gates of transistors M 1 , M 2 , M 3 , and M 4 . In some embodiments, a first pulse width modulated (PMW) control signal  202  is connected to a gate terminal of transistor M 3 , a second PMW control signal  202  is connected to a gate terminal of transistor M 1 , a third PMW control signal  202  is connected to a gate terminal of transistor M 4 , and a fourth PMW control signal  202  is connected to a gate terminal of transistor M 2 . 
     In some embodiments, a first current measurement circuit  210  includes a current mirror amplifier  211  (e.g., a current sensing circuit), n-channel MOS transistors S 1  and S 2 , a shielding switch  224 , and a pull-down resistor  225 . In this arrangement, the current Ispk traveling through speaker load  235  is represented by an equivalent measured current Isensep and Isensen. 
     Current mirror amplifier  211  includes two input terminals, non-inverting input terminal  212  and inverting input terminal  214 . Non-inverting input terminal  212  is connected to a source terminal of shielding switch  224 . A drain terminal of shielding switch  224  is connected to source terminal of transistor M 3  (e.g., a first transistor switch) and drain terminal of transistor M 1  (e.g., a second transistor switch). Inverting input terminal  214  of current mirror amplifier  211  is connected to source terminal of transistor S 1  and drain terminal of transistor S 2 . Current mirror amplifier  211  output signal  216  is connected to gate terminal of transistor S 1  to drive transistor S 1 . Source terminal of transistor S 2  is connected to ground signal  221 . Drain terminal of transistor S 1  is connected to Isensep current signal. 
     Shielding switch  224  gate terminal is connected to gate terminal of low side transistor M 1 . As second PWM control signal  202  turns on transistor M 1 , shielding switch  224  turns on in response to second PWM control signal  202  and provides a small signal DC voltage at node Va that is equivalent to the voltage at drain terminal of M 1 . In some embodiments, small signal DC voltage is approximately fifty to one hundred millivolts. Node Va is connected to non-inverting input terminal of  212  of current mirror amplifier  211  to provide voltage Va to current mirror amplifier  211 . Current mirror amplifier  211  output signal  216  controls a gate voltage of S 1  to adjust a drain-source voltage across S 2 . In this regard, the voltage across transistor M 1 , and equivalently at node Va, is mirrored across transistor S 1  to provide a Isensep current signal flowing through switches S 1  and S 2  that is approximately equal to load current Ispk. In some embodiments, current mirror amplifier  211  is implemented as a laterally diffused metal oxide semiconductor circuit. Pull-down resistor  225  is connected between node Va (e.g., at source terminal of shielding switch  224 ) and ground signal  221  to provide for a fast transition to zero volts at node Va when shielding switch  224  is turned off. 
     In some embodiments, a complementary second current measurement circuit  210 B includes a current mirror amplifier  211 B, n-channel MOS transistors S 3  and S 4 , a shielding switch  224 B, and a pull-down resistor  225 B. Current Ispk traveling through speaker load  235  at the H-bridge complementary transistor pair (e.g., M 4  and M 2 ) is represented by an equivalent measured current Isensen. 
     Current mirror amplifier  211 B includes two input terminals, non-inverting input terminal  215  and inverting input terminal  217 . Non-inverting input terminal  215  is connected to a source terminal of shielding switch  224 B. A drain terminal of shielding switch  224 B is connected to source terminal of transistor M 4  (e.g., a third transistor switch) and drain terminal of transistor M 2  (e.g., a fourth transistor switch). Inverting input terminal  217  of current mirror amplifier  211 B is connected to source terminal of transistor S 3  and drain terminal of transistor S 4 . Current mirror amplifier  211 B output signal  219  is connected to gate terminal of transistor S 3  to drive transistor S 3 . Source terminal of transistor S 4  is connected to ground signal  221 . Drain terminal of transistor S 1  is connected to Isensen current signal. 
     Shielding switch  224 B gate terminal is connected to gate terminal of low side transistor M 2 . As fourth PWM control signal  202  turns on transistor M 2 , shielding switch  224 B turns on in response to fourth PWM control signal  202  and provides a small signal DC voltage of approximately fifty millivolts at node Vab connected to source terminal of shielding switch  224 B. Node Va is connected to non-inverting input terminal of  215  of current mirror amplifier  211 B to provide voltage Va to current mirror amplifier  211 B. Current mirror amplifier  211 B output signal  219  controls a gate voltage of S 3  to control a drain-source voltage at S 4  and provide current Isensen that mirrors load current Ispk. Pull-down resistor  225 B is connected between node Vab and ground signal  221  to provide for a fast transition to zero volts at node Vab when shielding switch  224   b  is turned off. Power supply Avdd is connected to gates of transistors S 2  and S 4  to turn on transistor S 2  and S 4  when audio amplifier output driver  200  is powered on. 
     As shown in  FIG. 2 , transistors S 2  and S 4  mirror the current flowing in speaker load  235 , as discussed herein. Speaker load  235  is connected between source of M 3  and drain of M 1  on a first end and source of M 4  and drain of M 2  on a second end. Transistor S 2  mirrors a current flowing through transistor M 1  (e.g., a second transistor switch) during the PWM cycles when M 1  is conducting. Transistor S 4  mirrors a current flowing through transistor M 2  (e.g., a fourth transistor switch) during the PWM cycles that M 2  is conducting. In this regard, a speaker load  235  current is sensed for the complete range of Ispk current when Ispk flows through combined transistors M 1  and M 2 . 
       FIG. 3  illustrates plots of control voltages and sensed current of an audio amplifier output driver in accordance with an embodiment of the disclosure.  FIG. 3  shows a plot  305  of gate voltage, Vgate, at gate terminals of transistor switch M 1  and shielding switch  224  during a first transition  340  and a second transition  340 B. As shown, first transition  340  illustrates Vgate transitioning from zero volts to five volts. Second transition  340 B illustrates Vgate transitioning from five volts to zero volts. During first transition  340 , transistor switch M 3  (e.g., first transistor switch) is turning off and transistor switch M 1  (e.g., second transistor switch) is turning on. During second transition  340 B, transistor switch M 3  (e.g., first transistor switch) is turning on and transistor switch M 1  (e.g., second transistor switch) is turning off. Second PWM control signal  202  controls turn on and turn off of transistor switch M 1  and shielding switch  224 . Fourth PWM control signal  202  controls turn on and turn off of transistor switch M 2  and shielding switch  224 B. 
     Plot  310  illustrates a voltage, Vout, at source terminal M 3  and drain terminal M 1  during the same transitions of plot  305 . Plot  310  shows Vout transitioning from twelve volts (e.g., Pvdd) to zero volts caused by PWM control signal  202  turning off transistor switch M 3  and turning on transistor switch M 1 . Referring again to plot  305 , the voltage at Vgate moves from zero volts to approximately 1.3 volts and finally to five volts during this same first transition  340 . 
     Plot  315  illustrates node voltage Va during the same transitions  340  and  340 B. As shown in plot  315 , Va is a steady fifty millivolts during all transitions and including a transistor switch M 1  “on” time during transition  341 . In this regard, shielding switch  224  provides a small signal voltage (e.g., fifty millivolts) at non-inverting input terminal of current mirror amplifier  211  during transitions  340 ,  341 , and  340 B unaffected by voltage transitions of Vout and Vgate. Thus, as shown by plot  320 , current measurement circuit  210  provides a measured Isensep value that is accurate and stable during transitions  340 ,  341 , and  340 B. 
       FIG. 4  illustrates a schematic diagram of an exemplary audio amplifier output driver  200  including a sample and hold circuit  425  in accordance with an embodiment of the disclosure. Sample and hold circuit  425  is arranged to receive the small DC signal voltage (e.g., such as small signal DC voltage of approximately fifty millivolts) from the source of shielding switch  224  and provide the small DC signal voltage to the current mirror amplifier  211  for a pre-determined sample time period. As shown in  FIG. 4 , sample and hold circuit  425  is coupled between source terminal of shielding switch  224  and non-inverting input terminal  212  of current mirror amplifier  211 . 
     In some embodiments, sample and hold circuit  425  is implemented as a capacitor, field effect transistor switch and an operational amplifier. For example, the operational amplifier charges or discharges the capacitor to approximately the voltage level at the input, such as the small signal voltage. The charged voltage is switched to an output of sample and hold circuit  425  and provided to non-inverting input terminal  212  of current mirror amplifier  211  for the pre-determined sample time period. 
     Sample and hold circuit  425  includes a trigger circuit  420  configured to provide the small signal voltage to the current mirror amplifier  211  in response to second modulated pulse control signal  202 . In some embodiments, sample and hold circuit  425  is operable to provide the small signal voltage for a time equal to second modulated pulse control signal  202  time period. In other embodiments, the small signal voltage is provided to current mirror amplifier  211  for a time less than the time period of the second modulated pulse control signal  202 . In this regard, sample and hold circuit  425  holds the small signal voltage at the current mirror amplifier  211  to enable measurement of current Isensep (e.g., or Isensen for the complementary circuit) for a pre-determined sample time period. Second current measurement circuit  210 B includes a second sample and hold circuit  425 B and its corresponding trigger circuit  420 B connected between shielding switch  224 B and current mirror amplifier  211 B to perform the sample and hold function described herein. 
       FIG. 5  illustrates an exemplary process flow for an audio amplifier output driver speaker protection system in accordance with an embodiment of the disclosure. In block  510 , an amplified audio signal is received at an output of audio amplifier output driver  200 . Audio amplifier output driver  200  includes an H-bridge output stage  201  including two high side/low side output transistor switch pairs, each pair connected to a respective end of speaker load  235  to conduct a current through speaker load  235 . In some embodiments, each high side transistor switch is connected to a twelve volt DC power supply and each low side transistor switch is connected to ground signal  221  to drive speaker load  235 . 
     In block  520 , the flow diagram continues with providing the amplified audio signal to a speaker load  235 . For example, a first pulse width modulated control signal is coupled to a gate terminal of a first transistor switch (e.g., high side switch M 3 ) to control an “on” and “off” state of the first transistor switch. A second pulse width modulated control signal is coupled to a gate terminal of a second transistor switch (e.g., low side switch M 1 ) to control an “on” and “off” state of the second transistor switch. H-bridge output stage  201  includes a complementary high side/low side transistor switch pair (e.g., M 4 /M 2 ) connected to a second end of speaker load  235  and are controlled by complementary pulse width modulated control signals  202 . 
     In block  530 , the flow diagram continues with biasing current mirror circuitry using a shielding switch  224 . Shielding switch  224  provides a small signal DC voltage (e.g., approximately fifty millivolts) at non-inverting input terminal of current mirror amplifier  211  to provide the small signal DC voltage at current mirror amplifier  211  non-inverting input terminal  212  during transitions between “off” and “on” states of the first and second switching transistors, and “on” state of second transistor switch (e.g., low side transistor switch). In this regard, shielding switch  224  provides a small signal voltage (e.g., fifty millivolts) unaffected by switching voltage transitions of transistor switches. 
     In block  540 , the flow diagram continues with current measurement circuit  210  sensing the current traveling through speaker load  235 . Current mirror amplifier  211  provides for accurate current measurements of a representative current signal, Isensep, that is approximately equal to the current flowing through speaker load  235 . Current measurement circuit  210  provides a measured current value that is accurate and stable during transitions of switching transistors as a result of shielding switch  224 , as discussed herein. H-bridge output stage  201  includes a complementary second current measurement circuit  210 B configured to sense the equivalent speaker current Isensen at the complementary high side/low side switch pair. In this regard, a speaker load  235  current is sensed for the complete range of speaker current comprising Isensep and Isensen. 
     In block  550 , current measurement circuit  210  provides the measured currents Isensep and Isensen to an overcurrent protection circuit  117 . In some embodiments, overcurrent protection circuit  117  may adjust a frequency of the first and second pulse width modulated control signals to reduce the current traveling through the speaker load  235  when speaker current Ispk exceeds an upper current threshold. 
     In some embodiments, current measurement circuit  210  may provide analog voltage signals of measured currents Isensep and Isensen to an ADC  113  for conversion to digital sense signals that are passed to a speaker protection circuit  111 . Speaker protection circuit  111  may process the digital sense signals and provide gain adjustments to DAC  107  to adjust a speaker load  235  current at outputs of audio amplifier output driver  200 . 
     Where applicable, various embodiments provided by the present disclosure may be implemented using hardware, software, or combinations of hardware and software. Also, where applicable, the various hardware components and/or software components set forth herein may be combined into composite components comprising software, hardware, and/or both without departing from the spirit of the present disclosure. Where applicable, the various hardware components and/or software components set forth herein may be separated into sub-components comprising software, hardware, or both without departing from the scope of the present disclosure. In addition, where applicable, it is contemplated that software components may be implemented as hardware components and vice-versa. 
     Software, in accordance with the present disclosure, such as program code and/or data, may be stored on one or more computer readable mediums. It is also contemplated that software identified herein may be implemented using one or more general purpose or specific purpose computers and/or computer systems, networked and/or otherwise. Where applicable, the ordering of various steps described herein may be changed, combined into composite steps, and/or separated into sub-steps to provide features described herein. 
     The foregoing disclosure is not intended to limit the present disclosure to the precise forms or particular fields of use disclosed. As such, it is contemplated that various alternate embodiments and/or modifications to the present disclosure, whether explicitly described or implied herein, are possible in light of the disclosure. Having thus described embodiments of the present disclosure, persons of ordinary skill in the art will recognize that changes may be made in form and detail without departing from the scope of the present disclosure. Thus, the present disclosure is limited only by the claims.