Audio amplifier with duty ratio control

An audio amplifier with duty ratio control is provided. The audio amplifier comprises a pulse width modulation modulator, a power stage, and a voltage converter. The pulse width modulation modulator is configured to receive an input signal for generating a pulse width modulation signal. The power stage is configured to output an output signal according to a supply voltage and the pulse width modulation signal. The voltage converter is configured to adjust voltage level of the supply voltage according to the pulse width modulation signal. The audio amplifier is configured to adjust the voltage level of the supply voltage when duty ratio of the pulse width modulation signal is greater than a duty ratio threshold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to audio amplifiers, and in particular to an audio amplifier that adaptively adjusts the supply voltage PVDD to make the audio amplifier operate at an optimal operating point.

2. Description of the Related Art

Audio amplifiers are useful in electronic circuits that are used to reproduce input audio signals at sound voice output, so the amplifier must have volume level control, lower harmonic distortion, and higher efficiency performance. Beside, Class D is currently the optimal solution for better possible efficiency, low-cost, and light weight than other classes. Usually, pulse width modulation is applied to control the output devices tightly at Class-D amplifier system.

For improved efficiency concepts, Class-H amplifier is variations upon the theme of Class A/B, which works on power supply voltage section. The Class-H amplifier system utilizes a lower rail voltage, significantly reducing power consumption at low-power conditions, and the system dynamically controls rail voltage to driver high amplitude transients as high-power conditions.

Refer toFIG.1A, which is a drawing illustrating a conventional Class-D amplifier circuit. As shown inFIG.1A, the conventional Class-D amplifier100comprises an audio pulse width modulation (PWM) modulator110, a pre-driver120, and a voltage converter130.

The audio pulse width modulation (PWM) modulator110receives an audio signal (IN) at its input. The output of the audio pulse width modulation (PWM) modulator110is electrically connected to the input of the pre-driver120. The pre-driver120amplifies the audio signal and outputs the amplified audio signal to a pair of power switches140,150.

The voltage converter130receives an input voltage VIN and outputs a fixed supply voltage PVDD. The output is connected to an upper power switch140.

One end of an inductor (L)160is connected between the upper power switch140and the lower power switch150. The other end of the inductor160is connected to a speaker180to allow the audio to be output by the speaker. A capacitor170is connected in parallel with the speaker.

Refer toFIG.1B, which is a drawing illustrating a standalone Class-H amplifier circuit based on a conventional Class-D amplifier.

Some of the components of the Class-H amplifier circuit200illustrated inFIG.1Bare the same as the components illustrated inFIG.1A. The Class-H amplifier circuit200differs from the Class-D amplifier circuit100in that inFIG.1B, the audio signal (IN) is fed to a voltage converter131by an envelope detector190so that the envelope of the audio signal can be tracked. In addition, the voltage converter131is configured to output a supply voltage PVDD variably according to an output of the envelope detector190.

Refer toFIG.1C, which illustrates the signals of supply voltages PVDD of the circuits ofFIG.1AandFIG.1Band the audio signal.

As shown inFIG.1C, the supply voltage PVDD of Class-D is a fixed voltage, represented by dashed lines.

In contrast, the supply voltage PVDD, represented by solid curved lines, of the Class-H amplifier is a voltage that tracks the envelope of the audio signal (IN), represented by a gray curve.

Refer toFIG.2, which is a graph illustrating an efficiency comparison of a Class-H amplifier versus a Class-D amplifier in an example.

As shown inFIG.2, the Class-H amplifier improves the power conversion efficiency about 15% in this example over the Class-D amplifier designed around a fixed voltage. Accordingly, Class-H amplifier technology is an excellent design choice for the applications with battery voltage limitations. The Class-H amplifier utilizes typical audio tracking to adjust the supply voltage PVDD that can reduce the power switching loss caused by higher cross-voltage and improve the power conversion efficiency as compared with the Class-D amplifier.

As such, inFIG.1B, the amplifier circuit200has multiple-level switchable or variable supply voltage PVDD, as compared to a fixed supply voltage used inFIG.1A. The Class-H amplifier has high power efficiency over the Class-D amplifier.

BRIEF SUMMARY OF THE INVENTION

An objective of the present disclosure is to provide an audio amplifier with duty cycle control. The audio amplifier can be implemented based on a Class-H configuration with a closed loop control for adjusting a supply voltage for the audio amplifier according to a pulse width modulation signal of the audio amplifier. The audio amplifier is capable of operating at an optimal operating point and controlling the duty cycle of the pulse width modulation signal.

According to embodiments of the invention, an audio amplifier with duty ratio control is provided. The audio amplifier comprises a pulse width modulation modulator, a power stage, and a voltage converter. The pulse width modulation modulator is configured to receive an input signal for generating a pulse width modulation signal. The power stage is configured to output an output signal according to a supply voltage and the pulse width modulation signal. The voltage converter is configured to adjust voltage level of the supply voltage according to the pulse width modulation signal. The audio amplifier is configured to adjust the voltage level of the supply voltage when duty ratio of the pulse width modulation signal is greater than a duty ratio threshold.

According to embodiments of the invention, an audio amplifier with duty ratio control is provided. An audio amplifier with duty ratio control comprises a pulse width modulation modulator, a power stage, and a voltage converter. The pulse width modulation modulator is for generating a pulse width modulation signal. The power stage is coupled to the pulse width modulation modulator. The voltage converter is coupled to the power stage and configured to output a supply voltage to the power stage, wherein the voltage converter is configured to adjust a voltage level of the supply voltage according to duty ratio of the pulse width modulation signal.

In some embodiments of the audio amplifier, the audio amplifier further comprises a pulse width modulation detector which is electrically coupled between the power stage and the voltage converter and is for outputting an indication signal to the voltage converter, wherein the indication signal indicates whether the duty ratio of the pulse width modulation signal is greater than the duty ratio threshold and the voltage converter is configured to adjust the voltage level of the supply voltage according to the indication signal.

In some embodiments of the audio amplifier, the pulse width modulation detector is configured to output the indication signal according to the pulse width modulation signal and the duty ratio threshold.

In some embodiments of the audio amplifier, the voltage converter is configured to adjust the voltage level of the supply voltage when the indication signal indicates that the duty ratio of the pulse width modulation signal is greater than the duty ratio threshold.

In some embodiments of the audio amplifier, the audio amplifier is configured to limit the duty ratio of the pulse width modulation signal to a percentage based on the duty ratio threshold.

In some embodiments of the audio amplifier, a closed loop is formed between the power stage and the voltage converter.

DETAILED DESCRIPTION OF THE INVENTION

The supply voltage PVDD adjustment (rising/falling slope, gain) of a Class-H amplifier is critical to total harmonic distortion (THD) and power conversion efficiency. Capturing the pulse width modulation (PWM) signal of a Class-D amplifier for voltage modulation allows the supply voltage PVDD to be adaptively adjusted to make the audio amplifier operate at an optimal point.

Refer toFIG.3, which is a drawing illustrating a closed loop control Class-H audio amplifier according to an embodiment of the present invention.

As shown inFIG.3, the closed loop control Class-H audio amplifier400comprises an audio pulse width modulation (PWM) modulator420and a pre-driver430that cooperate form the basis for a Class-D amplifier. The audio pulse width modulation (PWM) modulator420receives an input signal such as an audio signal (IN). The audio pulse width modulation (PWM) modulator420converts the audio signal (IN) into a pulse width modulation signal. The pulse width modulation (PWM) signal is output by the audio pulse width modulation (PWM) modulator420and input into the pre-driver430. The pre-driver430amplifies the pulse width modulation (PWM) signal and outputs the amplified pulse width modulation (PWM) signal to a power stage. The power stage comprises a first switch450and a second switch440. The pre-driver430comprises two outputs. One output of the pre-driver430is electrically connected to the first switch450and the other output of the pre-driver430is electrically connected to the second switch440. For example, the audio pulse width modulation (PWM) modulator420and the pre-driver430can be implemented based on using a comparator to compare the input signal with a triangular wave. A low pass filter comprising an inductor460and a capacitor470has one end of the inductor460electrically connected between the first switch450and the second switch440. A speaker480is connected in parallel with the capacitor470of the low pass filter.

One end of the first switch450is connected to the output of a voltage converter410. The voltage converter410receives an input voltage VIN and outputs a supply voltage PVDD according to a control signal. For example, the voltage converter410can be implemented by using a step-up converter, a step-down converter, or a low dropout regulator, which can output a supply voltage variably according to a control signal. The control signal can be obtained, for example, according to the pulse width modulation signal.

The closed loop control Class-H audio amplifier400further comprises a PWM detector415. An input to the PWM detector415is electrically connected to the output of the pre-driver430that is electrically connected to the first switch450. The output of the PWM detector415is connected to a feedback input of the voltage converter410.

As shown inFIG.3, a closed loop is formed between the output of the voltage converter410, through the first switch450, into the input of the PWM detector415, and into the feedback input of the voltage converter410.

The PWM detector415senses the duty ratio of the pulse width modulation (PWM) signal at the input to the PWM detector415. By using the PWM detector415, the percentage of the duty ratio of the pulse width modulation (PWM) signal can be limited based on a duty ratio threshold. Since a louder audio volume has a larger duty ratio, the pulse width modulation (PWM) signal can be limited to ensure that the audio signal does not clip or distort.

In an embodiment of the present invention, the duty ratio threshold is preset in hardware or firmware.

In an embodiment of the present invention, the duty ratio threshold is controllable or variable by a user of the hardware or firmware.

The PWM detector415outputs a feedback control signal to the feedback input of the voltage converter410. The received feedback control signal is utilized by the voltage converter410to continually or adaptively adjust the supply voltage. In this way, the adaptively adjusted supply voltage PVDD ensures that the audio amplifier operates at the optimal point.

Referring toFIG.4, an embodiment of the PWM detector415is illustrated. InFIG.4, an embodiment of the PWM detector415includes a filter510, an error amplifier520, and an output stage530. The filter510receives the pulse width modulation signal (e.g., indicated by pwm(t)) and outputs a filtered signal indicating a duty ratio D(t) of the pulse width modulation signal. The error amplifier520compares the filtered signal indicating the duty ratio D(t) with a duty ratio reference Dref. The output stage530outputs an indication signal A(t) according to the output of the error amplifier520. When the duty ratio D(t) is equal to or greater than the duty ratio reference Dref, the error amplifier520outputs an output through the output stage530, so that the PWM detector415outputs the indication signal A(t) indicating that the duty ratio of the pulse width modulation signal is going to be equal to or greater than the duty ratio reference Dref.

Refer toFIG.5, which illustrates waveforms of the circuit ofFIG.4with pulse width modulation (PWM) control according to an embodiment of the present invention.

Referring toFIG.5, when the audio signal IN(t) is internally modulated, the duty ratio D(t) of the PWM signal varies. When the duty ratio D(t) is equal to or going to be greater than the duty ratio reference Dref, the voltage converter410adjusts the voltage level of the supply voltage PVDD(t) according to the indication signal A(t) output by the PWM detector415. For example, the voltage converter410increases the voltage level of the supply voltage PVDD(t). The audio amplifier400has the characteristic that when the voltage level of the supply voltage PVDD(t) is increased, the duty ratio of the PWM signal is reduced. By way of the characteristic, the duty ratio of the PWM signal is controlled, such as to be locked at the duty ratio reference Dref. As a result, the circuit of the embodiment operates at an optimal operating point to obtain the optimal system efficiency.

Also, as shown inFIG.5, the duty ratio of the PWM signal is limited to a duty ratio threshold, such as the duty ratio reference Dref which corresponds to a percentage of duty ratio (e.g., 80%, 85%, or any suitable value). When the duty ratio of the PWM signal is less than the duty ratio threshold, the PWM signal varies according to the audio signal IN(t) and the supply voltage PVDD(t) remains at a specific voltage level. When the duty ratio of the PWM signal is equal to or is going to be greater than the duty ratio threshold, the duty ratio of the PWM signal is reduced while the supply voltage PVDD(t) is increased by way of the characteristic of the audio amplifier400. In this manner, the duty ratio of the PWM signal is limited to the duty ratio threshold as a whole.

In some embodiments, the voltage converter can be implemented so that the duty ratio threshold can be controllable or variable by way of hardware, firmware or software implementation.

In some embodiments, the PWM detector415can be implemented by or based on a frequency-locked loop or feedback control module.

Refer toFIG.6, which is a schematic diagram illustrating a voltage converter according to an embodiment of the present invention. In the embodiment inFIG.6, the voltage converter410can be implemented to include a summation circuit610(e.g., indicated by a sigma sign (Σ)), an error amplifier620, and an output stage630. The summation circuit610outputs a summation signal according to the indication signal A(t) from the PWM detector415and a feedback signal FB of the voltage converter410(e.g., the supply voltage PVDD(t)). The error amplifier620outputs a comparison signal according to the summation signal and a reference signal (indicated by Vref). The output stage630outputs the supply voltage PVDD(t) according to the input voltage VIN(t) and the comparison signal. For example, the input voltage VIN(t) is a DC voltage signal and the output stage630of the voltage converter410adjusts the supply voltage PVDD(t) in response to the comparison signal.

Refer toFIGS.7A,7B, and7C, which are drawings illustrating closed loop control Class-H audio amplifiers (e.g.,400A,400B,440C) according to embodiments of the present invention.

The closed loop control Class-H audio amplifiers illustrated inFIGS.7A,7B, and7Coperate similarly to the closed loop control Class-H audio amplifier illustrated inFIG.3.

However, in these embodiments the inputs to the PWM detector415are electrically connected to alternative nodes in the circuit. For example, inFIG.7B, the input to the PWM detector415is electrically connected to a node connected to the load inductor L460. InFIG.7C, the input to the PWM detector415is electrically connected at a node between a resistor Ron and the second switch440. In the embodiment ofFIG.7A, the input to the PWM detector415is electrically connected to a feedback output of the Class-D amplifier425and not connected between the pre-driver430and the first switch450.

In the embodiments illustrated inFIGS.7A,7B, and7C, the Class-D amplifier425is formed by a combination of the audio pulse width modulation (PWM) modulator420and the pre-driver430.

According to some embodiments of the invention, an audio amplifier with duty ratio control is provided. The audio amplifier (e.g.,400,400A,400B, or400C inFIG.3,7A,7B, or7C) comprises a pulse width modulation modulator (e.g.,420inFIG.3,7A,7B, or7C), a power stage (e.g.,440and450inFIG.3,7A,7B, or7C; or430,440and450inFIG.3,7A,7B, or7C), and a voltage converter (e.g.,410inFIG.3,7A,7B, or7C). The pulse width modulation modulator is configured to receive an input signal for generating a pulse width modulation signal. The power stage is configured to output an output signal according to a supply voltage and the pulse width modulation signal. The voltage converter is configured to adjust voltage level of the supply voltage according to the pulse width modulation signal. The audio amplifier is configured to adjust the voltage level of the supply voltage when duty ratio of the pulse width modulation signal is greater than a duty ratio threshold.

According to other embodiments of the invention, an audio amplifier with duty ratio control is provided. The audio amplifier (e.g.,400,400A,400B, or400C inFIG.3,7A,7B, or7C) comprises a pulse width modulation modulator (e.g.,420inFIG.3,7A,7B, or7C), a power stage (e.g.,440and450inFIG.3,7A,7B, or7C; or430,440and450inFIG.3,7A,7B, or7C), and a voltage converter (e.g.,410inFIG.3,7A,7B, or7C). The pulse width modulation modulator is for generating a pulse width modulation signal. The power stage is coupled to the pulse width modulation modulator. The voltage converter is coupled to the power stage and configured to output a supply voltage to the power stage, wherein the voltage converter is configured to adjust a voltage level of the supply voltage according to duty ratio of the pulse width modulation signal.

In some embodiments of the audio amplifier, the audio amplifier further comprises a pulse width modulation detector (e.g.,415inFIG.3,7A,7B, or7C) which is electrically coupled between the power stage and the voltage converter and is for outputting an indication signal (e.g., A(t) inFIG.3,7A,7B, or7C) to the voltage converter, wherein the indication signal indicates whether the duty ratio of the pulse width modulation signal is greater than the duty ratio threshold and the voltage converter is configured to adjust the voltage level of the supply voltage according to the indication signal.

In some embodiments of the audio amplifier, the pulse width modulation detector is configured to output the indication signal (e.g., A(t) inFIG.3,7A,7B, or7C) according to the pulse width modulation signal (e.g., pwm(t)) and the duty ratio threshold (e.g., Dref), for example, as shown inFIG.4.

In some embodiments of the audio amplifier, the voltage converter is configured to adjust the voltage level of the supply voltage when the indication signal (e.g., A(t) inFIG.3,7A,7B, or7C) indicates that the duty ratio of the pulse width modulation signal is greater than the duty ratio threshold.

In some embodiments of the audio amplifier, the audio amplifier is configured to limit the duty ratio of the pulse width modulation signal to a percentage based on the duty ratio threshold (e.g., as shown inFIG.5).

In some embodiments of the audio amplifier, a closed loop is formed between the power stage and the voltage converter (e.g., as illustrated inFIG.3,7A,7B, or7C).