JP2008048305A - Class-d acoustic amplifier with half-swing pulse-width-modulation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To allow a class-D amplifier to be used in portable equipment by decreasing system and solution costs by obviating an output filter required by a conventional class-D amplifier. <P>SOLUTION: The amplifier includes a first comparator 116a, a second comparator 116b, and an output switch 120a, 120b. The first and second comparators respectively compare a pair of differential signals with a half-swing modulation signal to generate first and second pulse-width modulation (PWM) control signals, wherein a voltage swing of the half-swing modulation signal is smaller than the voltage swing of the differential signals. The output switch includes a pair of inputs coupled to receive the PWM control signals to provide a ternary encoded output signal in response to the PWM control signals. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates generally to amplifiers, and more particularly to class D acoustic amplifiers and modulation schemes therefor.

  A pulse width modulation (PWM) amplifier, also known as a class D amplifier, operates on a principle similar to that of a switching power supply, except that the reference voltage of the PWM amplifier is not a fixed voltage but a variable signal. In general, class D amplifiers are classified as analog input class D and digital input class D, i.e., fully digital acoustic amplifiers.

  Class D amplifiers are much more power efficient than class AB amplifiers. Because of its good efficiency, a class D amplifier requires a small power supply, eliminating or reducing the need for a heat sink and greatly reducing overall system cost, size, and weight. Other advantages include long-term battery operation, a quiet and good listening environment, and an integrated acoustic amplifier with high output power (> 20 W / channel).

  Conventional class D amplifiers require an output filter, which increases the size of the system and the solution cost, limiting its use in portable devices. A filterless class D amplifier eliminates the output filter while maintaining the efficiency advantage. According to the filterless modulation system, a class D amplifier having a large efficiency advantage can be obtained in spite of the cost and size substantially the same as those of a class AB amplifier.

  One way to achieve filterless class D operation, high efficiency, and low cost is to send current to the load only when needed, and maintain the current once sent so that the input signal is sent. If not, the energy to remove current from the load is not attenuated or wasted. As one of the methods, there is a four-phase modulation method having four operation states. The modulation scheme uses four states to drive a load, such as a speaker, depending on the acoustic input signal. This four-phase system is described in detail, for example, in US Pat. No. 6,262,632 by Corsi et al., All of which are incorporated herein.

  By eliminating the filter, electromagnetic interference (EMI) is radiated from the class D amplifier. This EMI phenomenon is also described in detail in US Pat. No. 6,614,297 by Score et al., All of which are incorporated herein. Score et al. Describe a system in which three-phase PWM encoding is used rather than conventional two-phase PWM encoding or four-phase PWM encoding. EMI increases when ΔV for three-phase PWM is | VDD | while | 2VDD | for two-phase PWM. Three-phase PWM encoding can be achieved by a four-phase switching operation, but the common mode EMI component of the three-phase PWM encoding by the four-phase switching operation is larger than that by the three-phase switching operation.

  The Score et al. Modulation scheme and amplifier increase the efficiency at small inputs for EMI performance and filterless operation, but to achieve these benefits, the Score et al. Technique uses a four-state switching signal (4-phase ) Is required to encode a three-state switching signal (three phases).

  There is a need for an improved modulation scheme for generating a three-phase PWM code for a class D amplifier, particularly for an analog input class D amplifier.

  An amplifier having first and second comparators for comparing a pair of differential signals and a half swing modulation signal to generate first and second pulse width modulation (PWM) control signals, respectively, is provided, and half swing modulation is provided. The voltage swing of the signal is smaller than the voltage swing of the differential signal. The output switch has a pair of inputs connected to receive the PWM control signal and provides a three-phase encoded output signal in response to the PWM control signal.

  In an embodiment, the amplifier is a class D acoustic amplifier. The class D acoustic amplifier includes a differential amplifier that generates a pair of differential signals in response to an acoustic input signal; a first and second pulse width modulation by comparing the pair of differential signals and a half swing modulation signal (PWM) first and second comparators for generating control signals, respectively, wherein the voltage swing of the half swing modulation signal is smaller than the voltage swing of the differential signal; and a pair of outputs And an H-bridge output stage that provides an amplified three-phase encoded acoustic output signal to a load between the pair of outputs in response to a PWM control signal.

The foregoing and other features of the present invention will be further understood from the following detailed description of preferred embodiments of the invention given in conjunction with the accompanying drawings.
The accompanying drawings illustrate preferred embodiments of the invention and other information relevant to the disclosure.

3 shows a three-phase PWM of an analog input signal.

3 shows a three-phase PWM of an analog input signal.

1 shows a circuit diagram of a prior art three phase PWM encoded analog input amplifier.

FIG. 3 is a circuit diagram of a three-phase PWM encoded analog input acoustic amplifier including a half swing PWM according to an embodiment of the present invention.

A pulse width modulation of an analog input signal using a full wave modulation signal and a half swing modulation signal is schematically compared.

A pulse width modulation of an analog input signal using a full wave modulation signal and a half swing modulation signal is schematically compared.

It is a model of the feedback differential operational amplifier of the PWM amplifier of FIG.

A small signal model of a differential operational amplifier is shown.

It is a circuit diagram of a half swing triangular wave generator.

  This description of specific embodiments is to be read in connection with the accompanying drawings, which are considered as part of the entire description given. In the description, such as "low", "upper", "horizontal", "vertical", "upper", "lower", "upper", "lower", "top", and "bottom" Relative terms, as well as their derivatives (eg, “horizontal”, “downward”, “upwardly”, etc.) are used in the direction indicated in the drawings or shown in the drawings. Should be construed as referring. These relative terms are for convenience of explanation and do not require that the device be configured or operated in a particular direction. Terms related to attachment, connection, etc., such as “connected” and “interconnected”, unless explicitly stated otherwise, the components may be directly or via intervening components. Indirectly, we refer to a relationship that is fixed or attached to each other, and that both are movable or rigid attachment members or relationships.

  1A and 1B show three-phase pulse width modulation (PWM) of an analog input signal. According to three-phase PWM, the PWM encoded signal is one of three states related to the amplitude of the sample analog input signal: (i) + VDD, (ii) ground, or (iii) −VDD. FIG. 2 is a circuit diagram of an analog input class D amplifier 10 with three-phase PWM encoding of the prior art. An example of a class D amplifier with three-phase PWM encoding is shown in Howatt US Pat. No. 5,077,539, all of which is incorporated herein.

D-class amplifier 10 is connected to the fixed gain amplifiers 12a and _12b, a pair of differential inputs are identified as _{V IP} and _{V IN.} Gain amplifiers 12a and 12b are preamplifiers and are optional. If the analog input signal is too small, additional signal gain may be applied. In certain implementations, gain amplifiers 12a and 12b are designed to accommodate a variety of analog input signals with selectable gains. However, these preamplifiers do not affect the PWM operation of the amplifier 10 regardless of whether they are used. The outputs of the amplifiers 12a and 12b are connected to the positive and negative inputs of the differential operational amplifier 14 via a resistor R1. The operational amplifier 14 combines the input signal with the components of the feedback output signal to form a closed loop configuration or system, improving the overall frequency response and stability of the system to reduce errors due to non-linearities, This reduces noise distortion.

The differential signal output from the amplifier 14 is provided to a pair of comparators 16a and 16b for modulation by a timing signal, in particular, a full swing (ie, a full swing of the differential signal) between 0 V and VDD. The PWM output control signal is generated. The resulting digital signal is provided to the three-phase power switch drive logic 18 to provide a state of an output selection switch circuit, ie, H-bridge 20, for providing an amplified differential output connected to a load, eg, a speaker 22. To control. The H bridge circuit 20 is connected to a single unipolar power supply (VDD2) for providing an amplification switch output signal to the load 22. The output signal applied to the load is a faithful replica of the input signal, but a large amount of power is provided by the power supply, i. As shown in FIG. 2, the output 3-phase encoded PWM waveform output signal is the difference, or subtraction, of the two PWM differential outputs (identified as V _OP and V _OP ).

  Since the third state, i.e. the zero output state, is used, the output circuitry dissipates only power in proportion to the output signal. For this reason, power loss is small for small signal inputs. For a zero state signal, there is almost no current flowing through the speaker 22, so no loss occurs. Since heat generation is reduced by reducing power loss, heat removal can be achieved using a small conductive heat sink for the amplifier package or without the heat sink in certain cases of wire interconnect alone.

  However, the circuit of FIG. 2 has one problem that the three-phase PWM encoding method requires the three-phase logic block 18. The signal output from the comparators 16a and 16b cannot be directly applied to the H-bridge module 20 to provide a three-phase encoded PWM output signal. Reducing EMI is an important concern in the design of class D amplifiers. The outputs of the comparators 16a and 16b cause a four-phase switching operation in the H-bridge 20, which has a large common mode EMI component compared to an H-bridge having a three-phase switching operation.

FIG. 3 is a class D analog output acoustic amplifier 100 having a half swing PWM according to an embodiment of the present invention. Amplifier 100 has a pair of differential inputs for receiving a pair of differential signals shown as V _IP and V _IN. Differential signal, the gain amplifier 102 which operates similarly to the gain amplifiers 12a, 12b described above have a feedback resistor _{R 4,} is provided through a resistor _{R 3.} The differential output of the amplifier 102 is given to the input of the operational amplifier 114 via a resistance element R1. As described above in connection with circuit 10 of FIG. 2, operational amplifier 114 of amplifier 100 combines the input signal with the feedback signal to form a closed loop configuration that reduces noise distortion. The feedback loop has a differential integrator having resistors R ₁ , R ₂ , capacitor C, and operational amplifier 114. A feedback closed loop configuration is preferred but not necessary.

  As will be described in detail below, the amplifier 100 also includes a pair of comparators 116 a and 116 b that receive the differential output and the modulation signal of the operational amplifier 114 as inputs, as with the amplifier 10. The amplifier 100 also has a full bridge output topology 120 having a first or positive half portion 120a and a second or negative half portion 120b. Each half of the H-bridge 120 has a pair of transistors connected in series between VDD and ground. One skilled in the art will appreciate that a pair of transistors can have two NMOS transistors, two PMOS transistors, or NMOS and PMOS transistors. Different types of H-bridges require different drive circuits that connect the comparator outputs to the H-bridge. In the most efficient MOSFET design, N-channel MOSFETs are used on both the high side and the low side. This is because it has a lower ON resistance than a P-channel MOSFET. However, this design is more complicated because the gate of the high side MOSFET usually needs to be driven by a charge pump circuit. The half portions 120a and 120b of the bridge topology 120 of the embodiment of FIG. 3 are constituted by NMOS transistors and PMOS transistors, respectively, and the comparator output is amplified to a desired voltage level.

The amplifier 100 of FIG. 3 does not use a full swing triangular wave as a modulation signal, but uses a half swing triangular wave (shown as V _{saw in} FIG. 3) as a modulation signal. "Half swing" means that the voltage swing of the modulation signal does not swing completely between ground and VDD, i.e. smaller than the voltage swing of the actuation signal, as will be explained in more detail below. It means that. The voltage swing of the modulation signal is between V _CM and the largest (ie most positive) supply rail, or between V _CM and the smallest (ie most negative) supply rail, where V _CM is the largest Any level between the supply rail and the minimum supply rail can be taken. V _CM, in order to maximize the dynamic range of the signal is set to the common mode voltage of the differential signal. Half-swing amplification is may be less than the swing of the peak-to-peak of the modulation signal, to set the swing of the modulation signal to half the supply range, and, by setting the V _CM common mode voltage of the integrator, Better performance can be achieved. More specifically, in one embodiment, the modulation signal V _saw is (a) between V _CM and (V _CM + V _SW ), where V _CM is the integrator common mode voltage and V _SW is between the amplitude of the sawtooth waveform, or _(b) and _{V CM} and _(V CM _{-V SW),} a triangular signal oscillating.

In general, if the supply voltage rail is from 0V to VDD, the designer will choose a common mode voltage of VDD / 2 to maximize the dynamic range of the signal, but the signal of the common mode voltage will be at other levels. It may be. In this _embodiment, the _{V CM} is set to VDD / 2, the modulation signal _{V SW} is between VDD / 2 and VDD, or between VDD / 2 and 0V, thereby vibrating. The theory behind the proposed modulation method is shown in FIGS. 4A and 4B, which shows a sine wave signal (or other input signal) modulated with a prior art full swing triangular wave, 4B shows a novel modulation system using a half-swing triangular wave modulation signal oscillating between V _CM and VDD.

The modulation signal can be generated using a triangular wave / ramp generator. A circuit diagram of the triangular wave / ramp generator 200 is shown in FIG. If the switch SW1 is turned on, the current source _{I 1} charges the capacitor C. At the operational amplifier output V _OUT , a rising edge with a slope equal to I ₁ / C is obtained. If V _OUT > V _H (the high voltage limit is set by the high / low level limit module 202), SW1 is off and SW2 is on. This results in a falling edge with a slope equal to I ₂ / C at V _OUT . If V _OUT <V _L (low voltage limit set by high / low level limit module 202), SW2 is off and SW1 is on. By repeating these operations, a triangular wave having a voltage level between V _H and V _L can be obtained. Control signals for SW1 and SW2 are generated from the high / low level limiting module 202, which also sets the swing range of the generated triangular wave.

Importantly, the use of the half-swing modulation signal V _SW makes it possible to directly generate three-phase PWM encoding while maintaining good EMI performance as in the case of the logic circuit 18 of FIG. That's what it means. However, unlike the circuit of FIG. 2, the outputs of comparators 116a and 116b can be connected directly to output bridge 120 without further encoding, specifically to the gates of NMOS and / or PMOS transistors in bridge 120. is there. The subtraction of the outputs from the two comparators is switched to three states with the H bridge connected directly to the comparator stage without the need for coding logic. This is because only one output of the comparator can be VDD (if it is present) during an arbitrarily given switching period. Thus, no additional logic operation is required. This further reduces the complexity of the device and provides the benefits of cost, power consumption, and heat generation.

  Assuming that the triangular wave frequency is the same, two PWM pulses are generated for each PWM conversion by the conventional full swing modulation technique. According to the half-swing modulation technique proposed herein, only one PWM pulse is generated for each PWM conversion. That is, the effective PWM switching rate is halved by the proposed technique, thereby reducing power dissipation due to switching losses. Referring to FIG. 3, the amplified differential outputs of the first and second halves 120a, 120b of the H-bridge 120 are connected to a load element (eg, speaker 122) to provide an amplified three-phase encoded PWM output signal. As stated, the proposed method reduces circuit complexity by eliminating the need for three-phase logic required in the prior art. Further, the comparators 116a, 116b need not be rail-to-rail comparators, i.e., the comparator need not receive a rail-to-rail input signal whose signal swing is between VDD and ground. Rail-to-rail comparators require both NMOS and PMOS input stages to process rail-to-rail signals. Thus, the design complexity of a rail-to-rail comparator is higher than a comparator with only an NMOS input stage or a PMOS input stage and neither. According to the half swing technique, the triangular wave is approximately between VDD and VCM or between VCM and GND (ie, VDD / 2), so that a rail-to-rail comparator is not necessary. Also, while this design exhibits low sensitivity to triangular wave non-linearity, prior art full wave modulation is subject to non-linear mismatch between the positive and negative triangular wave cycles.

  5A and 5B further develops why the half swing PWM technique works. As shown in FIG. 5A, according to half swing PWM, only one feedback signal in the differential path is “valid” in the differential operational amplifier in each PWM switching period. However, the opposite polarity signal of the other path is automatically constructed by the operation of the differential operational amplifier. Thus, the closed loop feedback works well to produce a high performance output, such as an acoustic output. FIG. 5B shows a small signal model of the differential operational amplifier 114 to illustrate the half-swing PWM modulation technique described above.

  The above modulation method is shown using a differential input signal. However, since the operational amplifier 114 providing a closed loop is a differential type, the method can be applied without changing to a single-wire installation input signal. It is. Further, the half swing PWM can be applied to an open loop configuration, that is, by eliminating a feedback component. However, a closed loop configuration is preferred in order to suppress noise due to circuit nonlinearity. Further, a balanced triangular half swing modulation signal is preferred, but in the embodiment, a half swing sawtooth or sine waveform may be used as the modulation signal. Still further, by using a digital-to-analog converter (DAC) known to those skilled in the art, amplifier 100 can be used with a digital input, ie, as a digital class D acoustic amplifier.

  In specific applications, the class D acoustic amplifier described herein is used in applications such as televisions, mobile phones, portable radios, portable multimedia players, notebooks, DVD players, speakers, and the like. The Although described in connection with a class D amplifier, the modulation switching scheme described herein may also be applied to thermoelectric cooler drivers, motor controllers, and the like.

  Although the invention has been described with reference to specific embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variations and embodiments of the invention that can be made by those skilled in the art without departing from the equivalent scope of the invention.

Claims (27)

An amplifier,
A first and second comparator for comparing a pair of differential signals and a half swing modulation signal to generate first and second pulse width modulation (PWM) control signals, respectively, First and second comparators having a voltage swing smaller than the voltage swing of the differential signal;
An output switch having a pair of inputs connected to receive the first and second PWM control signals and providing a three-phase encoded output signal in response to the first and second PWM control signals; ,amplifier.   The amplifier of claim 1, wherein the output switch has an H-bridge output stage.   The first input from the pair of inputs is connected to a first half of the H-bridge, and the second input from the pair of inputs is connected to a second half of the H-bridge. Amplifier.   The amplifier of claim 1, further comprising a feedback loop connecting the output signal to the first and second comparators.   The feedback loop has a differential integrator, the output switch has a pair of operating output nodes, and the output signal from the pair of differential output nodes is the differential integration by the pair of differential signals. The amplifier of claim 4, wherein the amplifier is connected to the instrument.   The amplifier of claim 1, further comprising a ramp generator for providing the half swing modulation signal.   The amplifier of claim 1, wherein the half swing modulation signal has a voltage swing between a common mode voltage (Vcm) of the pair of differential signals and a preset voltage that is smaller than a voltage swing of the differential signal. .   The amplifier according to claim 7, wherein the preset voltage is a maximum voltage of the differential signal or a minimum voltage of the differential signal.   The amplifier according to claim 1, wherein the half swing modulation signal is a half swing triangular wave. A class D acoustic amplifier,
A differential amplifier for generating a pair of differential signals in response to an acoustic input signal;
First and second comparators for comparing the pair of differential signals and a half swing modulation signal to generate first and second pulse width modulation (PWM) control signals, respectively, the half swing modulation signal The first and second comparators having a voltage swing smaller than the voltage swing of the differential signal;
An H-bridge output stage having a pair of outputs, the H-bridge output stage providing an amplified three-phase encoded acoustic output signal to a load between the pair of outputs in response to the PWM control signal; Acoustic amplifier.   The acoustic amplifier of claim 10, wherein the H-bridge has a pair of inputs for receiving the first and second PWM control signals.   The first half of the H-bridge has a pair of switches responsive to the first PWM control signal, and the second half of the H-bridge has a pair of switches responsive to the second PWM control signal. The acoustic amplifier described in 1.   The acoustic amplifier of claim 10, further comprising a feedback loop connecting the output signal to the first and second comparators.   The feedback loop includes a differential integrator having the operational amplifier, and the pair of outputs of the H-bridge output stage are connected to the differential integrator by the acoustic input signal. Acoustic amplifier.   The acoustic amplifier of claim 10, further comprising a ramp generator for providing the half swing modulation signal. 11. The half swing modulation signal according to claim 10, wherein the half swing modulation signal has a voltage swing between a common mode voltage ( _Vcm ) of the pair of differential signals and a preset voltage that is smaller than a voltage swing of the differential signal. Acoustic amplifier.   The acoustic amplifier according to claim 16, wherein the preset voltage is a maximum voltage of the differential signal or a minimum voltage of the differential signal.   The acoustic amplifier according to claim 10, wherein the half swing modulation signal is a half wing triangular wave.   A digital-to-analog converter (DAC) connected to the differential amplifier for converting digital acoustic data into the acoustic input signal, wherein the class D acoustic amplifier processes the digital acoustic data; The acoustic amplifier according to claim 10, which is possible. A method of amplifying an input signal using three-phase modulation,
Comparing a pair of differential signals with a half swing modulation signal to generate first and second pulse width modulation (PWM) control signals, respectively, wherein the voltage swing of the half swing modulation signal is the differential signal; Steps smaller than the voltage swing of
Applying the PWM control signal to an output switch and providing a three-phase encoded output signal in response to the PWM control signal.   21. The method of claim 20, wherein the providing step comprises driving an H-bridge circuit having first and second PWM control signals to provide an amplified three-phase encoded differential output signal to a load.   21. The method of claim 20, further comprising generating the pair of differential signals in response to the input signal.   21. The method of claim 20, further comprising feeding back the three-phase encoded differential output signal to the pair of differential signals.   21. The method of claim 20, wherein the input signal is an acoustic signal. 21. The method of claim 20, wherein the half swing modulation signal has a voltage swing between a common mode voltage (V _cm ) of the pair of differential signals and a preset voltage that is less than a voltage swing of the differential signal. .   26. The method of claim 25, wherein the preset voltage is a maximum voltage of the differential signal or a minimum voltage of the differential signal.   21. The method of claim 20, wherein the half swing modulated signal is a half swing triangular wave.

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