Apparatus and method for negative boost audio amplifier

An audio amplifier with a negative DC/DC converter is provided. The audio amplifier receives VDD and VEE as power supply signals. The negative DC/DC converter is arranged to provide VEE from VDD, such that VEE is a negative voltage that is lower in magnitude than VDD. Accordingly, the power loss can be minimal with lower Vee, even though the audio power is increased. The audio amplifier further receives an audio input signal, and a reference voltage that is the arithmetic mean of VDD and VEE.

FIELD OF THE INVENTION

The invention is related to power converters, and in particular but not exclusively, to a audio amplifier with a negative DC/DC converter that provides VEE for an audio amplifier such that VEE is a negative voltage that is lesser in magnitude that VDD, to provide the appropriate headroom voltage.

BACKGROUND OF THE INVENTION

An audio amplifier is a power amplifier that may be used to amplify low-power audio signals (e.g. 20 Hz-20 KHz signals) for driving a speaker load. The speaker load value typically varies from 4 ohm to 32 ohm. Audio amplifiers may be used to drive loudspeakers, but may also be used for driving other types of speaker loads such as audio headphones and earpieces. Traditionally, audio amplifiers are class AB amplifiers. However, other types of amplifiers, such as class D amplifiers, may also be employed.

DETAILED DESCRIPTION

Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” includes plural reference, and the meaning of “in” includes “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. As used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. The term “based, in part, on”, “based, at least in part, on”, or “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the words “gate”, “drain”, and “source” includes “base”, “collector”, and “emitter”, respectively, and vice versa.

Briefly stated, the invention is related to an audio amplifier with a negative DC/DC converter. The audio amplifier receives VDD and VEE as power supply signals. The negative DC/DC converter is arranged to provide VEE from VDD, such that VEE is a negative voltage that is lower in magnitude than VDD. The audio amplifier further receives an audio input signal, and a reference voltage that is the arithmetic mean of VDD and VEE.

FIG. 1shows a block diagram of an embodiment of circuit100. Circuit100includes converter120and audio amplifier110. Audio amplifier110is arranged to receive audio amplifier input signal Ain, to receive reference voltage Ref, and to provide audio amplifier output signal Aout for driving a speaker load (not shown inFIG. 1). Signal Ain is based, at least in part, on an audio input signal. In one embodiment, signals Ain and Aout are both single-ended signals. In other embodiments, one or both of signals Ain and Aout are differential signals.

Audio amplifier110is arranged to receive positive power supply signal VDD as an upper power supply input and negative power supply signal VEE as a lower power supply input. Converter120is arranged to provide voltage VEE from voltage VDD such that the polarity of VEE is the opposite of the polarity of VDD (e.g. VEE is negative while VDD is positive). For example, in embodiment, VDD is 3.3V and VEE is −1.5V. In other embodiments, other values of VDD and VEE may be employed. In some embodiments, VEE is adjustable. In one embodiment, converter120is an inverting DC-DC power converter.

Reference voltage Ref is substantially equal to (VDD+VEE)/2. That is, reference voltage Ref is substantially the arithmetic mean of VDD and VEE. By using Ref=(VDD+VEE)/2 instead of ground, it is possible to avoid clipping that might otherwise cause poor sound quality.

FIG. 2Aillustrates a block diagram of an embodiment circuit200, which may be employed as an embodiment of circuit100ofFIG. 1. Negative DC/DC converter220is an embodiment of converter120ofFIG. 1. In the embodiment shown, circuit200further includes capacitor C0, resistors R1and R2, and speaker load230.

Audio amplifier210has an inverting input that is arranged to receive audio amplifier input signal Ain, a noninverting input coupled to reference voltage Ref, a positive power supply input that is arranged to receive voltage VDD, a negative power supply input that is arranged to receive signal VEE, and an output that is arranged to provide audio amplifier output signal Aout. Audio amplifier signal Ain is based on audio signal Vin (which is the input audio signal to be amplified).

AlthoughFIG. 2illustrates a single-ended configuration embodiment of circuit200, in other embodiments, other configurations may be employed, such as a bridge-tied load (BTL) configuration, or the like.

FIG. 2Billustrates a block diagram of an embodiment of circuit200B, which is a BTL configuration embodiment of circuit200ofFIG. 2A.FIG. 2Bfurther includes audio amplifier250, resistor R5, and resistor R6.

FIG. 3shows a block diagram of an embodiment of circuit300, which may be employed as an embodiment of circuit200B ofFIG. 2B. Circuit300further includes voltage divider340. Voltage divider340includes resistor R3and resistor R4. Resistor R4has substantially the same resistance as resistor R3. For example, in one embodiment, resistor R3is a 50 KOhm resistor, and resistor R4is a 50 KOhm resistor.

FIG. 4shows a block diagram of an embodiment of circuit400, which may be employed as an embodiment of circuit300ofFIG. 3. Circuit400further includes battery450, which is used to provide voltage VDD.

An embodiment of circuit400may be used in a portable handheld device having a power level that is usable up to 2.5V or 2.8V. When battery450is fully charged, negative DC/DC converter420need not be used. However, when the battery voltage (VDD) less than 3.3V, voltage VDD need not be boosted. Instead, negative converter420is enabled to increase the total headroom VDD-VEE available to the portable handheld device. Using negative DC/DC converter420doubles the user's battery time in one embodiment. In one embodiment, VEE is adjustable according to how discharged battery450is, so that the necessary amount of headroom is provided to audio amplifier410.

In some embodiments, use of VEE may be desirable even if battery450is fully charged. For example, in one embodiment, battery450operates at 3.8V, and a maximum power of about 600 mW-700 mW would be achieved even negative DC/DC converter420were not used. However, by employing DC/DC converter420and providing −1.5V at VEE, 1W of power can be achieved with lower battery voltage conditions, such as 3V or less than 3V.

FIG. 5illustrates an embodiment of circuit500, which may be employed as an embodiment of circuit100ofFIG. 1.FIG. 5and Table 1 illustrate the example of correlation between the Vee voltage control and audio gain/battery condition.

In one embodiment, the audio gain is adjustable, and power converter520is enabled/disabled based on the battery voltage and audio gain. In various embodiments, audio gain and power converter520can be controlled by serial or parallel control port, or the like. Further, the vdd reference can be set by an additional resistor, external control signal also, and/or the like (as shown by optional resistor Rvdd inFIG. 5).

In one embodiment, the correlation is as follows. If the battery (e.g. battery450ofFIG. 4) is over 3.5V, Vee is not required until the audio gain is 12 dB. Accordingly, in one embodiment, if the battery voltage is greater than 3.5V and the audio gain is less than 12 dB, power converter520is disabled so that Vee is not provided. However, if the battery voltage is less than 3.5V, Vee is required for 12 dB gain.