JP5035634B2 - Audio amplifier - Google Patents

Description

  The present invention relates to an audio amplifier that amplifies an audio signal to generate a driving signal for a sounding device such as a speaker, and more particularly to a technique that is effective for use in preventing a popping sound that occurs when the audio amplifier is released from standby. .

  For portable electronic devices, audio amplifiers are used to decode and expand audio streams read from memory, disks, etc. with a microcomputer (hereinafter referred to as microcomputer), extract audio data, and perform DA conversion. Some have the function of amplifying and driving the speaker. In a portable electronic device having such a function and operating on a battery, the audio amplifier is frequently turned on and off in order to reduce current consumption.

As described above, in a system having a function of turning on / off the audio amplifier, an unpleasant noise is generated particularly when the audio amplifier is turned on. Therefore, in order to suppress the noise generated when the audio amplifier is released from standby (at the time of turning on), for example, an invention described in Patent Document 1 has been proposed.
JP 2006-025246 A

  The invention described in Patent Document 1 generates an analog reference voltage generation circuit that generates a reference voltage AGND of an analog signal supplied to an audio amplifier, a reference voltage (VREF) generation circuit, and a waveform that rises at a constant speed. By configuring the ramp waveform generating circuit, the filter circuit for smoothing the rising waveform, and the saturation time shortening circuit, the noise generated when the standby is canceled is suppressed.

  However, since the invention of the prior application is to suppress the noise by smoothing the rising waveform of the analog reference voltage with a filter, the start of the operation of the audio amplifier is delayed even if the noise can be suppressed. Therefore, when the microcomputer outputs an audio signal simultaneously with the standby release signal, there is a problem that an accurate sound may not be generated.

  An object of the present invention is to suppress an annoying noise generated when an audio amplifier is released from standby in a system having a function for turning on / off an audio amplifier, and to quickly raise a reference voltage supplied to the audio amplifier. It is to provide audio amplifier control technology.

  To achieve the above object, the present invention provides a differential amplifier circuit in which an audio signal is input to one input terminal and a reference voltage is input to the other input terminal, a reference voltage generation circuit that generates the reference voltage, An audio amplifier comprising a power switch capable of supplying / shutting off a power supply voltage to each of the differential amplifier circuit and the reference voltage generation circuit, and amplifying an audio signal by the differential amplifier circuit to generate a driving signal for a sound generator The reference voltage generation circuit generates ½ of the power supply voltage in a steady state and generates the ½ voltage when the power switch is turned from an off state to an on state. A charge-up circuit that raises the potential of the node to a predetermined potential close to the ½ voltage within a time shorter than a period corresponding to the maximum frequency of the audible region It is obtained by way.

  Here, preferably, the charge-up circuit is configured such that when the power switch is turned from an off state to an on state, the potential of the node at which the half voltage is generated is set to the predetermined value in 10 μs to 40 μs. It is configured to rise to a potential.

  According to the above-described means, in a system having a function of turning on / off the audio amplifier, it is possible to suppress an unpleasant noise generated when the audio amplifier is released from standby (off → off) and to supply the reference to the audio amplifier. The voltage can be raised quickly.

  Preferably, the reference voltage generation circuit includes a first voltage dividing circuit that divides the power supply voltage in half, a node supplied with the power supply voltage, and an output node of the first voltage dividing circuit. A second voltage dividing circuit that divides the power supply voltage to generate a potential lower than the power supply voltage by a predetermined level, and a potential generated by the second voltage dividing circuit and the voltage dividing circuit. And a second differential amplifier circuit for driving the control terminal of the transistor in accordance with the potential difference between them. Thereby, it is possible to generate a reference voltage to be supplied to the differential amplifier circuit for signal amplification with a relatively simple circuit configuration.

  Preferably, a capacitor connected between an output node of the first voltage dividing circuit and a ground point or an external terminal for connecting a capacitor to the output node is provided. Thereby, the generated reference voltage can be stabilized.

  Further, preferably, the reference voltage generation circuit sets the potential generated by the second voltage dividing circuit and input to the second differential amplifier circuit so that the reference voltage is a half of the power supply voltage. After rising, it is configured to be able to shift to a potential lower than the predetermined potential. As a result, the charge-up circuit can have a hysteresis characteristic so that it does not malfunction even when the power supply voltage fluctuates.

  Preferably, the second voltage dividing circuit is supplied with a plurality of resistance elements connected between a node to which the power supply voltage is supplied and a ground point, and the power supply voltage among the plurality of resistance elements. A switching element provided in parallel with any one of the resistive elements except for the resistive element connected to the node, the switching element is turned off immediately after the power is turned on, and the reference voltage is It is configured to be turned on after rising to half the voltage. As a result, a charge-up circuit having hysteresis characteristics can be realized with a relatively simple circuit configuration.

  Further, preferably, the reference voltage generation circuit receives a third differential having the potential generated by the second voltage dividing circuit and a potential of a node generating a voltage that is ½ of the power supply voltage as inputs. An amplifying circuit is provided, and the switching element is controlled to be turned on / off by the output of the third differential amplifying circuit. Thereby, the potential input to the second differential amplifier circuit can be quickly shifted.

  According to the present invention, in a system having a function for turning on / off an audio amplifier, it is possible to suppress an annoying noise generated when the audio amplifier is released from standby and to quickly start up a reference voltage supplied to the audio amplifier. There is an effect of becoming.

  Preferred embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows an embodiment of an audio amplifier to which the present invention is applied. In FIG. 1, a portion surrounded by an alternate long and short dash line A is an audio amplifier circuit, and elements constituting the circuit are formed on one semiconductor chip, and a semiconductor integrated circuit (hereinafter referred to as an audio amplifier IC). It is comprised as).

  The audio amplifier IC 10 includes an external terminal SW to which an on / off control signal CS from the microcomputer 20 is input, an external terminal AIN to which an audio signal output from the microcomputer 20 is input, and a sounding device 30 such as a speaker or a headphone. A positive phase side output terminal OUT (+) and a negative phase side output terminal OUT (−) to be driven, an external element connection terminal T0 to which a capacitor C0 for stabilizing the analog reference voltage AGND is connected, and a power supply voltage Vdd are applied. And a ground terminal GND to which a ground potential is applied.

  The audio amplifier IC 10 also amplifies the audio signal output from the microcomputer 20 to generate a positive phase side drive signal for the speaker, and a negative phase side for generating a negative phase side drive signal for the speaker. The amplifier circuit 12 includes a reference voltage generation circuit 14 that generates an analog reference voltage AGND supplied to the input terminals of these amplifier circuits.

  Further, the reference voltage generation circuit 14 is connected in series between the power supply terminal VDD and the ground terminal GND, and generates a resistance R1 that generates a potential (Vdd / 2) of the power supply voltage Vdd by a resistance ratio. , R2 and a charge-up circuit 42 having an output terminal connected to a connection node N1 between the resistors R1 and R2, and quickly raising the potential of the node N1 when the standby is released (on transition). It is constituted by.

  The positive phase side amplifier circuit 11 includes a differential amplifier AMP1 in which an audio signal input from an external terminal AIN is input to a non-inverting input terminal via a resistor R3 and the analog reference voltage AGND is applied to an inverting input terminal; P-channel MOSFETs (insulated gate field effect transistors: hereinafter referred to as MOS transistors) Q11 and N connected in series between the power supply terminal VDD and the ground terminal GND and driven by the differential output of the differential amplifier AMP1 The push-pull type output stage is composed of a channel type MOS transistor Q12.

  In the negative phase side amplifier circuit 12, the audio signal input from the external terminal AIN is input to the inverting input terminal via the resistors R3, R4, and R5, and the analog reference voltage AGND is applied to the non-inverting input terminal. Push-pull comprising a differential amplifier AMP2, a P-MOS transistor Q21 and an N-MOS transistor Q22 connected in series between the power supply terminal VDD and the ground terminal GND and driven by the differential output of the differential amplifier AMP2. And a mold output stage.

  The audio amplifier according to the present embodiment can be stopped by a command from the microcomputer 20, and between the power supply voltage terminal VDD and the voltage dividing circuit 41 and the charge-up circuit 42, between the power supply voltage terminal VDD and the positive phase side amplification. Switches SW1, SW2, SW3, and SW4 are provided between the circuit 11 and the power supply voltage terminal VDD and the negative phase side amplification circuit 12, respectively, and are turned on / off by a signal φ based on the control signal CS from the microcomputer 20 It is configured to be.

  Furthermore, when the operation is stopped (when the power is shut off), a switch SW0 for cutting off the input line of the audio signal, and switches SW11, SW12, SW13 and SW21, SW22 for fixing the potential so that the internal node does not float. SW23 is provided. SW0 is turned on / off by a signal φ based on the control signal CS, and SW11 to SW23 are configured to be turned on / off complementarily to SW0 and SW1 to SW4 by a signal / φ obtained by inverting the control signal CS by an inverter INV. ing.

  FIG. 2 shows a specific circuit example of the charge-up circuit 42.

  The charge-up circuit 42 of this embodiment includes resistors R11 to R14 connected in series between a power supply terminal T1 to which a power supply voltage Vdd is applied via the switch SW4 and a ground terminal GND, a power supply terminal T1, and a reference. The potential of the connection node between the P channel MOS transistor Q1 connected between the voltage output terminal T2 (AGND) and the resistors R11 and R12 is input to the inverting input terminal, and the voltage AGND of the output terminal T2 is applied to the non-inverting input terminal. And a differential amplifier AMP3 to be applied.

  Further, the charge-up circuit 42 has an N-channel MOS transistor Q2 connected in series between the connection node of the resistors R13 and R14 and the ground terminal GND, and the potential of the connection node of the resistors R12 and R13 at the inverting input terminal. And a differential amplifier AMP4 to which the voltage AGND of the output terminal T2 is applied to the non-inverting input terminal. The MOS transistor Q1 is controlled by the output of the differential amplifier AMP3, and the MOS transistor Q2 is controlled by the output of the differential amplifier AMP4.

  Next, the operation of the charge up circuit 42 will be described. In a state where the switches SW1 and SW2 are turned off and the power supply is shut off, all nodes in the circuit and the terminals T1 and T2 are set to the ground potential. In this state, when the control signal CS changes to the high level and the switches SW1 and SW2 are turned on, the terminal T1 first rises to Vdd, and the differential amplifiers AMP3 and AMP4 are activated to start operation. In the absence of the charge-up circuit 42, the potential of the node N1 is gradually increased by charging the external capacitor C0 with the current flowing through the resistor R1. At this time, if the resistance value of R1 (= R2) is lowered, the rise will be faster, but the through current of R1 and R2 will increase, so that the resistance value cannot be made very small.

  In the charge-up circuit 42 of this embodiment, immediately after the switches SW1 and SW2 are turned on, the potential at the inverting input terminal of the differential amplifier AMP4 is higher than the potential at the terminal T2, so that the output is at a low level. Q2 is initially turned off, and current flows through resistors R11 to R14. At this time, the resistance ratio of R11 to R14 is set in advance so that the potential of the connection node between the resistors R11 and R12 is about 100 mV to 1 V lower than Vdd / 2 so as not to be affected by power supply ripple noise. Has been.

  When the terminal T1 rises to Vdd, the potential of the connection node between the resistors R11 and R12 becomes higher than the potential of the terminal T2, the output of the differential amplifier AMP3 becomes low level (potential of the terminal T2), and the MOS transistor Q1 is turned on. Is done. Then, the potential of the terminal T2 rises toward the potential Vdd of the terminal T1. When the potential of the terminal T2 reaches Vdd / 2-20 to 100 mV where the output of the differential amplifier AMP3 is low level, the output of the differential amplifier AMP3 is inverted to high level and the MOS transistor Q1 is turned off.

  The conductance (size) of the MOS transistor Q1 and the driving force of the differential amplifier AMP3 are set so that this time is about 20 μsec. Therefore, as shown in FIG. 3, the potential AGND of the terminal T2 rises rapidly from Vdd / 2-20 to 100 mV, and thereafter is determined by the resistance value of the resistor R1 of the voltage dividing circuit 41 and the capacitance value of the capacitor C0 of the node N1. Slowly increases to Vdd / 2 according to a time constant (for example, 1 second). When this potential AGND is supplied to the inverting input terminals of the differential amplifiers AMP1 and AMP2, as shown in FIGS. 4B and 4C, the output voltages OUT (+) and OUT (−) are AGND in the non-signal state. (Vdd / 2).

  Here, 20 μs of the rapid charge-up time is 50 kHz in terms of frequency, and 50 kHz is outside of 20 Hz to 20 kHz which is called an audible region of the human ear. For this reason, when the potential AGND of the terminal T2 is raised by the charge-up circuit 42 of this embodiment, the noise caused by canceling standby is suppressed. Further, since the potential at the connection node between the resistors R12 and R13 is lower than the potential at the connection node between the resistors R11 and R12, when the potential at the terminal T2 approaches Vdd / 2-20 to 100 mV, the output of the differential amplifier AMP4 is Inverted to high level, the MOS transistor Q2 is turned on. Then, current flows through the resistors R11 to R13, Q2.

  As a result, the current flowing through the resistors R11 to R14 increases, and the potential of the connection node between the resistors R11 and R12 increases. Therefore, even if the potential at the terminal T2 becomes slightly lower than Vdd / 2−100 mV due to power supply noise or the like, the output of the differential amplifier AMP3 does not invert to the low level. That is, the differential amplifier AMP4 and the MOS transistor Q2 give a hysteresis characteristic to the charge-up circuit 42 so that malfunction due to noise can be avoided.

  When the switches SW3 and SW4 are turned off by the control signal CS from the microcomputer 20 and the operation is stopped, the internal node potential is maintained while maintaining (the potential of the connection node between the resistors R11 and R12> the potential of the terminal T2). Since it is lowered, the MOS transistor Q1 is not turned on in the middle, and the potential AGND of the terminal T2 is slowly lowered as shown in FIG.

  At this time, the switches SW11, SW12 and SW21, SW22 are turned on, so that the gate terminals of the output MOSFETs of the amplifier circuits 11, 12 are quickly fixed to the power supply voltage on the high side and to the ground potential on the low side. Is done. As a result, the output MOS transistors Q11, Q12 and Q21, Q23 are forcibly turned off and are not affected by the outputs of the differential amplifiers AMP1, AMP2.

  Furthermore, in the audio amplifier of this embodiment, the switches SW13 and SW23 are turned on, so that the output terminals of the amplifier circuits 11 and 12 are fixed to the power supply voltage Vdd. The rising speed at this time is about 3 μsec (300 kHz). For this reason, no humming occurs when the power is turned off. In addition, the output terminal becomes high impedance (floating), and it is possible to prevent the noise from being generated from the speaker due to fluctuations in the potential of the output terminal due to external noise.

  In the audio amplifier, the size of the P-MOSFET is larger than the size of the N-MOSFET (about 3 times). For this reason, when the same voltage is applied between the source and the drain, the leakage current of the P-MOSFET becomes larger than the leakage current of the N-MOSFET. However, in this embodiment, since each output terminal is fixed to the power supply potential when the audio amplifier is turned off as described above, the leakage current is reduced to 1/3 compared to the case where the output terminal is fixed to the ground potential. There is an advantage that can be.

  Furthermore, when the power switches SW3 and SW4 are turned off, the switch SW0 on the audio signal input line is also turned off. As shown in FIG. 1, the audio signal input line is connected to the output stage node via resistors R21 to R24, but when the power switches SW3 and SW4 are turned off, the switch SW0 is also turned off. Therefore, it is possible to prevent a backflow from flowing from the node of the output stage fixed to the power supply voltage to the input side of the audio signal.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the above embodiment. For example, in the above-described embodiment, the power switches (SW1, SW2, SW3, SW4) for supplying and shutting off the power supply voltage to the positive phase side amplification circuit 11, the negative phase side amplification circuit 12, and the reference voltage generation circuit 14 are separately provided. However, it is possible to replace all of them with a common switch, or to replace the two power switches SW1 and SW2 in the reference voltage generation circuit 14 with one switch.

  In the embodiment, the MOS transistor is used as the charge-up transistor Q1 that rapidly raises the potential of the output node N1 of the voltage dividing circuit when the power is turned on. However, a bipolar transistor may be used. Similarly, the transistor Q2 for providing the charge-up circuit 42 with hysteresis characteristics can be replaced with a bipolar transistor instead of a MOS transistor.

  Further, a voltage dividing circuit for generating a voltage (Vdd / 2−100 mV) applied to the inverting input terminal of the differential amplifier AMP3 in the charge-up circuit 42 is shown in which four resistors R11 to R14 are connected in series. However, any of these resistors can be replaced with a diode. Further, the switch element Q2 is connected in parallel with the resistor R14 in order to give the charge-up circuit 42 hysteresis characteristics. However, the switch element Q2 may be provided in parallel with the resistor R12 or R13.

  Moreover, in the said embodiment, although 20 microseconds (equivalent to 50 kHz) was shown as an example of the quick charge up time when a power switch is switched from an OFF state to an ON state, it corresponds to a frequency outside the audible region (20 kHz or more). For example, it may be set to 10 μsec or 40 μsec. However, in order to shorten the rapid charge-up time, it is necessary to increase the size of the MOS transistor Q1 or increase the driving force of the differential amplifier AMP3. Since the potential of the connection node between R1 and R2 may cause overshoot, the range of 10 μs to 40 μs is desirable.

  In the above description, the case where the invention made mainly by the present inventor is applied to an audio amplifier which is a field of use that has been used as the background has been described. However, an analog signal is amplified and a positive phase side drive signal and a negative phase side drive signal are Can be used in an amplifier circuit that generates and outputs

1 is a circuit diagram showing an embodiment of an audio amplifier to which the present invention is applied. FIG. 3 is a circuit diagram illustrating a configuration example of a charge-up circuit used in the audio amplifier according to the embodiment. It is a graph which shows the mode that the analog reference voltage rises at the time of power-on in the audio amplifier of the embodiment. 4 is a time chart showing a state of potential change of each part when the power is turned on in the audio amplifier of the embodiment. 6 is a time chart showing a state of potential change of each part when the power is turned off in the audio amplifier of the embodiment.

Explanation of symbols

10 Audio amplifier IC
DESCRIPTION OF SYMBOLS 11 Positive phase side amplifier circuit 12 Negative phase side amplifier circuit 14 Reference voltage generation circuit 20 Microcomputer 30 Speaker (sound generator)
41 Voltage divider circuit 42 Charge-up circuit

Claims (7)

A differential amplifier circuit in which an audio signal is input to one input terminal and a reference voltage is input to the other input terminal;
A reference voltage generating circuit for generating the reference voltage;
A power switch that can supply / shut off a power supply voltage to each of the differential amplifier circuit and the reference voltage generation circuit;
An audio amplifier that amplifies an audio signal by the differential amplifier circuit to generate a driving signal for a sound generator,
The reference voltage generation circuit includes:
In a steady state, a voltage that is ½ of the power supply voltage is generated, and when the power switch is turned from the OFF state to the ON state, the potential of the node that generates the ½ voltage is set in the audible region. An audio amplifier comprising a charge-up circuit that rises to a predetermined potential close to the ½ voltage within a time shorter than a period corresponding to a maximum frequency.   When the power switch is turned from an off state to an on state, the charge-up circuit raises the potential of a node that generates a voltage half of the power source voltage to the predetermined potential in 10 μsec to 40 μsec. The audio amplifier according to claim 1, wherein the audio amplifier is configured to be raised. The reference voltage generation circuit includes:
A first voltage dividing circuit for dividing the power supply voltage by half;
A transistor connected between the node to which the power supply voltage is supplied and the output node of the first voltage dividing circuit; and a second that generates a potential lower than the power supply voltage by a predetermined level by dividing the power supply voltage. A voltage dividing circuit; and a second differential amplifying circuit configured to input a potential generated by the second voltage dividing circuit and a potential of an output node of the voltage dividing circuit and drive a control terminal of the transistor according to a potential difference therebetween A charge-up circuit comprising:
The audio amplifier according to claim 1, further comprising:   4. The audio amplifier according to claim 3, further comprising a capacitor connected between an output node of the first voltage dividing circuit and a ground point, or an external terminal for connecting a capacitor to the output node. The reference voltage generation circuit includes:
The potential generated by the second voltage dividing circuit and input to the second differential amplifier circuit is set to a potential lower than the predetermined potential after the reference voltage rises to ½ of the power supply voltage. The audio amplifier according to claim 3 or 4, wherein the audio amplifier is configured to be shiftable.   The second voltage dividing circuit is connected to a plurality of resistance elements connected between a node to which the power supply voltage is supplied and a ground point, and a node to which the power supply voltage is supplied among the plurality of resistance elements. A switching element provided in parallel with any one of the resistive elements except the resistive element, the switching element is turned off immediately after the power is turned on, and the reference voltage is ½ of the power supply voltage. 6. The audio amplifier according to claim 5, wherein the audio amplifier is turned on after rising to a voltage of. The reference voltage generation circuit includes:
A third differential amplifying circuit having a potential generated by the second voltage dividing circuit and a potential of a node generating a voltage half the power supply voltage as inputs; The audio amplifier according to claim 6, wherein the switch element is configured to be turned on / off by an output of a circuit.

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