JP2006217106A - Class d amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a class D amplifier which is free from variations in manufacture and prevents pop noise although the amplifier has low-cost and high efficiency. <P>SOLUTION: In the class D amplifier wherein a transistor M1 for high-side driving and a transistor M2 for low-side driving switch alternately according to an input PWM signal to drive a speaker SP through a capacitor C1 for AC coupling with a signal generated through the switching, a transistor M3 is connected to an inverter INV1 driving the gate of the transistor M1 in series and a transistor M4 is connected to an inverter INV2 driving the gate of the transistor M2 in series. Then a time constant circuit of a resistance R1 and a capacitor C3 relaxes the conduction of the transistor M3 and a time constant circuit of a resistance R2 and a capacitor C4 relaxes the conduction of the transistor M4 when the amplifier is actuated or at a stop, or returns from a standby state. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a class D amplifier that outputs an audio signal based on a rectangular wave such as a PWM method, and is generated by an inrush current flowing through an output AC coupling capacitor, particularly at startup, stop, and return from a standby state. The present invention relates to a class D amplifier that has taken measures to prevent pop noise.

  In an amplifier, a sudden change occurs in the output signal in a transient state such as when the power is turned on. Particularly in an audio amplifier, this sudden fluctuation appears as an unpleasant noise called pop noise. Therefore, in general, a mute circuit is provided between the output of the amplifier and the speaker, and the speaker is cut off or grounded until the amplifier reaches a steady state.

  Further, as shown in Conventional Example 1 in FIG. 4, a mute transistor M9 is connected in series to the power supply side of the high-side drive and low-side drive transistors M1 and M2, and after the power supply VDD is turned on, the capacitor C5 and the resistor R3 are connected. There is also a method of reducing pop noise by gradually reducing the on-resistance of the mute transistor M9 by a time constant circuit consisting of (see, for example, Patent Document 1). In FIG. 4, SP is a speaker, C1 is an AC coupling capacitor, L1 is a low-pass filter inductor, C2 is a low-pass filter capacitor, and 1 is a PWM modulation circuit for PWM-modulating an audio signal and outputting it. There is also a method in which a mute transistor is similarly connected to the ground side of the output transistors M1 and M2 to reduce pop noise by gradually reducing the on-resistance when the power is turned on.

  Further, as shown in Conventional Example 2 of FIG. 5, when the power is turned on, the switch SW1 is turned off and the switch SW2 is turned on by the control circuit 2, and the AC coupling capacitor C1 is supplied to the buffer amplifier 4 by the voltage generated by the output reference voltage circuit 3. There is also a method in which the control circuit 2 turns on the switch SW1 and turns off the switch SW2 to switch to the PWM amplifier 5 to reduce pop noise after the charging to the stable operation region is completed (for example, Patent Document 2). reference).

JP 2001-223538 A JP 2003-110441 A

  However, the mute transistor M9 is connected in series to the power supply side or the ground side of the high-side drive and low-side drive transistors M1 and M2 as described with reference to FIG. In the method of gradually lowering the power, loss occurs in the mute transistor M9. Therefore, in order to increase the efficiency, it is necessary to make the on-resistance of the mute transistor M9 sufficiently lower than the on-resistances of the transistors M1 and M2. The mute transistor M9 becomes very large, and even if integrated, the chip size increases and the cost increases. Further, when the power supply is suddenly turned on, the gate potential of the mute transistor M9 instantaneously drops, and pop noise cannot be suppressed depending on the timing.

  Further, in the method of switching between the buffer amplifier 4 and the PWM amplifier 5 as described with reference to FIG. 5, it is necessary to make the midpoint of the output voltage of the buffer amplifier 4 coincide with the midpoint of the output voltage of the PWM amplifier 5. Becomes pop noise. Therefore, when an offset voltage is generated in the buffer amplifier 4 due to manufacturing variations or the like, a problem that pop noise cannot be suppressed occurs.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a class D amplifier that solves the above-described problems and is low-cost and high-efficiency while preventing pop noise without being affected by manufacturing variations.

  In order to solve the above-mentioned problem, in the invention according to claim 1, the high-side drive transistor and the low-side drive transistor alternately perform a switching operation according to the input PWM signal, and the AC coupling capacitor is set by the switching signal. A first drive circuit for driving the gate of the high-side drive transistor, a second drive circuit for driving the gate of the low-side drive transistor, and a start-up Alternatively, there is provided control means for starting up the operations of the first and second drive circuits with a time constant upon return.

  According to a second aspect of the present invention, in the class D amplifier according to the first aspect, as the control means, a first control transistor connected between the first drive circuit and a power source or a ground, and at the time of starting or returning A first time constant circuit for conducting the first control transistor with a time constant; a second control transistor connected between the second drive circuit and a power supply or ground; and And a second time constant circuit for conducting the control transistor with a time constant.

  According to the present invention, since there is no element inserted in series with the transistors for high side drive and low side drive, the efficiency does not decrease during normal operation, and even if integrated, the chip size is not greatly affected. Can be realized at low cost. In addition, since the amplifier does not switch in the output line immediately after the power is turned on until the stable operation, there is an advantage that there is no occurrence of pop noise due to reference voltage fluctuation at the time of switching due to an offset generated due to manufacturing variation or the like. For this reason, it is very effective even when used for headphones such as portable devices, where even a faint pop noise feels uncomfortable to the ear.

  FIG. 1 is a circuit diagram showing a configuration of a class D amplifier according to an embodiment of the present invention. SP is a speaker or headphone as a load, C1 is an AC coupling capacitor, L1 is an inductor, and C2 is a capacitor. The inductor L1 and the capacitor C2 constitute a low pass filter. M1 is a high-side drive PMOS transistor, and M2 is a low-side drive NMOS transistor. INV1 is an inverter (first drive circuit) that drives the gate of the transistor M1, and INV2 is an inverter (second drive circuit) that drives the gate of the transistor M2. M3 is an NMOS transistor (first control transistor) connected between the inverter INV1 and the ground, and M4 is a PMOS transistor (second control transistor) connected between the inverter INV2 and the power supply VDD. M5 is a PMOS transistor that is turned on during mute operation and normal operation, and M6 is an NMOS transistor that is turned on during mute operation and normal operation. R1 and R2 are resistors, and C3 and C4 are capacitors. The resistor R1 and the capacitor C3 constitute a first CR time constant circuit, and the resistor R2 and the capacitor C4 constitute a second CR time constant circuit. M7 is an NMOS transistor for discharging the charge of the capacitor C3 during the reset period, and M8 is a PMOS transistor for discharging the charge of the capacitor C4 during the reset period. INV3 to INV6 are inverters, EN is an enable terminal, RESET is a reset terminal, and SIG is an input signal terminal.

  In general, the operation of the class D amplifier is such that the high-side drive transistor M1 and the low-side drive transistor M2 are alternately switched at a switching frequency of 10 times or more of the audio frequency band. When the output of the speaker SP is silent, the voltage on the positive side of the AC coupling capacitor C1 is an average value of VDD / 2.

  However, at the moment when the operation is started at the time of turning on the power and returning from the standby state, the voltage of the step response of the rectangular wave is output and even after passing through the low-pass filter constituted by the inductor L1 and the capacitor C2. Then, a sudden charge fluctuation occurs on the positive electrode side of the AC coupling capacitor C1. This becomes pop noise, and particularly in a portable audio playback device using headphones, it becomes a very unpleasant sound.

  Therefore, in this embodiment, the transistor M3 is connected to the ground side of the inverter INV1 that drives the gate of the high-side drive transistor M1, and the gate voltage Vg3 of the transistor M3 is determined by the time constant of the resistor R1 and the capacitor C3 at the start of operation. Is gradually increased, the peak value of the rectangular wave applied to the gate of the high-side drive transistor M1 is gradually increased, and the on-resistance value Rm1 of the high-side drive transistor M1 is gradually decreased.

  More specifically, after the power is turned on, during a period until the power supply voltage VDD is stabilized, the level of the RESET terminal is set to the H level and the EN terminal is set to the L level, whereby the transistor M5 is turned off and M7 is turned on. The electric charge of the capacitor C3 is discharged by M7, the transistor M3 is turned off, and the high side drive transistor M1 is also turned off.

  Thereafter, by setting the EN terminal to the H level and the RESET terminal to the L level, the transistor M5 is turned on, M7 is turned off, and the capacitor C3 is gradually charged through the resistor R1. When the voltage of the capacitor C3 rises, the gate potential Vg3 of the transistor M3 rises, the peak value of the rectangular wave applied to the gate of the high-side drive transistor M1 gradually rises, and the high side when the rectangular wave is at the H level The on-resistance Rm1 of the driving transistor M1 gradually decreases.

  For this reason, the AC coupling capacitor C1 is charged by a pulse with a gradually increasing peak value, so that the charging voltage Va also changes gradually, and no sudden change appears in the voltage Vb applied to the speaker SP. The voltage change applied to is a frequency component (f = 1 / tpop) below the audible range, and no pop noise is generated. Note that the operation of the low-side drive transistor M2 is the same as that of the above-described high-side drive transistor M1, and a description thereof will be omitted.

  The above operation waveforms are shown in FIG. In FIG. 3, Vg3 is the gate voltage of the transistor M3, Vg4 is the gate voltage of the transistor M4, Vg1 is the gate voltage of the transistor M1, Vg2 is the gate voltage of the transistor M2, Rm1 is the on-resistance of the transistor M1, and Rm2 is the on-state of the transistor M2. Resistance, Va is a voltage of the capacitor C2, and Vb is a voltage applied to the speaker SP.

  This operation is performed not only when the power is turned on, but also when the AC coupling capacitor C1 leaks during standby for a long time and decreases from VDD / 2. Therefore, there is an advantage that pop noise does not occur when returning. Furthermore, when this circuit is integrated into an IC, the current value of the capacitors C3 and C4 can be made very small, and the capacity of the capacitors C3 and C4 can be reduced accordingly. It can be realized at low cost. Further, the resistors R1 and R2 in FIG. 1 operate similarly even if they are replaced with current sources I1 and I2 as shown in FIG. In this case, since the capacitors C3 and C4 are charged with a constant current, the gate voltages Vg3 and Vg4 change with a constant gradient. Further, in the above description, MOS transistors are connected between the inverter INV1 and the ground, and the inverter INV2 is connected to the power source. However, the inverters INV1 and INV2 are connected to either the power source or the ground. So that the transistor is turned on with a time constant at start-up or return.

It is a circuit diagram which shows the structure of the class D amplifier of the Example of this invention. It is a circuit diagram which shows the structure of another class D amplifier of the Example of this invention. It is a waveform diagram of the mute operation of the class D amplifier of the embodiment of the present invention. It is a circuit diagram which shows the mute circuit of the prior art example 1. FIG. 6 is a circuit diagram showing a mute circuit of a conventional example 2.

Claims (2)

In a class D amplifier in which a high-side drive transistor and a low-side drive transistor alternately perform switching operations according to an input PWM signal, and a speaker or a headphone is driven via an AC coupling capacitor by a signal by the switching.
A first drive circuit for driving the gate of the high-side drive transistor, a second drive circuit for driving the gate of the low-side drive transistor, and at the time of start-up or return, the first and second drive circuits A class D amplifier comprising a control means for starting up the operation with a time constant. The class D amplifier according to claim 1, wherein the control means includes:
A first control transistor connected between the first drive circuit and a power source or the ground; a first time constant circuit for conducting the first control transistor with a time constant at start-up or return; and A second control transistor connected between the driving circuit and the power source or the ground; and a second time constant circuit for conducting the second control transistor with a time constant at the time of start-up or return. Class amplifier.

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