JP2019180044A - Audio amplifier, audio output device and electronic equipment using the same, and protection method of audio amplifier - Google Patents

Abstract

To provide an audio amplifier in which reliability is improved.SOLUTION: An operational amplifier 110 has a push-pull type output stage 112. An overcurrent detection circuit 150 compares a source current I, flowing to a high-side transistor of the output stage, with an overcurrent threshold level, and asserts a detection signal when the source current Iexceeds the overcurrent threshold level. A protection circuit 170 becomes a first protection state when a detection signal is asserted in the first mode, and becomes a second protection state when the detection signal is asserted in the second mode.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an amplifier that drives a speaker or headphones.

An audio amplifier (also referred to as a power amplifier) is used to amplify an audio signal and drive an electroacoustic transducer such as a speaker or headphones. FIG. 1 is a circuit diagram of an audio output device 200R including an audio amplifier 100R. An electroacoustic transducer 202 as a load is connected to the output of the audio amplifier 100R via a DC block output capacitor _COUT .

For simplification of description, taking the non-inverting audio amplifier 100R as an example, the input signal V _IN ′ is input to the non-inverting input terminal (+) of the operational amplifier 110, and the output signal V _OUT is input to the inverting input terminal (−). A feedback signal V _{FB that} is _divided is input.

The bias circuit 120 generates a bias voltage V _BIAS near the midpoint of the power supply voltage V _DD and the ground voltage V _GND (0 V), and supplies the bias voltage V _BIAS to the non-inverting input terminal of the operational amplifier 110. The audio amplifier 100, audio signal _{V IN} of the AC through the input capacitor (DC blocking _{capacitor) C IN} is inputted, it is superimposed on the bias voltage _{V BIAS.} The input voltage V _IN ′ of the operational amplifier 110 swings according to the AC component of the audio signal _VIN , with the bias voltage V _BIAS as the center.

The audio amplifier 100R includes an overcurrent protection (OCP) circuit 130. The OCP circuit 130 monitors the output current I _OUT of the operational amplifier 110 during audio reproduction, and when the overcurrent protection threshold (overcurrent threshold) I _OCP is exceeded, overcurrent protection (latch stop of the output stage) is performed. Call. The overcurrent threshold I _OCP is defined in consideration of the rated current of the electroacoustic transducer 202 and the absolute maximum rated current of the transistor in the output stage of the operational amplifier 110.

In the shutdown state (standby state) before starting up the audio amplifier 100R, the output voltage _VOUT is 0V. When the power of the audio amplifier 100R is turned on, the bias circuit 120 gradually increases the bias voltage V _BIAS in order to suppress pop noise. As a result, the output voltage V _OUT gradually increases in proportion to the bias voltage V _BIAS , and pop noise is suppressed.

FIG. 2 is a time chart for explaining a startup sequence of the audio amplifier 100R of FIG. Note that the vertical axis and horizontal axis of the waveform diagrams and time charts referred to in this specification are appropriately expanded or reduced for easy understanding, and each waveform shown is also simplified for easy understanding. Or exaggerated or emphasized. The power supply voltage _{V DD} rises at time _{t 0.} When the shutdown state of the audio amplifier 100R at time _{t 1} is released, the bias voltage _{V BIAS} increases, the input signal _{V IN} 'also increases so as to follow it. When the time _{t 2} the bias voltage _{V BIAS} reaches a predetermined level (e.g., _V DD / 2), audio amplifier 100R becomes amplified ready audio signal. Time _{t 2} later, it is inputted audio signal _{V IN,} the output terminal OUT, and the output voltage _{V OUT} obtained by amplifying the audio signal _{V IN} is generated.

Activation completion after That overcurrent protection in the normal state is indicated at time t ₃ or later. Output when the terminal OUT is ground at time _{t 3,} the output current _{I OUT} of the audio amplifier 100R is increased. The output current _{I OUT} at time _{t 4} it takes overcurrent protection exceeds the over current threshold _{I OCP,} operation of the audio amplifier 100R is stopped.

  As a result of studying the audio amplifier 100R of FIG. 1, the present inventor has recognized the following problems.

FIG. 3 is an equivalent circuit diagram of the audio amplifier 100R at the time of a ground fault. The operational amplifier 110 has a push-pull type output stage 112. R _LOAD is the resistance value of the electroacoustic transducer 202, R _OUT is the resistance value between the output stage 112 and the output terminal OUT, and R _SHORT is the resistance value between the output terminal OUT and the ground GND at the time of short circuit. To express. R _OUT includes, for example, a parasitic resistance of a bonding wire.

FIG. 4 is a diagram for explaining overcurrent protection during the startup sequence. Here will be described a case where the output terminal OUT at before time t ₁ of the shutdown release is grounded. When R _SHORT << R _LOAD holds, the output current I _OUT flowing through the output stage 112 is I _OUT = V _OUT '/ (R _OUT + R _SHORT ). After the shutdown is released, the bias voltage V _BIAS gradually increases with time, and V _OUT '≈V _BIAS is established until the audio signal _VIN is input, and the output current I _{OUT at} this time is I _OUT ≈V _BIAS / (R _OUT + R _SHORT ). Therefore, overcurrent protection becomes effective when V _BIAS / (R _OUT + R _SHORT )> I _OCP .

For example, when R _OUT = 10 mΩ, R _SHORT = 40 mΩ, and I _OCP = 3.5 A, overcurrent protection is applied when the bias voltage V _BIAS exceeds 175 mV. Conversely, from time _{t 1} of the shutdown release until time _{t 2} when _the bias voltage _{V BIAS} reaches 175 mV, is less than 3.5A, the large current that can cause thermal problems for the high-side transistor _{M H} Continues to flow in a steady and direct manner. The potential difference between the drain and the source of the high side transistor _{M H} at the output ground fault is a power supply voltage _{V DD -V OUT '= V DD} -V BIAS, them if _V DD _{= 5.0V,} and V BIAS = 0.1 V For example, the power consumption of the high-side transistor _MH is (0.1 V ÷ 50 mΩ) × (5.0 V−0.1 V) = 9.8 W. If the overcurrent state continues for a long time, the reliability of the high-side transistor _MH is lowered.

The overcurrent state lengths t ₁ to t ₂ depend on the time constant of the bias voltage V _BIAS . Therefore, this problem can be solved by shortening the time constant of the bias voltage V _BIAS , but in this case, another problem that pop noise is likely to occur is caused.

Alternatively, the overcurrent threshold I _OCP can be solved by making it much lower than the maximum rated current of the transistor in the output stage, but in that case, the maximum output power during normal audio reproduction is reduced.

  Here, the case where the audio amplifier 100R is a non-inverting type is taken as an example, but the same problem may occur in the inverting amplification type or the voltage follower. It can also occur in BTL (Bridged Tied Load) audio amplifiers.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and one of exemplary purposes of an aspect thereof is to provide an audio amplifier with improved reliability.

  One embodiment of the present invention relates to an audio amplifier or a protection method. The audio amplifier includes an operational amplifier having a push-pull type output stage. When the audio amplifier is activated, the source current flowing through the high-side transistor is compared with the first threshold value. During startup, the source current is limited so as not to exceed the first threshold value. If the source current exceeds a second threshold value greater than the first threshold value after the start-up of the audio amplifier, the output stage is stopped.

  It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between methods, apparatuses, and the like are also effective as an aspect of the present invention.

  According to the present invention, the reliability of an audio amplifier can be improved.

It is a circuit diagram of an audio output device provided with an audio amplifier. 2 is a time chart for explaining a startup sequence of the audio amplifier of FIG. 1. It is an equivalent circuit diagram at the time of a ground fault of an audio amplifier. It is a figure explaining the overcurrent protection in a starting sequence. 1 is a block diagram of an audio output device including an audio amplifier according to an embodiment. FIG. 6 is a diagram for explaining the operation of the audio amplifier in FIG. 5. It is a figure which shows the structural example of a protection circuit and an overcurrent detection circuit. It is a figure which shows the more specific structural example of a protection circuit and an overcurrent detection circuit. It is a circuit diagram which shows the structural example of a protection execution part. It is a circuit diagram which shows the structural example of a bias circuit and a mode selector. 11A to 11D are circuit diagrams showing a mode selector according to a modification. It is a block diagram which shows the audio amplifier of a BTL format. It is a circuit diagram of the part relevant to the overcurrent protection of the audio amplifier which concerns on a modification. 14A to 14C are external views of electronic devices.

(Outline of the embodiment)
One embodiment disclosed herein relates to an audio amplifier. The audio amplifier can be switched between the first mode and the second mode. The audio amplifier has an operational amplifier having a push-pull type output stage, an overcurrent threshold value in the first mode is a first value, and an overcurrent threshold value is a second value larger than the first value in the second mode, A detection circuit that compares a source current flowing through the high-side transistor of the stage with an overcurrent threshold and asserts a detection signal when the source current exceeds the overcurrent threshold; and when the detection signal is asserted in the first mode A protection circuit that enters a first protection state and enters a second protection state when a detection signal is asserted in the second mode.

  When the bias voltage is low, the output stage can be protected from thermal problems by setting the first mode and lowering the overcurrent threshold. Also, when the bias voltage is high, the absolute maximum rated current is exceeded in the output stage transistor without sacrificing the maximum output power of the output stage by setting the second mode and increasing the overcurrent threshold. Overcurrent can be prevented from flowing.

  The protection circuit may limit the source current so as not to exceed the first value overcurrent threshold in the first protection state, and turn off the high-side transistor in the second protection state. As a result, in the first protection state, the source current is limited, the output voltage is increased as the bias voltage increases, and if there is no cause of an abnormality such as a ground fault during the first protection state, It can be returned to the state. When the cause of abnormality such as ground fault does not disappear, the high side transistor is turned off after the transition to the second protection state, and the circuit can be protected.

  The protection circuit may include a protection transistor provided between the gate source and the base emitter of the high side transistor. In the first protection state, the source current flowing through the high-side transistor can be limited to the limit current by adjusting the degree of ON of the protection transistor (gate-source voltage / base-emitter voltage). In the second protection state, the high-side transistor can be turned off by turning on the protection transistor.

  The protection circuit may change the gate voltage of the protection transistor in accordance with the signal level of the detection signal in the first protection state. As a result, feedback can be applied so that the source current does not exceed the limit current.

  The protection circuit may fully turn on the protection transistor in the second protection state. The protection circuit may latch assertion of the detection signal in the second protection state and maintain the second protection state.

  The audio amplifier may be in the first mode at startup (during the startup sequence) and in the second mode during normal operation of the audio amplifier (after completion of the startup sequence).

  The audio amplifier may further include a bias circuit that generates a bias voltage of the operational amplifier. The audio amplifier may be in the first mode when the bias voltage is lower than the threshold voltage, and may be in the second mode when the bias voltage is higher than the threshold voltage.

  The audio amplifier may be in the first mode before the elapse of a predetermined time from the activation of the audio amplifier, and may be in the second mode after the elapse of the predetermined time.

  The audio amplifier may further include a control pin that receives a control signal from the outside. The audio amplifier may be in the first mode when the control signal is at the first level, and may be in the second mode when the control signal is at the second level.

  The detection circuit includes a replica transistor having a gate or base connected in common to a high-side transistor, and a current flowing through the replica transistor based on a first threshold current in the first mode and a first threshold current in the second mode. And a comparison circuit for comparing with a large second threshold current.

  The comparison circuit includes a first resistor provided between the replica transistor and the power supply line, a second resistor having a first resistance value in the first mode and a second resistance value in the second mode, and a first resistor and a second resistor. A source / emitter load current mirror circuit, a first current source connected to the input of the current mirror circuit, and a second current source connected to the output of the current mirror circuit. You may respond | correspond according to the state of the connection node of a mirror circuit and a 2nd current source. The threshold current can be switched by switching the resistance value of the second resistor.

  The audio amplifier may be integrated on a single semiconductor substrate. “Integrated integration” includes the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated. A resistor, a capacitor, or the like may be provided outside the semiconductor substrate. By integrating the circuit on one chip, the circuit area can be reduced and the characteristics of the circuit elements can be kept uniform.

  One embodiment disclosed herein is an audio output device. The audio output device includes an electroacoustic transducer and an audio amplifier that drives the electroacoustic transducer. The audio amplifier includes a push-pull type output stage including a high-side transistor and a low-side transistor, a differential amplifier stage that controls the output stage according to an audio signal, and an overcurrent threshold value having a first value in the first mode, In the second mode, the overcurrent threshold becomes a second value larger than the first value, the source current flowing through the high-side transistor in the output stage is compared with the overcurrent threshold, and the source current exceeds the overcurrent threshold. And a detection circuit that asserts the detection signal, and a protection circuit that enters the first protection state when the detection signal is asserted in the first mode and enters the second protection state when the detection signal is asserted in the second mode. .

(Embodiment)
The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected. The case where it is indirectly connected through other members that do not affect the state or inhibit the function is also included.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. This includes cases where the connection is indirectly made through other members that do not affect the connection state or inhibit the function.

  FIG. 5 is a block diagram of an audio output device 200 including the audio amplifier 100 according to the embodiment. The audio output device 200 includes an electroacoustic conversion element 202 and an audio amplifier 100.

  The audio amplifier 100 mainly includes an operational amplifier 110, a bias circuit 120, an overcurrent detection circuit 150, a protection circuit 170, and a mode selector 180, and is a functional IC (Integrated Circuit) integrated on a single semiconductor substrate (die). .

The operational amplifier 110 forms an amplifier 102 together with a plurality of resistors R _{11 to} R ₁₄ . Although FIG. 5 shows the non-inverting amplifier 102, the invention is not limited to this, and an inverting amplifier 102 may be used.

  A shutdown (SDB) signal is input to the shutdown (SDB) pin of the audio amplifier 100. The audio amplifier 100 enters a shutdown state when the SDB signal is at a first level (for example, low), and is released from shutdown when the SDB signal is at a second level (for example, high).

The bias circuit 120 generates a bias voltage V _BIAS for the amplifier 102. The bias circuit 120, the capacitor _{C 11} is externally through a bias (BIAS) pin. The bias circuit 120 starts generating the bias voltage V _BIAS in response to the shutdown cancellation. Bias voltage _{V BIAS} gradually rises in accordance with a time constant determined by the capacitor _{C 11.} The bias voltage V _BIAS is supplied to the amplifier 102.

The operational amplifier 110 includes an output stage 112 and a differential amplifier stage 114. Output stage 112 is a push-pull, including high-side transistor _{M H} and the low-side transistor _{M L.} The differential amplifier stage 114 controls the output stage 112 so that the voltages of the non-inverting input terminal (+) and the inverting input terminal (−) are close to each other.

Audio amplifier 100 is configured to be switched first mode phi ₁ and the second mode phi _2. The mode selector 180 generates a mode signal MODE to instruct the first mode phi ₁ and a second mode phi _2. For example the mode signal MODE is low at first mode phi _1, is high in the second mode phi _2. The mode selector 180 selects the first mode φ ₁ during startup immediately after the audio amplifier 100 is turned on, and the second mode φ ₂ during subsequent normal operation. In this embodiment, the mode selector 180 monitors the bias voltage _{V BIAS,} the bias voltage _{V BIAS} is the first mode phi ₁ to lower than the predetermined threshold voltage _{V MODE,} the bias voltage _{V BIAS} is sill the higher the value the voltage _{V mODE} state and the second mode phi _2. For example, the threshold voltage V _MODE is defined to be several hundred mV lower than the target voltage of the bias voltage V _BIAS . For example, when the target voltage of the bias voltage V _BIAS is 2.5V, the threshold voltage V _MODE is preferably about 2.2 to 2.3V.

The overcurrent detection circuit 150 compares the source current I _SRC flowing through the high-side transistor _MH of the output stage 112 with the overcurrent threshold I _OCP and detects when the source current I _SRC exceeds the overcurrent threshold I _{OCP. Assert} signal V _DET (eg, low). The over-current threshold _{I OCP} in the first mode phi ₁ first value _{I OCP1} next, the overcurrent threshold _{I OCP} in the second mode phi ₂ becomes the first value _{I OCP1} greater than the second value _{I OCP2.}

The protection circuit 170 enters the first protection state when the detection signal V _DET is asserted in the first mode φ ₁ , and enters the second protection state when the detection signal V _DET is asserted in the second mode φ ₂ .

More specifically, the protection circuit 170 limits the source current I _SRC so as not to exceed the _overcurrent threshold I _OCP that is the first value I _OCP1 in the first protection state (OCP limiter). Also when the source current _{I SRC} in a second mode phi ₂ exceeds the over-current threshold _{I OCP} which is a second value _{I OCP2} becomes second protection state, turns off the high-side transistor _{M H} (OCP latch).

The above is the configuration of the audio amplifier 100. Next, the operation will be described. FIG. 6 is a diagram for explaining the operation of the audio amplifier 100 of FIG. FIG. 6 shows the operation when the OUT terminal is grounded when the shutdown is canceled. As in FIG. 3, the resistance value of the ground fault path is R _SHORT .

The power supply voltage _{V DD} is supplied to the time _{t 0.} SDB signal becomes high at time _{t 1,} when the shutdown state is released, the bias voltage _{V BIAS} starts to rise. As the bias voltage V _BIAS increases, the input voltage V _IN ′ and the output voltage _{VOUT of} the amplifier 102 also increase. Since the OUT terminal is grounded, the source current I _SRC = V _BIAS / (R _OUT + R _SHORT ) flows through the high side transistor _MH . Immediately after startup, V _BIAS <V _MODE , so the first mode φ ₁ is selected.

When the source current _{I SRC} at time _{t 2} reaches the first threshold _{I OCP1,} first protection state is validated, the source current _{I SRC} is limited to the first threshold _{I OCP1} as limit current. When the bias voltage V _BIAS reaches the threshold value V _MODE at time t ₃ , the mode is switched to the second mode φ ₂ .

When the time _{t 3} is switched to the second mode phi _2, the overcurrent threshold _{I OCP} is increased to a second value _{I OCP2.} As a result, immediately after that, I _SRC <I _OCP2 and the detection signal V _DET is negated. Result of the first protection state is released, the limit of the source current _{I SRC} is also released, the source current _{I SRC} increases, at time _{t 4} reaches a second value _{I OCP2.} As a result, the detection signal V _DET is asserted again, and the second protection state is entered. In the second protection state, the amplifier 102 stops latching, and the source current I _SRC becomes zero.

The above is the operation of the audio amplifier 100. According to this audio amplifier 100, when the shutdown state is canceled due to a ground fault, and the bias voltage V _BIAS immediately after startup is low, the first mode φ ₁ is set and the overcurrent threshold I _{OCP is set.} The output stage 112 can be protected from thermal problems. When the bias voltage V _BIAS is high, the second mode φ ₂ is set to increase the overcurrent threshold I _OCP2 , so that the maximum output power of the output stage 112 is not sacrificed. It is possible to prevent an overcurrent exceeding the absolute maximum rated current from flowing through the transistor _MH .

  The present invention is understood as the block diagram and circuit diagram of FIG. 5 or extends to various apparatuses and methods derived from the above description, and is not limited to a specific configuration. Hereinafter, more specific configuration examples and examples will be described in order not to narrow the scope of the present invention but to help understanding and clarify the essence and operation of the present invention.

  FIG. 7 is a diagram illustrating a configuration example of the protection circuit 170 and the overcurrent detection circuit 150.

The protection circuit 170 includes a protection transistor 172 and a protection execution unit 173. The protection transistor 172 is provided between the gate and source of the high side transistor _MH .

The signal level of the detection signal V _DET changes according to the source current I _SRC . The protection execution unit 173 of the protection circuit 170 changes the gate voltage V _G1 of the protection transistor 172 in accordance with the signal level of the detection signal V _DET in the first protection state. Due to this negative feedback, the source current I _SRC is limited to a limit amount based on I _OCP1 .

Further, the protection execution unit 173 fully turns on the protection transistor 172 in the second protection state. More preferably, in the second protection state, the protection execution unit 173 latches the assertion of the detection signal V _DET and maintains the second protection state. This provides a latch stop.

The overcurrent detection circuit 150 includes a replica circuit 151 of the output stage 112. Replica circuit 151 includes a high-side transistor _{M H} of the replica transistor 152, the replica transistor 153 of the low-side transistor _{M L.} The gate of the replica transistor 152 and 153, transistors _M H, are commonly connected to the gate of _{M L.} A detection current I _SRC ′ proportional to the source current flowing through the high side transistor _MH flows through the replica transistor 152.

Comparator circuit 154 'a, in the first mode phi ₁ first threshold current _{I OCP1'} current _{I SRC} flowing through the replica transistor 152, the larger in the second mode phi ₂ first threshold current _{I OCP1} ' Compared with the two-threshold current I _OCP2 ′, a detection signal V _DET corresponding to the comparison result is output.

FIG. 8 is a diagram illustrating a more specific configuration example of the protection circuit 170 and the overcurrent detection circuit 150. The protection execution unit 173 includes a timer latch circuit 174, a first switch SW1, and a second switch SW2. The first switch SW1 is provided between the gate of the output and protection transistors 172 of the comparison circuit 154, it turns on the first mode phi _1. As a result, the detection voltage V _DET is directly supplied to the gate of the protection transistor 172, a negative feedback path is formed, and the first protection state is established.

The timer latch circuit 174 latches the detection signal V _DET . The output of the timer latch circuit 174 is a binary signal of high level and low level. The second switch SW2 is provided between the gate of the output and protection transistors 172 of the timer latch circuit 174, the ON in the second mode phi _2. When the detection signal V _DET is asserted in the second mode phi _2, the output of the timer latch circuit 174 becomes a low level voltage, the protection transistor 172 is full-on, high-side transistor M _H is turned off completely.

The comparison circuit 154 includes a first resistor R ₂₁ , a second resistor R ₂₂ , a current mirror circuit 156, a first current source CS ₁ , and a second current source CS ₂ . The first resistor _{R 21} is provided between the replica transistor 152 and the power supply line. The second resistor R ₂₂ is a variable resistor having a first resistance value Ra in the first mode φ _{1 and a} second resistance value Rb (Rb> Ra) larger than the first resistance value in the second mode φ ₂ . The current mirror circuit 156 uses the first resistor R ₂₁ and the second resistor R ₂₂ as a source load (emitter load). The first current source CS1 is connected to the input of the current mirror circuit 156, and the second current source CS2 is connected to the output of the current mirror circuit 156. The signal level of the detection signal V _DET depends on the state of the connection node between the current mirror circuit 156 and the second current source CS2.

FIG. 9 is a circuit diagram illustrating a configuration example of the protection execution unit 173. The protection execution unit 173 includes a timer latch circuit 174. Timer latch circuit 174 includes a timer circuit 176 and a latch circuit 178. When the detection signal V _DET becomes low (asserted) in the second mode φ ₂ , the transistor Tr ₃₁ is turned on, and the capacitor C ₃₁ is charged via the resistor R ₃₁ . When the voltage V _C31 of the capacitor C ₃₁ exceeds the threshold value of the Schmitt buffer 177, the output of the Schmitt buffer 177 becomes high. The output of the Schmitt buffer 177 is inverted by the inverter INV1 and input to the latch circuit (flip-flop) 178. The high output of the latch circuit 178 is inverted by the inverter INV2, and a low level voltage is input to the gate of the protection transistor 172 via the second switch SW2. As a result, the protection transistor 172 is fully turned on.

FIG. 10 is a circuit diagram illustrating a configuration example of the bias circuit 120 and the mode selector 180. The bias voltage V _BIAS is set near the midpoint (V _DD / 2) of the power supply voltage V _DD and the ground voltage V _GND (0 V). The bias circuit 120 includes a voltage dividing circuit including resistors R ₄₁ and R ₄₂ and a transistor Tr ₄₁ . The transistor Tr ₄₁ is controlled according to the SDB signal, and is turned on when the shutdown is released. A connection point between the two resistors R ₄₁ and R ₄₂ is connected to a bias (BIAS) pin. An external capacitor C ₄₁ is connected to the BIAS pin. The time constant of the bias voltage _{V BIAS} is determined according to the capacitance of the capacitor _{C 41.}

Mode selector 180 includes a voltage comparator 182. The voltage comparator 182 compares the bias voltage V _BIAS with the threshold voltage V _MODE and outputs a mode signal MODE based on the comparison result.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. is there. Hereinafter, such modifications will be described.

In the embodiment, the mode is switched based on the bias voltage V _BIAS , but this is not the case. FIGS. 11A to 11D are circuit diagrams showing a mode selector 180 according to a modification.

(Modification 1)
The mode selector 180 in FIG. 11A includes a timer circuit 184. The timer circuit 184 monitors the SDB signal, a first mode phi ₁ until elapsed from standby release for a predetermined time, the after the second mode phi _2. Thus, the first mode φ ₁ can be set before the predetermined time has elapsed from the start of the audio amplifier 100, and the second mode φ ₂ can be set after the predetermined time has elapsed.

In FIG. 11B, the audio amplifier 100 includes a mode control pin MODE, and the function of the mode selector 180 is mounted outside the audio amplifier 100, for example, inside a microcomputer. Audio amplifier 100, depending on the state of the mode control pin MODE, the first mode phi ₁ and the second mode phi ₂ switch.

In FIG. 11C, the mode selector 180 includes an input detection circuit 186. Input detection circuit 186 monitors the audio signal inputted to the input pin IN, the undetected state of the audio signal as a first mode phi _1, when detecting the input of the audio signal is switched to the second mode phi _2.

In FIG. 11D, the mode selector 180 includes a voltage comparator 188. The voltage comparator 188 compares the output signal V _OUT of the amplifier 102 with the threshold voltage V _MODE and selects the first mode φ ₁ when V _OUT <V _MODE and the second mode φ ₂ when V _OUT > V _MODE. To do.

(Modification 2)
Although the single-end audio amplifier 100 has been described in the embodiment, the present invention can also be applied to a BTL audio amplifier. FIG. 12 is a block diagram showing a BTL audio amplifier 100A. The audio amplifier 100A includes amplifiers 102P and 102N. Amplifier 102P, 102N of each configuration is similar to the amplifier 102 described above, the resistor _{R 13} is connected to each other input without being grounded.

(Modification 3)
In the embodiment has described the protection of the high side transistor M _H, it may be similarly protected also the low-side transistor M _L. FIG. 13 is a circuit diagram of a portion related to overcurrent protection of an audio amplifier according to a modification. 13, the H of subscripts to the circuit block relating to the protection of the high side transistor M _H, are denoted by the L subscripts to the circuit block relating to the protection of the low-side transistor M _L. The circuit block corresponding to the high side and the low side has a configuration in which the polarity of the transistors is switched and the top and bottom are reversed. Overcurrent detection circuit 150L for low side, the sink current flowing through the low-side transistor M _L compared to over-current threshold, and generates a second detection signal. This overcurrent threshold is also switched between the first mode and the second mode. The low-side protection circuit 170L enters the first protection state when the second detection signal is asserted in the first mode, and enters the second protection state when the second detection signal is asserted in the second mode. An inverter is added to the input of the timer circuit 176L. Further, the inverter INV1 in FIG. 9 is replaced with a NAND gate. Others are the same as FIG.

(Modification 4)
In the embodiment, the audio amplifier composed of MOS transistors has been described. However, some or all of the transistors may be composed of bipolar transistors. In that case, the gate, the source, and the drain may be read as the base, the emitter, and the collector.

(Modification 5)
In the configuration example of FIG. 7 and the like, the common protection transistor 172 is used in the first protection state and the second protection state, but not limited thereto, the protection transistor for the first protection state, and the second protection state Two protective transistors may be provided separately in parallel.

(Use)
Finally, an application of the audio output device 200 will be described. 14A to 14C are external views of the electronic device 1. FIG. 14A shows a display device 600 that is an example of the electronic apparatus 1. The display device 600 includes a housing 602 and a speaker 2. The audio output device 200 is built in the housing and drives the speaker 2. The speaker 2 corresponds to the electroacoustic transducer 202.

  FIG. 14B shows an audio component 700 that is an example of the electronic apparatus 1. The audio component 700 includes a housing 702 and a speaker 2. The audio output device 200 is built in the housing 702 and drives the speaker 2.

  FIG. 14C illustrates a small information terminal 800 that is an example of the electronic apparatus 1. The small information terminal 800 is a smartphone, a tablet PC (Personal Computer), an audio player, a digital camera, a digital video camera, or the like. The small information terminal 800 includes a housing 802, a speaker 2, and a display 804. The audio output device 200 is built in the housing 802 and drives the speaker 2.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments merely show one aspect of the principle and application of the present invention. Many variations and modifications of the arrangement are allowed without departing from the spirit of the present invention defined in the scope.

DESCRIPTION OF SYMBOLS 100 Audio amplifier 102 Amplifier 110 Operational amplifier 112 Output stage 114 Differential amplification stage _MH High side transistor M _L Low side transistor 120 Bias circuit 150 Overcurrent detection circuit 152 Replica transistor 154 Comparison circuit 156 Current mirror circuit CS1 1st current source CS2 2nd Current source 170 Protection circuit 172 Protection transistor 173 Protection execution unit 174 Timer latch circuit 176 Timer circuit 178 Latch circuit 180 Mode selector SW1 First switch SW2 Second switch 200 Audio output device 202 Electroacoustic transducer

Claims (18)

An operational amplifier having a push-pull type output stage;
In the first mode, the overcurrent threshold is a first value, and in the second mode, the overcurrent threshold is a second value larger than the first value, and the source current flowing through the high-side transistor of the output stage is An overcurrent detection circuit that asserts a detection signal when the source current exceeds the overcurrent threshold compared to a current threshold;
A protection circuit that enters a first protection state when the detection signal is asserted in the first mode, and enters a second protection state when the detection signal is asserted in the second mode;
An audio amplifier comprising:   The protection circuit limits the source current so as not to exceed the first current overcurrent threshold value in the first protection state, and turns off the high-side transistor in the second protection state. The audio amplifier according to claim 1.   The audio amplifier according to claim 1, wherein the protection circuit includes a protection transistor provided between a gate source and a base emitter of the high side transistor.   The audio amplifier according to claim 3, wherein the protection circuit changes a gate voltage of the protection transistor in accordance with a signal level of the detection signal in the first protection state.   The audio amplifier according to claim 3 or 4, wherein the protection circuit fully turns on the protection transistor in the second protection state.   6. The audio amplifier according to claim 1, wherein the protection circuit latches assertion of the detection signal in the second protection state and maintains the second protection state.   7. The audio amplifier according to claim 1, wherein the first mode is set when the audio amplifier is activated, and the second mode is set when the audio amplifier is normally operated. A bias circuit for generating a bias voltage of the operational amplifier;
The first mode is set when the bias voltage is lower than a threshold voltage, and the second mode is set when the bias voltage is higher than the threshold voltage. Audio amplifier.   8. The audio amplifier according to claim 1, wherein the first mode is set before a predetermined time elapses from the activation of the audio amplifier, and the second mode is set after the predetermined time elapses. It further includes a control pin that receives an external control signal,
8. The audio amplifier according to claim 1, wherein the first mode is set when the control signal is at a first level, and the second mode is set when the control signal is at a second level.   8. The first mode when the output voltage of the operational amplifier is lower than a threshold voltage, and the second mode when the output voltage is higher than the threshold voltage. Audio amplifier as described in. The overcurrent detection circuit includes:
A replica transistor having a gate or base commonly connected to the high-side transistor;
A comparison circuit for comparing a current flowing through the replica transistor with a first threshold current in the first mode and a second threshold current larger than the first threshold current in the second mode;
The audio amplifier according to claim 1, comprising: The comparison circuit is
A first resistor provided between the replica transistor and a power supply line;
A second resistance having a first resistance value in the first mode and a second resistance value in the second mode;
A current mirror circuit using the first resistor and the second resistor as source / emitter loads;
A first current source connected to an input of the current mirror circuit;
A second current source connected to the output of the current mirror circuit;
The audio amplifier according to claim 12, wherein the detection signal depends on a state of a connection node between the current mirror circuit and the second current source. In the first mode, the overcurrent threshold is a first value, and in the second mode, the overcurrent threshold is a second value larger than the first value, and the sink current flowing through the low-side transistor of the output stage is reduced. A low-side overcurrent detection circuit that asserts a second detection signal when the sink current exceeds the overcurrent threshold compared to a current threshold;
A low-side protection circuit that enters a first protection state when the second detection signal is asserted in the first mode, and enters a second protection state when the second detection signal is asserted in the second mode;
The audio amplifier according to claim 1, further comprising:   The audio amplifier according to claim 1, wherein the audio amplifier is integrated on a single semiconductor substrate. An electroacoustic transducer;
An audio amplifier that drives the electroacoustic transducer;
With
The audio amplifier is
A push-pull type output stage including a high-side transistor and a low-side transistor;
A differential amplification stage for controlling the output stage according to an audio signal;
In the first mode, the overcurrent threshold is a first value, and in the second mode, the overcurrent threshold is a second value larger than the first value, and the source current flowing through the high-side transistor of the output stage is An overcurrent detection circuit that asserts a detection signal when the source current exceeds the overcurrent threshold compared to a current threshold;
A protection circuit that enters a first protection state when the detection signal is asserted in the first mode, and enters a second protection state when the detection signal is asserted in the second mode;
An audio output device comprising:   An electronic apparatus comprising the audio output device according to claim 16. An audio amplifier protection method,
The audio amplifier includes a push-pull type output stage including a high-side transistor and a low-side transistor,
The protection method is:
Limiting the source current flowing through the high-side transistor so as not to exceed a first threshold when the audio amplifier is activated;
Stopping the output stage when the source current exceeds a second threshold value greater than the first threshold value after completion of startup of the audio amplifier;
A protection method comprising:

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