Abstract:
An output-stage circuit is disclosed. The output-stage circuit includes high-side output driver, first body selector, low-side output driver, second body selector and inductance. When output current is larger than current threshold value so as to make the low-side output driver generate overcurrent, the low-side output driver controlled by second control signal is disabled, and the high-side output driver controlled by first control signal is enabled so as to create first current channel. When output current is larger than current threshold value so as to make the high-side output driver generate overcurrent, the high-side output driver controlled by the first control signal is disabled, and the low-side output driver controlled by the second control signal is enabled so as to create second current channel to avoid current flowing through low-side output driver&#39;s body, and thus reduce the output current and voltage spiking of the output voltage.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    1. Field of the Disclosure 
         [0002]    The instant disclosure relates to an output-stage circuit: in particular, to an over-current protection method of the output-stage circuit. 
         [0003]    2. Description of Related Art 
         [0004]    In recent years, considering technology of semiconductor is getting more mature and portable consumer electronics devices are getting wide popularity, how to design an audio amplifier has become an important research topic. Generally speaking, an output-stage circuit is consisted of a high-side output driver and a low-side output driver, and the high-side output driver devices and low-side output driver device are respectively connected to the body selector to prevent a speaker from making a clicking sound, wherein the high-side output driver is a high-side P-type power transistor, and the low-side output driver is a low-side N-type power transistor. Regarding cause and control of the clicking sound, there is reference in content of disclosure in U.S. Pat. No. 7,265,614. 
         [0005]    Generally speaking, an audio amplifier utilizes a load, such as a speaker, through an output-stage circuit. Once the current flowing through is larger than an affordable value of the high-side P-type power transistor or the low-side N-type power transistor, there will be an over-current fault, wherein there are a lot causes of the over-current fault, such as a short circuit or a spiking voltage. The faults may hurt the high-side P-type power transistor or the low-side N-type power transistor seriously and permanently, and further lead to a damage to the entire audio circuit. For example, in prior art, when an output current flowing into a body selector through the high-side P-type power transistor to release the output current, the spiking voltage of the output voltage will be too high because of a body diode inside of the high-side P-type power transistor and the body selector; on the other hand, when the output current flowing into another body selector through the low-side N-type transistor to release an output current, the spiking voltage of the output voltage will be too low because of a body diode inside of the N-type transistor and the body selector. 
         [0006]    Therefore, how to avoid sudden arising current from damaging circuit elements becomes a problem for an audio circuit design. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    The instant disclosure provides an output-stage circuit, and the output-stage circuit includes a high-side output driver, a first body selector, a low-side output driver, a second body selector, and an inductor. The high-side output driver is electrically connected to a system voltage and a first control signal. The first body selector is electrically connected to the high-side output driver, and the first body selector receives a first reference voltage and the system voltage. The low-side output driver is electrically connected to a ground voltage and a second control signal, wherein the high-side output driver is directly connected to the low-side output driver and generates an output voltage. The second body selector is electrically connected to the low-side output driver, and the second body selector receives a second reference voltage and the ground voltage. An end of the inductor is electrically connected to the output voltage, and another end of the inductor is electrically connected to a third reference voltage, and there is an output current flowing through the inductor. When the output current is larger than a current threshold value so that the low-side output driver generates an overcurrent, the low-side output driver controlled by the second control signal is disabled, and the high-side output driver controlled by the first control signal is enabled so as to establish an output end of low impedance to a first current channel of the system voltage, and to prevent the current flowing through a body of the high-side output driver, and accordingly to reduce the spiking voltage of the output voltage after an overcurrent prevention started by the low-side output driver; when the output current is larger than the current threshold value so that the high-side output driver generates an overcurrent, the high-side output driver controlled by the first control signal is disabled, and the low-side output driver controlled by the second control signal is enabled so as to establish an output end of low impedance to a second current channel of the system voltage, and a current path of the output current is changed so as to prevent the current flowing through a body of the high-side output driver, and accordingly to reduce the output current and a voltage spiking of the output voltage after an overcurrent prevention started by the high-side output driver. 
         [0008]    In an embodiment of the instant disclosure, when an output current reduces until becoming a zero current, a high-side output driver controlled by a first control signal is disabled to close a first current channel, wherein the first current channel has a better feature of low impedance in comparison to a first body selector. 
         [0009]    In an embodiment of the instant disclosure, when an output current reduces until becoming a zero current, a low-side output driver controlled by a second control signal is disabled to close a second current channel, wherein the second current channel has a better feature of low impedance in comparison to a second body selector. 
         [0010]    In an embodiment of the instant disclosure, an output-stage circuit further includes a detector and a controller. The detector is for detecting if an output current is larger than a current threshold value or a zero current, and accordingly to transmit a detecting result. The controller is electrically connected to the detector, and according to the detecting result received to transmit a first control signal and a second control signal respectively to a high-side output driver and a low-side output driver corresponded so as to control a state of being disabled or being enabled. 
         [0011]    In an embodiment of the instant disclosure, a third reference voltage is a system voltage or a ground voltage. 
         [0012]    In an embodiment of the instant disclosure, a high-side output driver includes a first P-type transistor, and a low-side output driver includes a first N-type transistor, wherein the first P-type transistor has a gate receiving a first control signal and accordingly determining to be open or closed, a source connected to a system voltage, a drain connected to output node, a body connected to a first body selector, and a embedded drain(P+) to body(N-Well) diode called as first body diode; wherein the first N-type transistor has a a gate receiving a second control signal and accordingly to determine whether to be open or closed, a source connected to a ground voltage, a drain connected to the same output node of the first P-type transistor, a body connected to a second body selector, and a embedded body(P-sub or P-well) to drain(N+) diode called as second body diode; wherein when a current value of an output current is larger than a current threshold value so that there is an overcurrent at the first N-type transistor, and the first N-type transistor controlled by a second control signal is disabled, and the first P-type transistor controlled by the first control signal is enabled so as to create a first current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, wherein the first current channel is a channel between a drain and a source of the first P-type transistor. 
         [0013]    In an embodiment of the instant disclosure, when a current value of an output current is larger than a current threshold value so that there is an overcurrent at the first P-type transistor, a first P-type transistor controlled by a first control signal is disabled, and a first N-type transistor controlled by a second control signal is enabled to create a second current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, wherein the second current channel is a channel between a drain and a source of the first N-type transistor. 
         [0014]    In an embodiment of the instant disclosure, a high-side output driver further includes a second P-type transistor, and a low-side output driver further includes a second N-type transistor, wherein the second P-type transistor has a third body diode having a source connected to a drain of the first P-type transistor, and having a gate receiving a third control signal so as to determine to be open or closed, and the second N-type transistor has a fourth body diode having a drain connected to a drain of the second P-type transistor and outputs an output voltage, and having a gate receiving a fourth control signal so as to determine to be open or closed, and having a source connected to a drain of the first N-type transistor. 
         [0015]    The present embodiment further provides an over-current protection method, the output-stage circuit includes a high-side output driver, a first body selector, a low-side output driver, a second body selector, and an inductor, wherein the high-side output driver is electrically connected to a system voltage and a first control signal, and the first body selector is electrically connected to a high-side output driver, and receives a first reference voltage and a system voltage, the low-side output driver is electrically connected to a ground voltage and a second control signal, and a high-side output driver is directly connected to the low-side output driver to generate an output voltage, and the second body selector is electrically connected to the low-side output driver and receives a second reference voltage and a ground voltage; an end of an inductor is electrically connected to the output voltage, and another end is electrically connected to a third reference voltage, and there is an output current flowing through the inductor. An over-current protection method includes steps as follows: detecting a current value of an output current; determining whether there is an overcurrent at a high-side output driver or a low-side output driver; when the current value of an output current is larger than a current threshold value so that there is an overcurrent at the low-side output driver, the low-side output driver is disabled and the high-side output driver is enabled to create a current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, and when the output current reduces until becoming a zero current, the high-side output driver is disabled; when the current value of the output current is larger than the current threshold value so that there is an overcurrent at the high-side output driver, the high-side output driver is disabled and the low-side output driver is enabled to create a second current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, and when the output current reduces until becoming a zero current, the low-side output driver is disabled. 
         [0016]    An embodiment of the instant disclosure further provides an audio amplify system, the audio amplify system includes a pre-stage amplifier, an output-stage circuit and a load. The pre-stage amplifier receives and amplifies an input signal. The output-stage circuit is electrically connected to the pre-stage amplifier. The load is electrically connected to the output-stage circuit. 
         [0017]    To sum up, the instant disclosure provides an output-stage circuit and an over-current protection method and an audio amplify system, which are able to utilize detecting if an output current is larger than a current threshold value to determine whether to change a current path of the output current, which means to bring the output current to a first current channel or a second current channel to reduce a current value of the output current, and further to reduce a spiking voltage of the output voltage to prevent the output voltage from damaging or striking the relevant electronic elements. 
         [0018]    For further understanding of the instant disclosure, reference is made to the following detailed description illustrating the embodiments and examples of the instant disclosure. The description is only for illustrating the instant disclosure, not for limiting the scope of the claim. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a schematic diagram of a circuit block of an output-stage circuit according to an embodiment of the instant disclosure; 
           [0020]      FIG. 2  shows a schematic diagram of a detailed circuit of an output-stage circuit according to an embodiment of the instant disclosure; 
           [0021]      FIG. 3  shows a waveform diagram according to the output-stage circuit driven in  FIG. 2 ; 
           [0022]      FIG. 4  shows a detailed diagram of an output-stage circuit according to the instant disclosure; 
           [0023]      FIG. 5  shows a waveform diagram according to the output-stage circuit driven and drawn in  FIG. 4 ; 
           [0024]      FIG. 6  shows a detailed diagram of an output-stage circuit according to an embodiment of the instant disclosure; 
           [0025]      FIG. 7  shows an over-current protection method of an output-stage circuit according to an embodiment of the instant disclosure; and 
           [0026]      FIG. 8  shows a block circuit diagram according to an audio amplify system according to an embodiment of the instant disclosure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    The aforementioned illustrations and following detailed descriptions are exemplary for the purpose of further explaining the scope of the instant disclosure. Other objectives and advantages related to the instant disclosure will be illustrated in the subsequent descriptions and appended drawings. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. 
         [0028]    It will be understood that, although the terms first, second, third, and the like, may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only to distinguish one element, component, region, layer or section from another region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the instant disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0029]    [Embodiment of an Output-Stage Circuit] 
         [0030]    Referring to  FIG. 1 ,  FIG. 1  shows a schematic diagram of a circuit block of an output-stage circuit according to an embodiment of the instant disclosure. As shown in  FIG. 1 , the output-stage circuit  100  includes a high-side output driver  110 , a first body selector  120 , a low-side output driver  130 , a second body selector  140 , and an inductor L. The high-side output driver  110  is electrically connected to a system voltage VCC and a first control signal CS 1 . The first body selector  120  is electrically connected to the high-side output driver  110 . The low-side output driver  130  is electrically connected to a ground voltage GND and a second control signal CS 2  and directly connected to the high-side output driver  110 . The second body selector  140  is electrically connected to the low-side output driver  130 . An end of the inductor L is electrically connected between the high-side output driver  110  and the low-side output driver  130  and there is an output voltage VOUT generated. Another end of the inductor L is electrically connected to a third reference voltage VREF 3 , and there is an output current IOUT flowing through the inductor L. 
         [0031]    In the present embodiment, the output-stage circuit  100  further includes a detector  150  and a controller  160 . The controller  160  is electrically connected to the detector  150 . First of all, the instant disclosure utilizes the detector  150  to detect if the output current IOUT of the inductor L is larger than a current threshold value or if the output current IOUT a zero current, and accordingly transmits a detecting result RS to the controller  160 . Afterward, the controller  160  respectively transmits the first control signal CS 1  and the second control signal CS 2  to the high-side output driver  110  and the low-side output driver  130  according to the detecting result RS received, so as to control a status to be disabled or enabled. 
         [0032]    In the prior art, when the output current IOUT flowing into the first body selector  120  to release the output current IOUT through the high-side output driver  110 , a spiking voltage of the output voltage VOUT will be too high; on the other hand, when the output current IOUT flowing into the second body selector  140  to release the output current IOUT through the low-side output driver  130 , the spiking voltage of the output voltage VOUT will be too low. Therefore, the instant disclosure provides a change of releasing path of the output current IOUT to prevent the spiking voltage of the output voltage 
         [0033]    VOUT from being too high and further damaging electronic elements surrounding a terminal of the output voltage VOUT or leading to an circuit error operation, wherein the current releasing path has a feature of less impedance in comparison to the first body selector  120  or the second body selector  140 . 
         [0034]    There is more instruction for further teaching in a work mechanism of the output-stage circuit  100 . 
         [0035]    In the present embodiment, when the third reference voltage VREF 3  is the system voltage, and when the detector  150  detects that the output current IOUT is larger than the current threshold value (which means that there is about to be an overcurrent at the low-side output driver  130 ), a detecting result RS will be transmitted to the controller  160 . Afterwards, according to the detecting result RS received, the controller  160  will transmit the first control signal CS 1  to the high-side output driver  110  to enable the high-side output driver  110  to create a first current channel; in the meantime, the second control signal CS 2  will be transmitted to the low-side output driver  130  to disable the low-side output driver  130  so as to further change a current path of the output current IOUT, and accordingly to bring the output current IOUT to a first current channel inside the high-side output driver  110  to release the output current IOUT to another end of the system voltage VCC. The current channel has the feature of low impedance compared to the first body selector  120 , and thus during a transient-state of releasing the output current IOUT, the spiking voltage of the output voltage VOUT will not surge too high, so that the electronic elements electrically connected to the output voltage VOUT are further protected. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT, the detector  150  will continuously detect a value change of the output current IOUT, when the detector  150  detects that the value of the output current IOUT reduces until becoming a zero current, the detector  150  will transmit the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  will transmit the first control signal CS 1  to the high-side output driver  110  to disable the high-side output driver  110  so as to prevent an overcurrent at the high-side output driver  110 . Then the output voltage VOUT and the system voltage VCC will be at the same voltage level. 
         [0036]    On the other hand, when the third reference voltage VREF  3  is the ground voltage, and when the detector  150  detects that the output current IOUT is larger than the current threshold value (which means that there is about to be an overcurrent at the high-side output driver  110 ), then there is the detecting result RS transmitted to the controller  160 . Afterwards, according to the detecting result RS received, the controller  160  will transmit the first control signal CS 1  to the high-side output driver  110  to disable the high-side output driver  110 ; in the meantime, the second control signal CS 2  is transmitted to the low-side output driver  130  to enable the low-side output driver  130  so as to create a second current channel, and the current path of the output current IOUT is further changed, and accordingly the output current IOUT is brought to the second current channel inside the low-side output driver  130  to release the output current IOUT to the ground voltage GND. The second current channel has a feature of low impedance in comparison to the second body selector  140 , and thus during the transient-state of releasing the output current IOUT, the spiking voltage of the output voltage VOUT will not surge too low, so that the electronic elements electrically connected to the output voltage VOUT are further protected. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT, the detector  150  will continuously detect the value change of the output current IOUT, and when the detector  150  detects that the value of the output current IOUT reduces until becoming a zero current, the detector  150  will transmit the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  will transmit the second control signal CS 2  to the low-side output driver  130  to disable the low-side output driver  130  so as to prevent an overcurrent at the low-side output driver  130 . Then the output voltage VOUT and the ground voltage GND will be at the same voltage level. 
         [0037]    There will be at least one embodiment for a further instruction in describing a specific operation process of the output-stage circuit  100  of the instant disclosure. 
         [0038]    In the following embodiments, there are only parts different from the embodiment in  FIG. 1  described, and the omitted parts are indicated to be identical to the parts of the embodiment in  FIG. 1 . In addition, for an easy instruction, similar numbers or symbols are referred to similar elements. 
         [0039]    [Embodiment of an Output-Stage Circuit] 
         [0040]    Referring to  FIG. 2 ,  FIG. 2  shows a schematic diagram of a detailed circuit of an output-stage circuit according to an embodiment of the instant disclosure. As shown in  FIG. 2 , different from  FIG. 1 , in the present embodiment, a high-side output driver  110  includes a first P-type transistor MP 1 , and a low-side output driver  130  includes a first N-type transistor MN 1 . The first P-type transistor MP 1  has a first body diode D 1 , a gate of the first P-type transistor MP 1  receives a first control signal CS 1  and accordingly to determine whether to be open or closed, and a source of the first P-type transistor MP 1  is connected to a system voltage VCC, and a drain of the first P-type transistor MP 1  outputs the output voltage VOUT, and a body of the first P-type transistor MP 1  is connected to a first body selector  120 . The first N-type transistor MN 1  has a body diode D 2 , and a gate of the first N-type transistor MN 1  receives a second control signal CS 2  and accordingly to determine whether to be open or closed, and a source of the first N-type transistor MN 1  is connected to a ground voltage GND, and a drain of the first N-type transistor MN 1  is connected to the drain of the first P-type transistor MP 1 , and a body of the first N-type transistor MN 1  is connected to a second body selector  140 . 
         [0041]    There is further instruction in teaching a work mechanism of the output-stage circuit  200  so as to understand the instant disclosure better. 
         [0042]    Referring  FIGS. 1-3  together,  FIG. 3  shows a waveform diagram according to the output-stage circuit driven in  FIG. 2 . Regarding the output-stage circuit  200  of the present embodiment, when an end of an inductor L electrically connected to a third reference voltage VREF 3  is the system voltage VCC; likewise, during a time from t 11  to t 12 , the output voltage VOUT continuously gets larger towards a voltage level of the system voltage VCC and an output current IOUT also continuously increases, wherein a current path of the output current IOUT is shown as a symbol of A. At t 12 , the detector  150  detects that the output current IOUT flowing through the inductor L is larger than a current threshold value (which means that the first N-type transistor MN 1  may be an overcurrent), then there is a detecting result RS transmitted to the controller  160 , wherein a designer may further design a current threshold value according to an actual circuit application demand. Afterwards, according to the detecting result RS received, the controller  160  transits the first control signal CS 1  of a high voltage level to a low voltage level and transmits the first control signal CS 1  to the first P-type transistor MP 1  to open the first P-type transistor MP 1  and to create a first current channel, wherein the first current channel having a feature of low impedance is a p-channel between a drain and a source of the first P-type transistor MP 1 ; meanwhile, the second control signal CS 2  at a high voltage level is also transited to a low voltage level and the second control signal CS 2  is transmitted to the first N-type transistor MN 1  to close the first N-type transistor MN 1  and to further change the current path of the output current IOUT, and accordingly the output current IOUT is brought to the first current channel inside the first P-type transistor MP 1  to release the output current IOUT to an end of the system voltage VCC, wherein a current path of the output current IOUT is shown as symbol B, and the current volume starts to present a sign of decreasing. The current path that the output current IOUT flows through the first current channel has a more obvious feature of lower impedance than the current path that the output current IOUT flows through a first body diode D 1  and the first body selector  120 ; therefore, during the time from t 12  to t 13 , which means during a transient-state of releasing the output current IOUT, a spiking voltage of an output voltage VOUT will not overly surge high, so that the most surge of the output voltage VOUT is not much more than the system voltage VCC, and further to protect the relevant electronic elements electrically connected to the output voltage VOUT. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT (i.e. from t 12  to t 13 ), the detector  150  still keeps detecting a value change of the output current IOUT; at t 13 , the detector  150  detects that the current value of the output current IOUT decreases until becoming zero current, and the detector  150  transmits the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  transits the first control signal CS 1  of the low voltage level to the high voltage level and transmits the first control signal CS 1  to the first P-type transistor MP 1  to close the first P-type transistor MP 1  to prevent the first P-type transistor MP 1  from becoming an overcurrent. Then, the output voltage VOUT and the system voltage VCC are at the same voltage level. 
         [0043]    On the other hand, referring to  FIGS. 1 ,  4 , and  5 ,  FIG. 4  shows a detailed diagram of an output-stage circuit according to the instant disclosure.  FIG. 5  shows a waveform diagram according to the output-stage circuit driven and drawn in  FIG. 4 . Before further instruction, it is to be clarified that in comparison to the output-stage circuit  200  in  FIG. 2 , the difference will be a current path of the output-stage circuit  400  from  FIG. 4 , like the current path A corresponds to a current path A′, and the current path B corresponds to a current path B′. 
         [0044]    In the present embodiment, another end of the inductor L of the output-stage circuit  400  electrically connected to the third reference voltage VREF 3  is the ground voltage GND; during a time from t 21  to t 22 , the output voltage VOUT continuously gets smaller towards a voltage level of the ground voltage GND and the output current IOUT also continuously increases, wherein a current path of the output current IOUT is shown as a symbol of A′. At the time of t 22 , the detector  150  detects that the output current IOUT flowing through the inductor L is larger than a current threshold value (which means that the first P-type transistor MP 1  may about to be an overcurrent), then there is a detecting result RS transmitted to the controller  160 , wherein a designer may further design a current threshold value according to an actual circuit application demand. Afterwards, according to the detecting result RS received, the controller  160  transits the first control signal CS 2  of a low voltage level to a high voltage level and transmits the first control signal CS 2  to the first N-type transistor MN 1  to open the first N-type transistor MN 1  and to create a second current channel, wherein the second current channel having a feature of low impedance is a n-channel between a drain and a source of the first N-type transistor MN 1 ; meanwhile, the first control signal CS 1  of low voltage level is transited to a high voltage level and the first control signal CS 1  is transmitted to the first P-type transistor MP 1  to close the first P-type transistor MP 1  and to further change the current path of the output current IOUT, and accordingly the output current IOUT is brought to the second current channel inside the first N-type transistor MN 1  to release the output current IOUT to an end of the ground voltage GND, wherein a current path of the output current IOUT is shown as symbol B′, and the current volume starts to present a sign of decreasing. The current path that the output current IOUT flows through the second current channel has a more obvious feature of lower impedance than the current path that the output current IOUT flows through a second body diode D 2  and a second body selector  140 ; therefore, during the time from t 22  to t 23 , which means during a transient-state of releasing the output current IOUT, a spiking voltage of an output voltage VOUT will not overly surge low, so that the most surge of the output voltage VOUT is not much more than the ground voltage GND, and further to protect the relevant electronic elements electrically connected to the output voltage VOUT. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT (i.e. from t 22  to t 23 ), the detector  150  still keeps detecting a value change of the output current IOUT; at t 23 , the detector  150  detects that the current value of the output current IOUT decreases until becoming zero current, and the detector  150  transmits the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  transits the first control signal CS 2  of the high voltage level to the low voltage level and transmits the second control signal CS 2  to the first N-type transistor MN 1  to close the first N-type transistor MN 1  to prevent the first N-type transistor MN 1  from becoming an overcurrent. Then, after getting into the steady-state period, the output voltage VOUT will be stabilized at the same voltage level as the ground voltage GND. 
         [0045]    In the following embodiments, there are only parts which are different from the embodiment in  FIG. 4  described, and which means the omitted parts are identical to the parts of the embodiment in  FIG. 4 . Besides, for an easy instruction, similar referred numbers or symbols are indicated to be similar elements. 
         [0046]    [An Embodiment of an Output-Stage Circuit] 
         [0047]    Referring to  FIG. 6 ,  FIG. 6  shows a detailed diagram of an output-stage circuit according to an embodiment of the instant disclosure. Different from the embodiment in  FIG. 4 , a high-side output driver  110  further includes a second P-type transistor MP 2 , and a low-side output driver  130  further includes a second N-type transistor MN 2 . The second P-type transistor MP 2  has a third body diode D 3 , and a source of the second P-type transistor MP 2  is connected to a drain of the first P-type transistor MP 1 , and a gate of the second P-type transistor receives a third control signal CS 3  and accordingly to determine to be open or closed, and the second N-type transistor MN 2  has a fourth body diode D 4 , and a drain of the second N-type transistor MN 2  is connected to a drain of the second P-type transistor MP 2  and outputs an output voltage VOUT, and a gate of the second N-type transistor MN 2  receives a fourth control signal CS 4  and accordingly determine to be open or closed, and a source of the second N-type transistor MN 2  is connected to a drain of the first N-type transistor MN 1 . Briefly, the embodiment constructs an output-stage circuit  600  by cascade; therefore, any idea which is not irrelevant to cascade is identified to be within the scope of the present embodiment of the instant disclosure. 
         [0048]    If it is needed, please refer to the waveform of driving in  FIG. 3  to further understand the present embodiment, wherein a waveform of the third control signal CS 3  is identical to the waveform of the first control signal CS 1 , and the waveform of the fourth control signal CS 4  is identical to the waveform of the second control signal CS 2 . In the output-stage circuit  600  of the present embodiment, when an end of an inductor L electrically connected to a third reference voltage VREF 3  is the system voltage VCC; likewise, the output voltage VOUT continuously gets larger towards a voltage level of the system voltage VCC and an output current IOUT also continuously increases. When a detector  150  detects that the output current IOUT flowing through the inductor L is larger than a current threshold value (which means that the first N-type transistor MN 1  or the second N-type transistor MN 2  may be overcurrent), then there is a detecting result RS transmitted to the controller  160 , wherein a designer may further design a current threshold value according to an actual circuit application demand. Afterwards, according to the detecting result RS received, the controller  160  transits the first control signal CS 1  and the third control signal CS 3  of a high voltage level to a low voltage level and transmits the first control signal CS 1  and the third control signal CS 3  to the first P-type transistor WW 1  and the second P-type transistor MP 2  correspondingly to open the first P-type transistor MP 1  and the second P-type transistor MP 2  to create a first current channel and a third current channel, wherein the first current channel and the third current channel both having a feature of low impedance are respectively a p-channel between a drain and a source of the first P-type transistor MP 1  and a p-channel between a drain and a source of the second P-type transistor MP 2 ; meanwhile, the controller  160  also transits the second control signal CS 2  and the fourth control signal CS 4  from the high voltage level to the low voltage level and transmits the second control signal CS 2  and the fourth control signal CS 4  to the first N-type transistor MN 1  and the second N-type transistor MN 2  to close the first N-type transistor MN 1  and the second N-type transistor MN 2  to further change the current path of the output current IOUT, and accordingly the output current IOUT is brought to the first current channel and the third current channel inside the first P-type transistor MP 1  and the second P-type transistor MP 2  to release the output current IOUT to an end of the system voltage VCC, wherein a current path of the output current IOUT is shown as symbol B, and the current volume starts to present a sign of decreasing. The current path that the output current IOUT flows through the first current channel and the third channel has a more obvious feature of lower impedance than the current path that the output current IOUT flows through a first body diode D 1  and a third body diode D 3  and the first body selector  120 ; therefore, during the time from t 12  to t 13 , which means during a transient-state of releasing the output current IOUT, a spiking voltage of an output voltage VOUT will not overly surge high, so that the most surge of the output voltage VOUT is not much more than the system voltage VCC, and further to protect the relevant electronic elements electrically connected to the output voltage VOUT. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT, the detector  150  still keeps detecting a value change of the output current IOUT; at t 13 , the detector  150  detects that the current value of the output current IOUT decreases until becoming zero current, and the detector  150  transmits the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  transits the first control signal CS 1  and the third control signal CS 3  of the low voltage level to the high voltage level and transmits the first control signal CS 1  and the third control signal CS 3  to the first P-type transistor MP 1  and the second P-type transistor MP 2  to close the first P-type transistor MP 1  and the second P-type transistor MP 2  to prevent the first P-type transistor MP 1  and the second P-type transistor MP 2  from becoming overcurrent. Then, the output voltage VOUT and the system voltage VCC are at the same voltage level. 
         [0049]    On the other hand, please continuously refer to  FIG. 6  if it is needed, please refer to the waveform of driving in  FIG. 5  to further understand the present embodiment, wherein a waveform of the third control signal CS 3  is identical to the waveform of the first control signal CS 1 , and the waveform of the fourth control signal CS 4  is identical to the waveform of the second control signal CS 2 . In the present embodiment, another end of the inductor L of the output-stage circuit  400  electrically connected to the third reference voltage VREF 3  is the ground voltage GND; during a time from t 21  to t 22 , the output voltage VOUT continuously gets smaller towards a voltage level of the ground voltage GND and the output current IOUT also continuously increases; likewise, when the detector  150  detects that the output current IOUT flowing through the inductor L is larger than a current threshold value (which means that the first P-type transistor MP 1  and the second P-type transistor MP 2  may about to be overcurrent), then there is a detecting result RS transmitted to the controller  160 , wherein a designer may further design a current threshold value according to an actual circuit application demand. Afterwards, according to the detecting result RS received, the controller  160  transits the first control signal CS 2  and the fourth control signal CS 4  of the low voltage level to the high voltage level and transmits the first control signal CS 2  and the fourth control signal CS 4  to the first N-type transistor MN 1  and the second N-type transistor MN 2  to open the first N-type transistor MN 1  and the second N-type transistor MN 2  and to create a second current channel and a fourth current channel, wherein the second current channel and the fourth current channel having a feature of low impedance are respectively a n-channel between a drain and a source of the first N-type transistor MN 1  and a n-channel between a drain and a source of the second N-type transistor MN 2 ; meanwhile, the controller  160  also transits the first control signal CS 1  and a third control signal CS 3  of the low voltage level from the low voltage level to the high voltage level and the first control signal CS 1  and the third control signal CS 3  are transmitted to the first P-type transistor MP 1  and the second P-type transistor MP 2  to close the first P-type transistor MP 1  and the second P-type transistor MP 2  and to further change the current path of the output current IOUT, and accordingly the output current IOUT is brought to the second current channel and the fourth current channel inside the first N-type transistor MN 1  and the second N-type transistor MN 2  to release the output current IOUT to an end of the ground voltage GND, wherein a current path of the output current IOUT starts to present a sign of decreasing. The current path that the output current IOUT flows through the second current channel and a fourth current channel has a more obvious feature of lower impedance than the current path that the output current IOUT flows through a second body diode D 2 , a fourth body diode D 4  and a second body selector  140 ; therefore, during the time a transient-state of releasing the output current IOUT, a spiking voltage of an output voltage VOUT will not overly surge low, so that the most surge of the output voltage VOUT is not much more than the ground voltage GND, and further to protect the relevant electronic elements electrically connected to the output voltage VOUT. It is to be noticed that, in the present embodiment, during the transient-state of releasing the output current IOUT, the detector  150  still keeps detecting a value change of the output current IOUT; the detector  150  detects that the current value of the output current IOUT decreases until becoming zero current, and the detector  150  transmits the detecting result RS to the controller  160 . Afterwards, according to the detecting result RS, the controller  160  transits the first control signal CS 2  and the fourth control signal CS 4  of the high voltage level to the low voltage level and transmits the second control signal CS 2  and the fourth control signal CS 4  to the first N-type transistor MN 1  and the second N-type transistor MN 2  to close the first N-type transistor MN 1  and the second N-type transistor MN 2  to prevent the first N-type transistor MN 1  and the second N-type transistor MN 2  from becoming an overcurrent. Afterwards, the output voltage VOUT and the ground voltage GND will be at the same voltage level. 
         [0050]    [An Embodiment of an Over-Current Protection Method of an Output-Stage Circuit] 
         [0051]    Referring to  FIG. 7 ,  FIG. 7  shows an over-current protection method of an output-stage circuit according to an embodiment of the instant disclosure. The method may be executed on every output-stage circuit in  FIGS. 1 ,  2 ,  4 , and  6 , and thus please refer to  FIGS. 1-3  for a better understanding. In the present embodiment, the output-stage circuit over-current protection method includes steps as follows: detecting a current value of an output current (S 710 ); determining whether is an overcurrent at a high-side output driver or a low-side output driver (S 720 ); when the current value of an output current is larger than a current threshold value so that there is an overcurrent at the low-side output driver, the low-side output driver is disabled and the high-side output driver is enabled to create a current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, and when the output current reduces until becoming a zero current, the high-side output driver is disabled (S 730 ); when the current value of the output current is larger than the current threshold value so that there is an overcurrent at the high-side output driver, the high-side output driver is disabled and the low-side output driver is enabled to create a second current channel so as to reduce the current value of the output current and a spiking voltage of the output voltage, and when the output current reduces until becoming a zero current, the low-side output driver is disabled (S 740 ). Briefly, any idea which is not irrelevant to “when an output current larger than a current threshold value a current path of an output current is changed to a current channel of a lower impedance to release an output current so as to prevent a spiking voltage of the output voltage from being over and damaging adjacent electronic elements” is identified to be within the scope of the present embodiment of the instant disclosure. 
         [0052]    Relevant details of steps of the over-current protection method of the output-stage circuit are described in the embodiments in  FIGS. 1-6 , and thus it is not repeated thereto. It is to be clarified that a sequence of steps of the embodiment in  FIG. 7  is simply for an easy instruction, and the sequence of the steps of the present embodiment is not used as a condition to embody the embodiments of the instant closure. 
         [0053]    [An Embodiment of an Audio Amplify System] 
         [0054]    Referring to  FIG. 8 ,  FIG. 8  shows a block circuit diagram according to an audio amplify system according to an embodiment of the instant disclosure. An audio amplify system  800  includes a pre-stage amplifier  810 , an output-stage circuit  820  and a load  830 . In the present embodiment, the pre-stage amplifier  810  receives and amplifies an input signal IN, and the output-stage circuit  820  is electrically connected to the pre-stage amplifier  820  to receive an output signal VOUT′, and to transmit an output voltage VOUT to the load  830 . The output-stage circuit  820  may be anyone of output-stage circuits  100 ,  200 ,  400 ,  600  in the embodiments as described above. 
         [0055]    To sum up, the instant disclosure provides an output-stage circuit and an over-current protection method and an audio amplify system, which are able to utilize detecting if an output current is larger than a current threshold value to determine whether to change a current path of the output current, which means to bring the output current to a first current channel or a second current channel to reduce a current value of the output current, and further to reduce a spiking voltage of the output voltage to prevent the output voltage from damaging or striking the relevant electronic elements. 
         [0056]    The descriptions illustrated supra set forth simply the preferred embodiments of the instant disclosure; however, the characteristics of the instant disclosure are by no means restricted thereto. All changes, alternations, or modifications conveniently considered by those skilled in the art are deemed to be encompassed within the scope of the instant disclosure delineated by the following claims.