Abstract:
The present disclosure provides a pair of NMOSFET switches connected in series, an output filter, a control circuit, a boot-strap capacitor and a disabling circuit. A high-side MOSFET switch is coupled to an input voltage. A low-side MOSFET switch is coupled to a ground. The high-side MOSFET switch and the low-side MOSFET switch have complementary duty cycles. The output filter is coupled to the NMOSFET switches to provide an output voltage. The boot-strap capacitor is coupled to the source of the high-side MOSFET switch. The voltage crossing the boot-trap capacitor is for making the gate voltage of the high-side MOSFET switch to be higher than the input voltage. The disabling circuit senses the voltage crossing the boot-strap capacitor, and generates a control signal to control the control circuit for continuously turning off the high-side MOSFET switch when the voltage crossing the boot-strap capacitor is less than a threshold voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The instant disclosure relates to a buck converter; in particular, to a buck converter and control method therefor. 
     2. Description of Related Art 
     Please refer to  FIG. 1  in conjunction with  FIG. 2A  and  FIG. 2B ,  FIG. 1  shows a circuit diagram of a conventional buck converter. The conventional buck converter  1  comprises a high-side N-type MOSFET  11 , an inductor L, a capacitor C, a low-side N-type MOSFET  12 , a control circuit  13 , a boot-strap capacitor Cboot and a charge controller  14 . An inductor L and a capacitor C of the buck converter  1  constitute an output filter for providing an output voltage VOUT. The control circuit  13  comprises a gate driving circuit  131 , a feedback comparator  132  and a feedback circuit constituted of resistors R 1 , R 2  (which dividing the output voltage VOUT to generate a feedback signal FB. The gate driving circuit  131  comprises a gate driving logic  1311 , a buffer  1312  and a buffer  1313 . The feedback comparator  132  compares the feedback FB and the reference voltage VREF. In the buck converter  1 , the gate of the high-side N-type MOSFET switch  11  is turned-on or turned-off by the voltage of the boot-strap capacitor Cboot, wherein the voltage of the boot-strap capacitor is charged by the input voltage VIN through the charge controller  14  when the low-side N-type MOSFET  12  is turned-on. When the buck converter is in a light load operation, the low-side N-type MOSFET would not be turned-on most of the time. Ideally, the voltages of a first terminal BST and a second terminal SW of the boot-strap capacitor Cboot are shown in  FIG. 2A . However, in practical, the voltage level of the boot-strap capacitor Cboot would decrease gradually due to leakage current, as shown in  FIG. 2B . Therefore, when the output voltage VOUT is not enough, the voltage of the gate driving signal (varying with the voltage of the capacitor Cboot) controlling the high-side N-type MOSFET would be not high enough. Then the resistance of the high-side N-type MOSFET becomes large, and the high-side N-type MOSFET may be damaged due to large power dissipation leading to burnout of circuit. 
     Please refer to  FIG. 1  in conjunction with  FIG. 2C ,  FIG. 2C  shows a curve diagram of the voltages at two terminals of a boot-strap capacitor of a conventional buck converter while the boot-strap capacitor being charged when the voltages across the two terminals of the boot-trap capacitor are less than a threshold voltage. U.S. Pat. No. 5,627,460 illustrates a technique avoiding the in-sufficient gate driving voltage of the high-side N-type MOSFET due to low voltage of the boot-strap capacitor Cboot. This technical solution turns on the low-side N-type MOSFET when the BST terminal voltage is below Vt, thus keeps the Cboot being re-charged frequently, as shown in  FIG. 2C . However, it may cause efficiency reduction of the buck converter. 
     SUMMARY OF THE INVENTION 
     The object of the instant disclosure is to provide a buck converter and control method therefor in order to avoid damaging the high-side N-type MOSFET of the buck converter during operation. 
     In order to achieve the aforementioned objects, according to an embodiment of the instant disclosure, a buck converter is provided. The buck converter steps-down an input voltage to an output voltage. The buck converter comprises a high-side N-type MOSFET switch, a low-side N-type MOSFET switch, an output filter, a control circuit, a boot-strap capacitor and a disabling circuit. The drain electrode of the high-side N-type MOSFET switch is coupled to the input voltage. The drain electrode of the low-side N-type MOSFET switch is coupled to the source electrode of the high-side N-type MOSFET switch. The source electrode of the low-side N-type MOSFET switch is coupled to a ground. The high-side N-type MOSFET switch and the low-side N-type MOSFET switch have complementary duty cycles. The output filter is coupled to the source electrode of the high-side N-type MOSFET switch and the drain electrode of the low-side N-type MOSFET switch for providing the output voltage. The control circuit controls the high-side N-type MOSFET switch and the low-side N-type MOSFET switch. A first terminal of the boot-strap capacitor is coupled to a regulating voltage. A second terminal of the boot-strap capacitor is coupled to the source electrode of the high-side N-type MOSFET switch. The boot-strap capacitor is charged by the regulating voltage. The voltage of the first terminal of the boot-strap capacitor is provided to the control circuit for generating a gate driving signal controlling the high-side N-type MOSFET switch, wherein the voltage of the gate driving signal is higher than the input voltage. The disabling circuit is coupled to the boot-strap capacitor and the control circuit. The disabling circuit senses the voltage across the boot-strap capacitor, and generates a control signal to control the control circuit for continuously turning off the high-side N-type MOSFET switch when the voltage crossing the boot-strap capacitor is less than a threshold voltage. 
     In order to achieve the aforementioned objects, according to an embodiment of the instant disclosure, a control method for a buck converter is provided. The buck converter comprises the high-side N-type MOSFET switch coupling to an input voltage and a low-side N-type MOSFET switch coupling to a ground, the control method comprising coupling a boot-strap capacitor between a regulating voltage and the source electrode of the high-side N-type MOSFET switch, wherein the source electrode of the high-side N-type MOSFET switch is coupled to the drain electrode of the low-side N-type MOSFET switch; turning-on the low-side N-type MOSFET switch, so as to make the regulating voltage charge the boot-strap capacitor through a path comprising the low-side N-type MOSFET switch; utilizing a gate driving signal to drive the high-side N-type MOSFET switch, wherein the voltage level of the gate driving signal is corresponding to the voltage across the boot-strap capacitor; sensing the voltage across the boot-strap capacitor; and turning-off the high-side N-type MOSFET switch continuously when the voltage across the boot-strap capacitor is less than a threshold voltage. 
     In summary, a buck converter and control method therefor are provided to avoid the high-side N-type MOSFET switch being turned on when the voltage across the boot-strap capacitor is less than a threshold voltage. Accordingly, dangerous situation of damage (e.g. burning) of the high-side N-type MOSFET due to in-sufficient voltage of gate driving signal may be avoided. 
     In order to further the understanding regarding the instant disclosure, the following embodiments are provided along with illustrations to facilitate the disclosure of the instant disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram of a conventional buck converter; 
         FIG. 2A  shows a curve diagram of the voltages at two terminals of a boot-strap capacitor of a conventional buck converter in the ideal case; 
         FIG. 2B  shows a curve diagram of the voltages at two terminals of a boot-strap capacitor of a conventional buck converter in the real case; 
         FIG. 2C  shows a curve diagram of the voltages at two terminals of a boot-strap capacitor of a conventional buck converter while the boot-strap capacitor being charged when the voltages at two terminals of the boot-trap capacitor are less than a threshold voltage; 
         FIG. 3  shows a circuit diagram of a buck converter according to a an embodiment of the instant disclosure; 
         FIG. 4  shows a curve diagram of the voltages at two terminals of the boot-strap capacitor of the buck converter of  FIG. 3 ; 
         FIG. 5  shows a circuit diagram of a buck converter according to another embodiment of the instant disclosure; 
         FIG. 6  shows a curve diagram of the voltages at two terminals of the boot-strap capacitor of the buck converter of  FIG. 5 ; and 
         FIG. 7  shows a flow chart of a control method for a buck converter according to an embodiment of the instant disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     [An Embodiment of a Buck Converter] 
     Please refer to  FIG. 3  showing a circuit diagram of a buck converter according to an embodiment of the instant disclosure. The buck converter  3  steps-down an input voltage VIN to an output voltage VOUT. The buck converter  3  comprises a high-side N-type MOSFET switch  31 , a low-side N-type MOSFET switch  32 , an output filter  37 , a control circuit  33 , a boot-strap capacitor Cboot, a disabling circuit  34  and a charge and discharge controller  35 . 
     The high-side N-type MOSFET switch  31 , the low-side N-type MOSFET switch  32 , and the output filter  37  constitute the typical buck converter. The output filter  37  comprises an inductor L and a capacitor C. A first terminal of the inductor L is coupled to the source of the high-side N-type MOSFET switch  31  and the drain of the low-side N-type MOSFET switch  32 . A second terminal of the inductor L provides the output voltage VOUT. A first terminal of the capacitor C is coupled to the second terminal of the inductor L, and a second terminal of the capacitor C is coupled to a ground GND. 
     The drain electrode of the high-side N-type MOSFET switch  31  is coupled to the input voltage VIN. The drain electrode of the low-side N-type MOSFET switch  32  is coupled to the source electrode of the high-side N-type MOSFET switch  31 . The source electrode of the low-side N-type MOSFET switch  32  is coupled to the ground GND. The high-side N-type MOSFET switch  31  and the low-side N-type MOSFET switch  32  have complementary duty cycles. The output filter  37  is coupled to the source electrode of the high-side N-type MOSFET switch  31  and the drain electrode of the low-side N-type MOSFET switch  32  for providing the output voltage VOUT. The control circuit  33  controls the high-side N-type MOSFET switch  31  and the low-side N-type MOSFET switch  32  to be turned-on or turned-off. A first terminal BST of the boot-strap capacitor Cboot is coupled to a regulating voltage PVDD of the charge and discharge controller  35 . A second terminal SW of the boot-strap capacitor Cboot is coupled to the source electrode of the high-side N-type MOSFET switch  31  (and the drain electrode of the low-side N-type MOSFET switch  32 ). The boot-strap capacitor Cboot is charged by the regulating voltage PVDD through a boot-strap switch  352  when the low-side N-type MOSFET  32  is turned-on. The voltage of the first terminal BST of the boot-strap capacitor Cboot is provided to the buffer  3312  for generating a gate driving signal S 1  controlling the high-side N-type MOSFET switch  31 , wherein the voltage of the gate driving signal S 1  is higher than the input voltage VIN. The disabling circuit  34  is coupled to the boot-strap capacitor Cboot and the control circuit  33 . The disabling circuit  34  senses the voltage across the boot-strap capacitor Cboot (i.e. voltage difference between the first terminal BST and the second terminal SW), and generates a control signal CT to control the control circuit  33  for continuously turning off the high-side N-type MOSFET switch  31  when the voltage crossing the boot-strap capacitor Cboot is less than a threshold voltage Vt. 
     Specifically, as shown in  FIG. 3 , the control circuit  33  comprises a gate driving circuit  331 , a feedback comparator  332  and a feedback circuit constituted of the resistors R 1 , R 2 . The feedback circuit generates a feedback voltage FB according to the output voltage VOUT. The feedback comparator  332  is coupled to the feedback circuit, and compares the feedback voltage FB and a reference voltage VREF for generating a comparing signal. The gate driving circuit  331  is coupled to the feedback comparator  332  and the gate electrode of the high-side N-type MOSFET switch  31  and the gate electrode of the low-side N-type MOSFET switch  32 , and controls the high-side N-type MOSFET switch  31  and the low-side N-type MOSFET switch  32  according to the comparing signal for maintaining the output voltage VOUT to a stable voltage. 
     In this embodiment, the gate driving circuit  331  comprises a gate driving logic  3311 , a buffer  3312  and a buffer  3313 . The gate driving logic  3311  generates a control signal controlling (e.g. turn-on or turn-off) the high-side N-type MOSFET switch  31 , and the buffer  3312  generates the gate driving signal S 1  according to the mentioned control signal for controlling the high-side N-type MOSFET switch  31 . For example, the buffer  3312  (or the buffer  3313 ) turns on the high-side N-type MOSFET switch  31  (or the low-side N-type MOSFET switch  32 ) when the gate driving logic  3311  generates a HIGH voltage level signal. The buffer  3312  (or the buffer  3313 ) turns off the high-side N-type MOSFET switch  31  (or the low-side N-type MOSFET switch  32 ) when the gate driving logic  3311  generates a LOW voltage level signal. 
     The buffer  3312  and the buffer  3313  are driving stages for providing sufficient variation range of the gate driving voltage. The variation range of the operation voltage of the buffer  3312  is between the voltage level of the first terminal BST of the boot-strap capacitor Cboot and the voltage level of the second terminal SW of the boot-strap capacitor Cboot, thus the gate driving signal S 1  outputted by the butter  3312  may be higher than the input voltage VIN. The variation range of the operation voltage of the buffer  3313  is between the voltage level of the regulating voltage PVDD and the voltage level of the ground GND. The gate driving logic  331  generates a control signal controlling (e.g. turn-on or turn-off) the low-side N-type MOSFET switch  32 , and the buffer  3313  generates the gate driving signal S 2  according to the mentioned control signal for controlling the low-side N-type MOSFET switch  32 . However, the control circuit  33  is not restricted thereto. 
     The charge and discharge controller  35  comprises a voltage regulator  351 , a boot-strap switch  352  and a charge and discharge logic  353 . The voltage regulator  351  converts the input voltage VIN to be a regulating voltage PVDD utilized for charging the boot-strap capacitor Cboot. In order to make the boot-strap capacitor Cboot have a proper voltage (which is the voltage difference between the first terminal BST and the second terminal SW) and avoid the voltage of the first terminal BST of the boot-strap capacitor Cboot feeding back to the regulating voltage PVDD (when the boot-strap switch  352  is turned off and the high-side N-type MOSFET switch  31  is turned on), the boot-strap switch  352  is utilized to control whether the boot-strap capacitor Cboot is charged. 
     Those skilled in the art will appreciate how to implement the boot-strap switch  352 , and modifications and alternations of the boot-strap switch  352  would be readily observed. For example, the boot-strap switch could be replaced by a diode or other kinds of transistor, and there is no need to go into details. The charge and discharge logic  353  controls the boot-strap switch  352  to charge the boot-strap capacitor Cboot. In this embodiment, the charge and discharge logic  353  also receives the control signal CT from the disabling circuit  34  in order to discharge the boot-strap capacitor Cboot, but this instant disclosure is not so restricted. The charge and discharge logic  353  may be divided into a charge logic and a discharge logic, wherein the charge logic may be a part of the charge and discharge controller  35  (only responsible for charging the boot-strap capacitor Cboot) and the discharge logic may be a part of the disabling circuit  34  (only responsible for discharging the boot-strap capacitor Cboot). However, the high-side N-type MOSFET switch  31  and the low-side N-type MOSFET switch  32  are switched according to the output voltage VOUT, thus it needs an integrated control mechanism for the charging and discharging of the boot-strap capacitor Cboot. Therefore, in this embodiment, the charge and discharge logic  353  is utilized to control the boot-strap switch  352  and the disabling circuit  34 . 
     The disabling circuit  34  comprises a determining unit  341  and a discharge unit  342 . The determining unit  341  is coupled to the boot-strap capacitor Cboot, and determines whether the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt, and generates the control signal CT accordingly. The discharge unit  342  is coupled to the boot-strap capacitor Cboot and the determining unit  341 , wherein the discharge unit  342  discharges the boot-strap capacitor Cboot to make the voltage across the boot-strap capacitor Cboot be zero (i.e. the voltage difference between the first terminal BST and the second terminal SW is zero volt) according to the control signal CT when the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt. Meanwhile, the operation voltage of the buffer  3312  of the control circuit  33  is about to zero volt (in which the maximum of the driving voltage S 1  is the voltage of the second terminal SW of the boot-strap capacitor), thus the high-side N-type MOSFET switch  31  would not be turned on. Accordingly, the buck converter  3  of this embodiment could avoid possible damage of the high-side N-type MOSFET switch  31  due to large resistance when the high-side N-type MOSFET switch  31  is not well conducted. 
     Specifically, the determining unit  341  comprises a comparator  3411  and a voltage source  3412 . A positive input terminal (+) of the comparator  3411  receives the voltage of the first terminal BST of the boot-strap capacitor Cboot. A negative input terminal (−) of the comparator  3411  receives the threshold voltage Vt. The mentioned threshold voltage Vt may be a predetermined voltage larger than zero volt, and the circuit designer may adjust the threshold voltage Vt arbitrarily as needed. The voltage source  3412  is coupled between the second terminal SW of the boot-strap capacitor Cboot and the negative input terminal (−) of the comparator  3411 . The comparator  3411  compares the voltage across the boot-strap capacitor Cboot with the threshold voltage Vt for generating the control signal CT. 
     The discharge unit  342  comprises a discharge switch  3422  and a discharge control circuit  3421 . In this embodiment, the discharge control circuit  3421  is a buffer, and the discharge control circuit is a P-type MOSFET. The discharge switch  3422  is coupled to the first terminal BST and the second terminal SW of the boot-strap capacitor Cboot. The discharge control circuit  3421  is coupled to a control terminal (which is a gate electrode of the P-type MOSFET) of the discharge switch  3422 . The discharge control circuit  3421  controls the discharge switch  3422  to discharge the boot-strap capacitor Cboot according to the control signal CT. Specifically, the discharge control circuit  3421  is controlled by the control signal CT receiving through the charge and discharge logic  353 . The discharge control circuit  3421  controls whether the discharge switch  3422  is turned on or turned off. When the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt, the control signal CT generated by the comparator  3411  controls the discharge control circuit  3421  to turn on the discharge switch  3422  in order to discharge the boot-strap capacitor Cboot. It is worth mentioning that the determining unit  341  in this embodiment is only one example to embody the determining unit, and the instant disclosure is not restricted thereto. An artisan of ordinary skill in the art will appreciate how to implement the determining unit as long as the determining unit could determine whether the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt. 
     Please refer to  FIG. 3  in conjunction with  FIG. 4 ,  FIG. 4  shows a curve diagram of the voltages at two terminals of the boot-strap capacitor of the buck converter of  FIG. 3 . The voltage difference between the first terminal BST and the second terminal SW of the boot-strap capacitor Cboot is the voltage across the boot-strap capacitor. During “OFF” duty, the high-side N-type MOSFET switch  31  is turned off, and the low-side N-type MOSFET switch  32  is turned on, thus the boot-strap capacitor Cboot is charged. During “ON” duty, the high-side N-type MOSFET switch  31  is turned on, and the low-side N-type MOSFET is turned off. When the voltage of the first terminal BST is less than the threshold voltage Vt, the voltage difference between the first terminal BST and the second terminal SW of the boot-strap capacitor Cboot becomes zero volt according the operation of the disabling circuit  34  disclosed in  FIG. 3 . Therefore, the high-side N-type MOSFET could not be turned on in the next “ON” duty, then in the next “OFF” duty the low-side N-type MOSFET switch  32  is turned on and the boot-strap Cboot could be charged again. Accordingly, the circuit operation thereafter returns to normal (i.e. the high-side N-type MOSFET switch  31  could be turned on in “ON” duty). 
     [Another Embodiment of a Buck Converter] 
     Please refer to  FIG. 5  showing a circuit diagram of a buck converter according to another embodiment of the instant disclosure. The buck converter  5  steps-down an input voltage VIN to an output voltage VOUT. The buck converter  5  comprises a high-side N-type MOSFET switch  51 , a low-side N-type MOSFET switch  52 , an output filter  57 , a control circuit  53 , a boot-strap capacitor Cboot, a disabling circuit  54  and a charge controller  55 . 
     The control circuit  53  comprises a gate driving circuit  531 , a feedback comparator  532  and a feedback circuit constituted of the resistors R 1 , R 2 . The gate driving logic  531  comprises a gate driving logic  5311 , a buffer  5312  and a buffer  5313 . The control circuit  53  is significantly identical to the control circuit  33  shown in  FIG. 3  except for differences specified in the follows. The output of the gate driving logic  5311  for controlling the high-side N-type MOSFET switch  51  is transmitted to the disabling circuit  54  but not to the buffer  5312 . 
     The charge controller  55  comprises a voltage regulator  551 , a boot-strap switch  552 , and a charge logic  553 . The voltage regulator  551  converts the input voltage VIN to be a regulating voltage PVDD utilized for charging the boot-strap capacitor Cboot. The boot-strap switch  552  is utilized to control whether the boot-strap capacitor Cboot is charged. The charge logic  553  controls the boot-strap switch  552  to charge the boot-strap capacitor Cboot. 
     The buck converter  5  is significantly identical to the buck converter  3  shown in  FIG. 3  except for differences between the disabling circuit  54  and the disabling circuit  34  of  FIG. 3 . The disabling circuit  54  comprises a determining unit  541  and a logic control unit  542 . The determining unit  541  is coupled to the boot-strap capacitor Cboot, and determines whether the voltage across the boot-strap capacitor Cboot is less than a threshold voltage Vt, and generates the control signal CT accordingly. The determining unit  541  comprises a comparator  5411  and a voltage source  5412 . The determining unit  541  is identical to the determining unit  341  of  FIG. 3 , thus the redundant information is not repeated. In this embodiment, the logic control unit  542  in the disabling circuit  54  replaces the discharge unit  342  of the disabling circuit  34 . Therefore, the control signal CT generated by the determining unit  541  is transmitted to the logic control unit  542 . The logic control unit  542  is coupled to the determining unit  541 . The logic control unit  542  may be a logic AND gate (as shown in  FIG. 5 ), and the logic AND gate receives the control signal CT from the disabling circuit  54  and the logic control signal from the gate driving logic  5311 . The logic control unit  542  controls the buffer  5312  to disable the high-side N-type MOSFET switch  51  according to the control signal CT when the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt. For example, when the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt, the control signal CT outputted by the comparator  5411  is at a LOW voltage level, then the logic control unit  542  continuously outputs a LOW voltage level signal, thus the buffer  5312  could not turn on the high-side N-type MOSFET switch  51 . Otherwise, when the voltage across the boot-strap capacitor Cboot is larger than the threshold voltage Vt, the control signal CT outputted by the comparator  5411  is at a HIGH voltage level, then if the gate driving logic  5311  provides a signal with a HIGH voltage level to the logic control unit  542  the buffer  5312  could turn on the high-side N-type MOSFET switch  51 . In other words, the logic control unit  542  of the disabling circuit  54  controls whether the high-side N-type MOSFET switch  51  could be turned on according to the control signal CT. 
     It is worth mentioning that the disabling circuit  54  in this embodiment is only one example to control the high-side N-type MOSFET switch  51 , and the instant disclosure is not restricted thereto. An artisan of ordinary skill in the art will appreciate how to implement the disabling circuit as long as the disabling circuit could disable the high-side N-type MOSFET switch  51  according to the control signal CT. 
     Please refer to  FIG. 6  in conjunction with  FIG. 5 ,  FIG. 6  shows a curve diagram of the voltages at two terminals of the boot-strap capacitor of the buck converter of  FIG. 5 . In normal operation, the high-side N-type MOSFET switch  51  is turned off and the low-side N-type MOSFET switch  52  is turned on during “OFF” duty, thus the boot-strap capacitor Cboot could be charged. During “ON” duty, the high-side N-type MOSFET switch  51  is turned on and the low-side N-type MOSFET switch  52  is turned off. According to the operation of the disabling circuit  54  shown in  FIG. 5 , the high-side N-type MOSFET switch  51  could not be turned on (i.e. continuously turned off) when the voltage of the first terminal BST is less than the threshold voltage Vt. Therefore, the high-side N-type MOSFET switch  51  could not be turned on in the next “ON” duty, in which the voltages of the first terminal BST and the second terminal SW are not changed during the next “ON” duty. Then, in the next “OFF” duty, the low-side N-type MOSFET switch  52  is turned on for charging the boot-strap capacitor Cboot again, and the follow-up circuit operation returns to normal (i.e. the high-side N-type MOSFET switch  51  could be turned on during the “ON” duty thereafter). 
     [An Embodiment of a Control Method for a Buck Converter] 
     Please refer to  FIG. 7  showing a flow chart of a control method for a buck converter according to an embodiment of the instant disclosure. The control method is utilized for controlling the high-side N-type MOSFET switch  31  of the buck converter  3  shown in  FIG. 3  or the high-side N-type MOSFET switch  51  of the buck converter  5  shown in  FIG. 5 . The buck converter  3  (or  5 ) comprises the high-side N-type MOSFET switch  31  (or  51 ) coupling to an input voltage VIN and a low-side N-type MOSFET switch  32  (or  52 ) coupling to a ground GND. The control method comprises following steps. Firstly, in step S 101 , coupling a boot-strap capacitor Cboot between a regulating voltage PVDD and the source electrode of the high-side N-type MOSFET switch  31  (or  51 ), wherein the source electrode of the high-side N-type MOSFET switch  31  (or  51 ) is coupled to the drain electrode of the low-side N-type MOSFET switch  32  (or  52 ). Then, in step S 103 , turning-on the low-side N-type MOSFET switch  32  (or  52 ), so as to make the regulating voltage PVDD charge the boot-strap capacitor Cboot through a path comprising the low-side N-type MOSFET switch  32  (or  52 ). Then, in step S 105 , utilizing a gate driving signal S 1  to drive the high-side N-type MOSFET switch  31  (or  51 ), wherein the voltage level of the gate driving signal S 1  is corresponding to the voltage across the boot-strap capacitor Cboot. Then, in step S 107 , sensing the voltage across the boot-strap capacitor Cboot. 
     Then, in step S 109 , turning-off the high-side N-type MOSFET switch  31  (or  51 ) continuously when the voltage across the boot-strap capacitor Cboot is less than the threshold voltage Vt. In step S 109 , the operation of the circuit shown in  FIG. 3  could discharge the boot-strap capacitor Cboot, so as to make the voltage across the boot-strap capacitor Cboot be zero volt. Alternatively, the operation of circuit shown in  FIG. 5  could disable the high-side N-type MOSFET switch ( 51 ). 
     According to above descriptions, a buck converter and control method therefor are provided to avoid the high-side N-type MOSFET switch being turned on when the voltage across the boot-strap capacitor is less than a threshold voltage. Accordingly, dangerous situation of damage (e.g. burning) of the high-side N-type MOSFET due to in-sufficient voltage of gate driving signal may be avoided. Further, in order to turn off the high-side N-type MOSFET switch continuously, the boot-strap capacitor is discharged or the high-side N-type MOSFET switch is disabled, thus the power dissipation could be saved and the efficiency could be improved. 
     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.