Abstract:
A level shift circuit and a DC-DC buck converter controller for using the same are disclosed. The level shift circuit is capable of detecting a state of a converting circuit, and avoids a current leakage when determining that the converting circuit is operating under a light-load. Therefore, the level shift circuit and the DC-DC converting controller provided by the present invention can reduce power consumption under the light-load and have power-saving advantage.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Chinese Application Serial Number 201310561010.8, filed Nov. 12, 2013, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The invention relates to a level shift circuit and a DC-DC buck converter controller for using the same, and more particularly relates to a level shift circuit with a power-saving function and a DC-DC buck converter controller for using the same. 
         [0004]    2. Description of Related Art 
         [0005]    In the conventional DC-DC buck converting circuit, a level shift circuit is required to adjust the signal level for correctly turn on and off the high-side transistor of an N-type MOSFET. However, the conventional current type level shift circuit has a large power consumption and leakage paths, and so is not suitable for light-load. On the other hand, the conventional pulse type level shift circuit has no problems of leakage paths and large power consumption, but the pulse type level shift circuit keeps levels of output signals thereof through a parasitic capacitance of a transistor. When a reference level of the level shift circuit flutters, it is easy to cause a logic state of the output signal of the level shift circuit to be changed. Thus, the anti-interference ability of the pulse type level shift circuit is very poor. 
         [0006]      FIG. 1  is a schematic diagram of a conventional current type level shift circuit. A first logic low level VS 1  and a first logic high level VP 1  are two logic levels of a first logic family. A second logic low level VS 2  and a second logic high level VP 2  are two logic levels of a second logic family. The function of the level shift circuit is used to transforming a high logic level and a low logic level of one logic family, i.e., the first logic high level VP 1  and the first logic low level VS 1 , into the other logic family, i.e. the second logic high level VP 2  and the second logic low level VS 2 . 
         [0007]    When a first input signal S is at the first logic high level VP 1  and a second input signal R is at the first logic low level VS 1 , a transistor MN 4  is turned on and a transistor MN 5  is turned off. At this moment, a current of a current source Ib flows through the current mirror composed of transistors MN 1  and MN 2  to make a current mirrored flow through the transistors MP 1 , MN 4  and MN 2 . A transistor MP 2  also simultaneously mirrors the current of the transistor MP 1  to make a level of a first output signal Q raise to the second logic high level VP 2 . Moreover, because the transistor MN 5  is cut off and leads to no current, the transistors MP 4  and MP 3  are also cut off. Because the level of the first output signal Q is at the second logic high level VP 2  and a transistor MN 7  is turned on, a potential of a second output signal QN is reduced to the second logic low level VS 2 . Similarly, when the first input signal S is at the first logic low level VS 1  and the second input signal R is at the first logic high level VP 1 , the level of the first output signal Q is at the second logic low level VS 2  and the level of the second output signal ON is at the second logic high level VP 2 . According to the level shift mentioned above, the first input signal S and the second input signal R of the first logic high level VP 1  and the first logic low level are transformed into the first output signal Q and the second output signal QN of the second logic high level VP 2  and the second logic low level VS 2 . 
         [0008]    For ensuring a transforming speed of the level shift, when the first input signal S is at a high level and the second signal R is at a low level, the current flowing from the second logic high level VP 2  via the transistors MP 1 , MN 4  and MN 2  to the first logic low level VS 1  is designed to be larger. Similarly, when the first input signal S is at a low level and the second input signal R is at a high level, the current flowing from the second logic high level VP 2  via the transistors MP 4 , MN 5  and MN 3  to the first logic low level VS 1  is also designed to be larger. This circuit design can ensure the transforming speed of the level shift in the current type level shift circuit. However, such circuit design has the higher power consumption. Especially, the DC-DC buck converting circuit operates under a light-load, such as the diode emulation mode. In this mode, the larger current continuously flowing through the current type level shift circuit is not conducive to power-saving. The second logic high level VP 2  may be provided by an extra boost circuit, not provided by an independent voltage source. The larger current continuously flowing leads to the second logic high level VP 2  falling down and so a voltage difference between the second high level VP 2  and the second logic low level VS 2  is decreased. 
         [0009]      FIG. 2  is schematic diagram of a conventional improved current type level shift circuit. Compared with that shown in  FIG. 1 , the improved current type level shift circuit extra increases transistors MN 8  and MN 9 . The main function of the transistors MN 8  and MN 9  is voltage suppression, and gate electrodes thereof are coupled to the first logic high level VP 1  for ensuring source electrodes of the transistors MN 8  and MN 9 , i.e., potentials of drain electrodes of the transistors MN 4  and MN 5  are clamped under the first logic high level VP 1 . Under this circuit design, both the transistors MN 4  and MN 5  can be low-voltage transistors, and it is conducive to raise the switching speed of the transistors MN 4  and MN 5 . However, large power consumption problem still exists. 
         [0010]      FIG. 3  is a schematic diagram of a conventional pulse type level shift circuit. Pulse signals VPS and VPR are pulse signals respectively triggered on rising-edges of the first input signal S and the second signal R, and have narrow pulse widths. The pulse signals VPS and VPR are respectively coupled to gate electrodes of transistors MN 2  and MN 3 .  FIG. 4  shows waveform diagrams of the pulse type level shift circuit shown in  FIG. 3 . When both the first input signal S and the pulse signal VPS are at the first logic high level VP 1  and the second input signal R is at the first logic low level VS 1 , a large current flows from the second logic high level VP 2  via the transistors MP 1 , MN 4  and MN 2  to the first logic low level VS 1 . The transistor MP 2  mirrors a current of the transistor MP 1  to make the first output signal Q raise to the second logic high level VP 2 , while the second output signal QN is at the second logic low level VS 2 . When the first input signal S is still at the first logic high level VP 1  and the pulse signal VPS is changed to be at the first logic low level VS 1 , the current of the transistor MP 1  is zero. At this moment, the whole level shift circuit does not consume any current. Similarly, when both the second input signal R and the pulse signal VPR are at the first logic high level VP 1  and the first input signal S at is the first logic low level VS 1 , the large current flows from the second logic high level VP 2  via the transistors MP 4 , MN 5  and MN 3  to the first logic low level VS 1 . The transistor MP 3  mirrors the current of the transistor MP 4  to make the second output signal QN raise to the second logic high level VP 2 , while the first output signal Q is at the second logic low level VS 2 . Soon after, when the second input signal R is still at the first logic high level VP 1  and the pulse signal VPR is changed to be at the first logic low level VS 1 . At this moment, the whole level shift circuit does not consume any current. 
         [0011]    Advantages of the pulse type level shift circuit are speeding up the translating speed of the level shift due to the large current flowing through the pulse type level shift circuit, and lowering power consumption due to no current consumption after level shift has completed. However, the pulse type level shift circuit still has defects. When both the pulse signals VPR and VPS are at the logic low level, the levels of the first output signal Q and the second output signal QN are kept by the parasitic capacitances of the transistors, which causes the pulse type level shift circuit has poor anti-noise ability. 
         [0012]      FIG. 5  is a schematic diagram of a level shift circuit disclosed in U.S. Pat. No. 7,839,197 of RICHTEK Technology Corporation. The level shift circuit shown in  FIG. 5  is designed with the advantages of the pulse type level shift circuit and the current type level shift circuit.  FIG. 6  shows waveform diagrams of the pulse type level shift circuit shown in  FIG. 5 . When the first input signal S and the pulse signal VPS are at the first logic high level VP 1  and the second input signal R is at the first logic low level VS 1 , a current flows from the second logic high level VP 2  via the transistors M 1 , M 5  and M 11  and a basic current source  642  to the first logic low level VS 1 . At this moment, a transistor M 8  mirrors a current of the transistor M 1  to make the first output signal Q be at the second logic high level VP 2  and the second output signal QN be at the second logic low level VS 2 . When the first input signal S is still at the first logic high level VP 1  and the pulse signal VPS is changed to be at the first logic low level VS 1 , the transistor M 111  is cut off and the basic current source  642  still maintains a small basic current flowing through the transistors M 5  and M 1 , and the small current is used to maintain the first output signal Q to continuously be at the second logic high level VP 2 . Similarly, when the second input signal R and the pulse signal VPR are at the first logic high level VP 1  and the first input signal S is at the first logic low level VS 1 , the current flows from the second logic high level VP 2  via the transistors M 4 , M 6  and M 12  and a basic current source  644  to the first logic low level VS 1 . At this moment, a transistor M 7  mirrors a current of the transistor M 4  to make the second output signal QN be at the second logic high level VP 2  and the first output signal Q become the second logic low level VS 2 . When the second input signal R is still at the first logic high level VP 1  and the pulse signal VPR is changed to be at the first logic low level VS 1 , the transistor M 12  is cut off and the basic current source  644  still maintains a small basic current flowing through the transistors M 6  and M 4 , and the small current is used to maintain the second output signal QN to be continuously at the second logic high level VP 2 . The transistors M 2  and M 3  are two mirror acceleration transistors. 
         [0013]    Advantages of the level shift circuit shown in  FIG. 5  involves the advantage of the high speed level shift of the pulse type level shift circuit and the good anti-noise ability of the current type level shift circuit.  FIG. 7  shows waveform diagrams of the second logic high level VP 2  and the second logic low level VS 2  of the level shift circuit shown in  FIG. 5 . In the continuous current mode, the converting circuit continuously operates to make the boost circuit retain the potential of the second logic high level VP 2 . However, in the diode emulation mode, the basic current sources  642  and  644  of the level shift circuit still provide the small current and continuously consumes the energy stored in the boost circuit, and it causes the voltage difference between the second logic high level VP 2  and the second logic low level VS 2  slowly dropping down and further is possible to cause the problem of the logic error level of the first output signal Q and the second output signal QN. 
       SUMMARY 
       [0014]    In view of the problems of the level shift circuit of the prior art, the level shift circuit of the present invention can avoid the level shift circuit consuming additional currents for achieving the advantages of reducing the power consumption, and further avoid the logic error level of the output signal when the converting circuit is operating under the light-load. 
         [0015]    To accomplish the aforementioned and other objects, the present invention provides a level shift circuit, comprising a signal input circuit, a signal output circuit and a state detecting circuit. The signal input circuit is coupled between a first level and a second level, configured to receive a first input signal and a second input signal. Levels of the first input signal and the second input signal are switched between the first level and a third level. The signal input circuit generates a first current when the first input signal is at the third level, and generates a second current when the second input signal is at the third level. The signal output circuit is coupled between the second level and a fourth level, configured to output a first output signal and a second output signal. Levels of the first output signal and the second output signal are switched between the second level and the fourth level. The first output signal is switched to the second level when the signal input circuit generates the first current. The second output signal is switched to the second level when the signal input circuit generates the second current. The state detecting circuit detects an operating state of a converting circuit, and accordingly determines whether generating a stop signal for stopping the signal input circuit generating the first current and the second current. 
         [0016]    The present invention also provides a DC-DC buck converter controller, adapted to control a first power switch and a second power switch of a converting circuit connected in series. The first power switch is coupled to an input voltage and a connection node, and the second power switch is coupled to the connection node and a common potential. The DC-DC buck converter controller comprises a feedback control circuit, a level shift circuit and a driver. The feedback control circuit generates a pulse width modulating signal according to a detecting signal indicative of a state of the converting circuit. A level of the pulse width modulating signal is switched between the common potential and a driving potential. The level shift circuit generates a level shift signal according to the pulse width modulating signal. The level shift circuit comprises a signal input circuit, a signal output circuit and a state detecting circuit. The signal input circuit is coupled between the common potential and a reference potential. The signal input circuit generates a first current when the pulse width modulating signal is at the driving potential, and generates a second current when the pulse width modulating signal is at the common potential. The signal output circuit is coupled between the reference potential and the connection node, configured to output the level shift signal. A level of the level shift signal is switched between the reference potential and a potential of the connection node. The level shift signal is switched to the reference potential when the signal input circuit generates the first current, and the level shift signal is switched to the potential of the connection node when the signal input circuit generates the second current. The state detecting circuit detects an operating state of the converting circuit and accordingly determines whether generating a stop signal for stopping the signal input circuit generating the first current and the second current. The driver is coupled to the level shift circuit and the feedback control circuit, and generates a high-side control signal and a low-side control signal according to the pulse width modulating signal and the level shift signal for respectively turning on and off the first power switch and the second power switch. 
         [0017]    Besides, the level shift circuit of the present invention also can additionally add a time delay for avoiding the noise interference and further raising anti-noise ability. 
         [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
           [0020]      FIG. 1  is a schematic diagram of a conventional current type level shift circuit. 
           [0021]      FIG. 2  is a schematic diagram of a conventional modified current type level shift circuit. 
           [0022]      FIG. 3  is a schematic diagram of a conventional pulse type level shift circuit. 
           [0023]      FIG. 4  shows waveform diagrams of the pulse type level shift circuit shown in  FIG. 3 . 
           [0024]      FIG. 5  is a schematic diagram of a level shift circuit disclosed in U.S. Pat. No. 7,839,197 of RICHTEK Technology Corporation. 
           [0025]      FIG. 6  shows waveform diagrams of the pulse type level shift circuit shown in  FIG. 5 . 
           [0026]      FIG. 7  shows waveform diagrams of the second logic high level VP 2  and the second logic low level VS 2  of the level shift circuit shown in  FIG. 5 . 
           [0027]      FIG. 8  is a schematic diagram of a level shift circuit according to a first preferred embodiment of the present invention. 
           [0028]      FIG. 9  shows waveform diagrams of the level shift circuit shown in  FIG. 8 . 
           [0029]      FIG. 10  is schematic diagram of a DC-DC buck converter controller applying a level shift circuit of a preferred embodiment of the present invention. 
           [0030]      FIG. 11  shows waveform diagrams of the level shift circuit shown in  FIG. 10 . 
           [0031]      FIG. 12  shows waveform diagrams of the reference potential VBS and the connection node potential VPH in the level shift circuit shown in  FIG. 10 . 
           [0032]      FIG. 13  is a schematic diagram of a level shift circuit according to a second preferred embodiment of the present invention. 
           [0033]      FIG. 14  is a schematic diagram of a level shift circuit according to a third preferred embodiment of the present invention. 
           [0034]      FIG. 15  shows waveform diagrams of the level shift circuit shown in  FIG. 14 . 
           [0035]      FIG. 16  is a schematic diagram of a current detecting circuit according to a preferred embodiment of the present invention. 
           [0036]      FIG. 17  is a schematic diagram of an inductance current detecting circuit according to a preferred embodiment of the present invention. 
           [0037]      FIG. 18  is a schematic diagram of a delay judging circuit according to a preferred embodiment of the present invention. 
           [0038]      FIG. 19  shows waveform diagrams of the delay judging circuit shown in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments, it will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
         [0040]      FIG. 8  is a schematic diagram of a level shift circuit according to a first preferred embodiment of the present invention. The level shift circuit comprises a signal input circuit  102 , a signal output circuit  104  and a state detecting circuit  110 . The signal input circuit  102  is coupled between a first level VSS 1  and a second level VPP 2 , configured to receive a first input signal S and a second input signal R. Also referring to  FIG. 9 ,  FIG. 9  shows waveform diagrams of the level shift circuit shown in  FIG. 8 . Levels of the first input signal S and the second input signal R are switched between the first level VSS 1  and a third level VPP 1 . In the present embodiment, the signal input circuit  102  comprises transistors MN 1 ˜MN 5 , MP 1  and MP 4 , a current switch MNO and a current source Ib. The current source Ib is coupled to the third level VPP 1  or the second level VPP 2 , and provides a current In flowing through the transistor MN 1  when the current switch MNO is turned on. The transistors MN 2  and MN 3  mirror the current In of the transistor MN 1  for respectively providing a first current I 1  and a second current I 2 . The signal input circuit  102  generates the first current I 1  flowing through the transistors MP 1 , MN 4  and MN 2  when the first input signal S is at the third level VPP 1 , and generates the second current I 2  flowing through the transistors MP 4 , MN 5  and MN 3  when the second input signal R is at the third level VPP 1 . The signal output circuit  104  is coupled between the second level VPP 2  and a fourth level VSS 2 , configured to output a first output signal Q and a second output signal QN. Levels of the first output signal Q and the second output signal QN are switched between the second level VPP 2  and the fourth level VSS 2 . 
         [0041]    The signal output circuit  104  comprises transistors MP 2 ˜MP 3  and MN 6 ˜MN 7 . When the signal input circuit  102  generates the first current I 1 , the transistor MP 2  of the signal output circuit  104  mirrors the first current I 1  to make the first output signal Q be switched to the second level VPP 2 . At this moment, the second output signal QN is switched to the fourth level VSS 2 . Similarly, the signal input circuit  102  generates the second current I 2 , the transistor MP 3  of the signal output circuit  104  mirrors the second current I 2  to make the second output signal QN be switched to the second level VPP 2 . At this moment, the first output signal Q is switched to the fourth level VSS 2 . 
         [0042]    The state detecting circuit  110  detects an operating state of a converting circuit (not shown), such as detecting the voltage and/or the current, for judging the operating state, and determines whether generating a stop signal Ssd for stopping the signal input circuit  102  to generate the first current I 1  and the second current I 2  according to the operating state of the converting circuit. In the present embodiment, detecting the current of the converting circuit is taken as an example to illustrate. 
         [0043]    The state detecting circuit  110  comprises a delay judging circuit  112  and a current detecting circuit  118 . The current detecting circuit  118  is coupled to the converting circuit for detecting a current of the converting circuit and generates a light-load notice signal Ss when detecting the current of the converting circuit is smaller than a light-load judging value. The state detecting circuit  110  determines whether generating the stop signal Ssd according to the light-load notice signal Ss. The delay judging circuit  112  is coupled to the current detecting circuit  118 , and generates the stop signal Ssd when the current detecting circuit  118  generates the light-load notice signal Ss lasting a preset delay time period. The delay judging circuit  112  comprises an AND gate  114  and a delay circuit  116 . The delay circuit  116  outputs a delay signal Sd when continuously receiving the light-load notice signal Ss for the preset delay time period. The AND gate  114  is coupled to the delay circuit  116  and the current detecting circuit  118 , and generates the stop signal Ssd when receiving both the light-load notice signal Ss and the delay signal Sd. 
         [0044]    If the stop signal Ssd is not generated, the current switch MNO is turned on. Under this condition, the current In provided by the current source Ib flows through the transistor MN 1 , and the transistors MN 2  and MN 3  mirror the current In and respectively generate the first current I 1  and the second current I 2 . However, if the converting circuit operates in a light-load state, the stop signal Ssd is generated to cut off the current switch MNO. Under this condition, the current of the transistor MN 1  is zero, and so both the transistors MN 2  and MN 3  of the signal input circuit  102  have no current. That is, the signal input circuit  102  stops generating the first current I 1  and the second current I 2 . Thus, when the converting circuit operates under the light-load state, for example: the discontinuous current mode, the diode emulation mode, and so on, the level shift of the present invention reduce the power consumption to achieve the power-saving advantage. 
         [0045]      FIG. 10  is schematic diagram of a DC-DC buck converter controller applying a level shift circuit of a preferred embodiment of the present invention. The converting circuit comprises a first power switch T 1  and a second power switch T 2  connected in series, an inductance L and a capacitance COUT. The first power switch T 1  is coupled to an input voltage VIN and a connection node PHASE, and the second power switch T 2  is coupled to the connection node PHASE and a common potential GND. The inductance L is coupled to the connection node PHASE and the output capacitance COUT, and the output capacitance COUT provides an output voltage VOUT. The DC-DC buck converter controller generates a high-side control signal UG and a low-side control signal LG for respectively turning on and off the first power switch T 1  and the second power switch T 2 . The DC-DC buck converter controller comprises a feedback control circuit, a level shift circuit and a driver  160 . The feedback control circuit comprises an error amplifier  130  and a PWM (pulse width modulated) logic circuit  140 . The error amplifier  130  receives a reference signal VREF and a detecting signal indicative of a state of the converting circuit, and generates an error amplification signal Sea according to the state of the converting circuit. In the present embodiment, the detecting signal represents the output voltage VOUT, and in actual application, it also can be a detecting signal indicative of an output current of the converting circuit. The PWM logic circuit  140  is coupled to the error amplifier  130  and generates a PWM (pulse width modulating) signal Sp according to the error amplification signal Sea. The PWM logic circuit  140  is coupled to a driving potential VDD and the common potential GND, and so a level of the PWM signal Sp is switched between the common potential GND and the driving potential VDD. The level shift circuit is coupled to a connection node potential VPH of the connection node PHASE, the driving potential VDD, the common potential GND and a reference potential VBS, and generates a level shift signal Sq (i.e., the first output signal Q or the second output signal QN of the embodiment in  FIG. 8 ) according to the PWM signal Sp. The reference potential VBS, providing a potential higher than the input voltage VIN, is used for ensuring that the DC-DC buck converter controller correctly control the first power switch T 1  to be turned on and off. The reference potential VBS may be provided by a voltage source independent of the input voltage VIN, or additionally adding a bootstrap circuit  150  as the present embodiment. The bootstrap circuit  150  is coupled to the connection node PHASE and the input voltage VIN, and provides the reference potential VBS through the switching process of the first power switch T 1 . 
         [0046]    The level shift circuit of the present invention may be the level shift circuit shown in  FIG. 8  or a level shift circuit shown in other embodiments. In the present embodiment, take the level shift circuit shown in  FIG. 8  to illustrate. For conveniently understand the operation of the DC-DC buck converter controller with respect to that shown in  FIG. 8 , relationships of the logic levels between the two embodiments are illustrated in the following: the first level VSS 1 , the second level VPP 2 , the third level VPP 1  and the fourth level VSS 2  respectively corresponding to the common potential GND, the reference potential VBS, the driving potential VDD and the connection node potential VPH. 
         [0047]    The level shift circuit comprises a level shift circuit  100  and a state detecting circuit  110 . The level shift circuit  100 , coupled to the common potential GND, the reference potential VBS, the driving potential VDD and the connection node potential VPH, comprises a signal input circuit  102 , a signal output circuit  104  and an inverter  106 . The inverter  106  is configured to receive the PWM signal Sp, and provides an inverted PWM signal Sp′. The PWM signal Sp and the inverted PWM signal Sp′ respectively serve as the first input signal S and the second input signal R for inputting into the signal input circuit  102 . The signal input circuit  102  generates a first current I 1  when the PWM signal Sp is at the driving potential VDD, and generates a second current I 2  when the PWM signal Sp is at the common potential GND. The signal output circuit  104  is configured to generate the level shift signal Sq, and the level shift signal Sq is switched to the reference potential VBS when the signal input circuit  102  generates the first current I 1 , and the level shift signal Sq is switched to the connection potential VPH when the signal input circuit  102  generates the second current I 2 . 
         [0048]    The state detecting circuit  110  detects an operating state of the converting circuit and accordingly determines whether generating a stop signal Ssd for stopping the signal input circuit  102  to generate the first current I 1  and the second current I 2 . In the present embodiment, the state detecting circuit  110  is coupled to the connection node PHASE for detecting the current of the inductance L. The state detecting circuit  110  comprises a delay judging circuit  112  and a current detecting circuit  118 . The current detecting circuit  118  detects the current of the inductance L and generates a light-load notice signal Ss when detecting that a current of the inductance L is lower than a current reverse threshold value. The delay judging circuit  112  is coupled to the current detecting circuit  118  and generates the stop signal Ssd when the current detecting circuit  118  continuously generates the light-load notice signal Ss for a preset delay time period Td. 
         [0049]    The driver  160  is coupled to the level shift circuit and the feedback control circuit and generates the high-side control signal UG and the low-side control signal LG according to the PWM signal Sp and the level shift signal Sq for respectively turning on and off the first power switch T 1  and the second power switch T 2 . The driver  160  comprises an upper driver  162  and a lower driver  164 . The upper driver  162  is coupled to the bootstrap circuit  150  and the connection node PHASE for receiving the reference potential VBS and the connection node potential VPH. The upper driver  162  is also coupled to the level shift circuit, and generates the high-side control signal UG according to the level shift signal Sq. The lower driver  164  is coupled to the feedback control circuit, and generates the low-side control LG according to the PWM signal Sp. 
         [0050]      FIG. 11  shows waveform diagrams of the level shift circuit shown in  FIG. 10 . Also referring to  FIG. 10 , the current detecting circuit  118  generates the light-load notice signal Ss when judging an inductance current flowing reversely. The delay judging circuit  112  generates the stop signal Ssd when the light-load notice signal Ss lasts the preset delay time period Td. Referring to  FIG. 8 , the stop signal Ssd cuts off the current switch MNO to make the current of the transistor MN 1  be zero, thereby stopping the first current I 1  and the second current I 2 . Besides, it is worth to notice that the lower driver  164  cuts off the second power switch T 2  for avoiding the inductance current flowing reversely against the coming reverse inductance current. At this moment, because both the first power switch T 1  and the second power switch T 2  are cut off, the connection node potential VPH of the connection node PHASE is oscillating. The bootstrap circuit  150  is coupled to the connection node PHASE, and so the oscillation of the connection node potential VPH affects the reference potential VBS. That leads to the noise interference. In the prior art, the current In is immediately cuts off to cause the erroneous level shift signal Sq of the level shift circuit  100 . In contrast, in the present invention, the current In within the preset delay time period Td from when both the first power switch T 1  and the second power switch T 2  are cut off still exists to solve the noise-interference problems. Moreover, after oscillation of the connection node potential VPH (i.e., passing the preset delay time period Td), the current In is stopped for power-saving. 
         [0051]      FIG. 12  shows waveform diagrams of the reference potential VBS and the connection node potential VPH in the level shift circuit shown in  FIG. 10 . When the current detecting circuit  118  detects the inductance current flowing reversely, i.e., the converting circuit enters into the light-load state, for example: DEM or DCM. After the preset delay time period, the state detecting circuit  110  generates the stop signal Ssd for cutting off the current In to make the level shift circuit  100  stop generating the first current I 1  and the second current I 2 . At this moment, no more current of the level shift circuit  100  flows from the reference potential VBS to the connection node potential VPH, and that ensures the level difference of the reference potential VBS and the connection node potential VPH being maintained. 
         [0052]      FIG. 13  is a schematic diagram of a level shift circuit according to a second preferred embodiment of the present invention. Compared with the embodiment shown in  FIG. 8 , the main differences are that a signal input circuit  202  adds transistors MN 8  and MN 9 , and a signal output circuit  204  adds transistors MP 5  and MP 6 . Gate electrodes of the transistors MN 8  and MN 9  are coupled to the third level VPP 1  to ensure source electrodes of the transistors MN 8  and MN 9 , i.e., the drain electrodes of the transistors MN 4  and MN 5 , being clamped below the third level VPP 1 . Thus, the transistors MN 4  and MN 5  can use the low-voltage transistor to reduce the cost of the level shift circuit. The transistors MP 5  and MP 6  functions as an accelerating circuit. A gate electrode of the transistor MP 5  is coupled to a gate electrode of the transistor MP 1 , and a drain electrode thereof is coupled to a gate electrode of the transistor MP 4 . When the first current I 1  is generated, the transistors MP 1  and MP 5  are simultaneously turned on. At this moment, the transistor MP 5  can quickly raise the gate electrode of the transistor MP 4  and completely cut off the transistor MP 4  for quickly cutting off the second current I 2 . Similarly, a gate electrode of the transistor MP 6  is coupled to the gate electrode of the transistor MP 4 , and a drain electrode thereof is coupled to the gate electrode of the transistor MP 1 . When the second current I 2  is generated, the transistors MP 4  and MP 6  are simultaneously turned on. At this moment, the transistor MP 6  can quickly raise the gate electrode of the transistor MP 1  and completely cut off the transistor MP 1  for quickly cutting off the first current I 1 . 
         [0053]      FIG. 14  is a schematic diagram of a level shift circuit according to a third preferred embodiment of the present invention. The embodiments shown in  FIG. 8  and  FIG. 13  transfer the first input signal S and the second input signal R with the lower logic levels of the first level VSS 1  and the third level VPP 1  into the first output signal Q and the second output signal QN with the higher logic levels of the fourth level VSS 2  and the second level VPP 2 . The level shift circuit shown in  FIG. 14  transfer the first input signal S and the second input signal R with the higher logic levels of the fourth level VSS 2  and the second level VPP 2  into the first output signal Q and the second output signal QN with the lower level of the first level VSS 1  and the third level VPP 1 . 
         [0054]      FIG. 15  shows waveform diagrams of the level shift circuit shown in  FIG. 14 . Also referring to  FIG. 14 , a signal input circuit  302  is coupled between the first level VSS 1  and the second level VPP 2  and receives the first input signal S and the second input signal R. The levels of the first input signal S and the second input signal R are switched between the fourth level VSS 2  and the second level VPP 2 . The signal input circuit  302  comprises the transistors MP 1 ˜MP 5 , MN 1  and MN 6 , the current switch MNO and the current source Ib. The current source Ib is coupled to the first level VSS 1 , and provides a current Ip flowing through the transistor MP 1  when the current switch MNO is turned on. A signal output circuit  304  is coupled between the third level VPP 1  and the first level VSS 1  and outputs the first output signal Q and the second output signal QN. The levels of the first output signal Q and the second output signal QN are switched between the third level VPP 1  and the first level VSS 1 . The level shift circuit of the present embodiment is similar to the level shift circuits shown in  FIG. 8  and  FIG. 13 , and the detailed description of the circuit operation can refer to the corresponding description in  FIG. 8  and  FIG. 13 , and it will not repeated in here. 
         [0055]      FIG. 16  is a schematic diagram of a current detecting circuit according to a preferred embodiment of the present invention. In order to clearly understand the operation of the current detecting circuit, the current detecting circuit is applied to the circuit of  FIG. 8  for illustrating. The current detecting circuit comprises a comparator Com and a RS flip-flop. The low-side control signal LG is used to enable and disable the comparator Com. The comparator Com is enabled when the low-side control signal LG is at a high level, i.e., the second power switch T 2  is turned on, and is disabled when the low-side control signal LG is at a low level. A non-inverting input end of the comparator Com receives a current detecting signal Ise, and an inverting end thereof receives a light-load judging value Vrc. In the present embodiment, the light-load judging value Vrc is the ground potential, i.e., the common potential GND. When the second power switch T 2  is turned on, the comparator Com is enabled for detecting whether the current of the second power switch T 2  flows reversely. When the current detecting signal Ise is higher than the light-load judging value Vrc, the comparator Corn generates a high level signal for triggering the RS flip-flop generating the light-load notice signal Ss. In actual application, the connection node potential VPH may serve as the current detecting signal Ise. When the connection node potential VPH is larger than zero, it represents that the inductance current flows reversely, i.e. from the inductance into the second power switch T 2  and so the converting circuit operates in the light-load state. At this moment, the current detecting circuit generates the light-load notice signal Ss. 
         [0056]      FIG. 17  is a schematic diagram of an inductance current detecting circuit according to a preferred embodiment of the present invention. The inductance current detecting circuit comprises a transconductance amplifier GM, a sample and hold circuit S/H, a detecting capacitance C and resistances Re and Rcsn. The inductance L is connected the series of the detecting capacitance C and the resistance Re in parallel. The inductance L has an inherent DC resistance DCR, and so a voltage across Vc of the capacitance C is proportional to an inductance current IL of the inductance L. A non-inverting input end of the transconductance amplifier GM is coupled to a connection node of the resistance Re and the detecting capacitance C, and an inverting input end thereof is coupled to the other end of the capacitance C through the resistance Rcsn. The transconductance amplifier GM generates an output current Icsn at an output end according to voltage levels at the non-inverting input end and the inverting input end. The non-inverting input end of the transconductance amplifier GM is coupled to the output end. Thus, the output current Icsn flows through the resistance Rcsn and form a voltage across of the resistance Rcsn to compensate the voltage across Vc of the capacitance C for making the voltage difference of the inverting input end and the non-inverting input end of the transconductance amplifier GM be zero. When the inductance current IL flows reversely, i.e., the inductance current IL flows back from the output voltage VOUT to the connection node PHASE, and the output current Icsn is smaller than or equal to zero. The sample and hold circuit S/H detects the voltage across Vc of the capacitance C at every cycle and accordingly generates the current detecting signal Ise. When the inductance current IL flows reversely, the current detecting signal Ise is larger than zero and so the inductance current detecting circuit generates the light-load notice signal Ss. 
         [0057]      FIG. 18  is a schematic diagram of a delay judging circuit according to a preferred embodiment of the present invention. The delay judging circuit comprises a switch Md, a current source Id, a capacitance Cd, a comparator Dd and an AND gate Ad. One end of the switch Md is coupled to the driving potential VDD, and the other end thereof is coupled to the current source Id. One end of the capacitance Cd is coupled to a connection node of the switch Md and the current source Id, and the other end thereof is coupled to the ground. A non-inverting input end of the comparator Dd receives a delay reference voltage Vr, and an inverting input end thereof is coupled to the capacitance Cd. 
         [0058]      FIG. 19  shows waveform diagrams of the delay judging circuit shown in  FIG. 18 . When the light-load notice signal Ss is at a low level, the switch Md is turned on for making a voltage of the capacitance Cd be raised to the driving potential VDD, which is higher than the delay reference voltage Vr. At this moment, the AND gate Ad stops outputting the stop signal Ssd (i.e., the stop signal Ssd is at a low level). When the light-load notice signal Ss is changed to a high level, the switch Md is cut off. At this moment, the current source Id starts discharging the capacitance Cd, and so the voltage of the capacitance Cd starts dropping from the driving potential VDD. After the preset delay time period Td, the voltage of the capacitance Cd is lower than the delay reference voltage Vr, the comparator Dd output a high level signal. At this moment, both two signals received by the two input ends of the AND gate Ad are at high levels and the AND gate Ad outputs the stop signal Ssd. When the light-load notice signal Ss is changed from the high level to the low level, the switch Md is turned on for immediately charging the voltage across of the capacitance Cd to be the driving potential VDD. At this moment, the AND gate Ad stops outputting the stop signal Ssd. 
         [0059]    While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.