Patent Publication Number: US-9891648-B2

Title: Switching converter with smart frequency generator and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of CN application 201410841384.X, filed on Dec. 30, 2014 and incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention generally relates to electronic circuits, and more particularly but not exclusively to switching converters. 
     BACKGROUND 
     Peak current control method is widely used in switching converters. In peak current control, a reference voltage and a feedback signal which indicates the output voltage of the switching converter are sent into an error amplifier to generate a compensation signal. The main transistor of the switching converter will become on once a clock signal comes, and become off when a current sensing signal indicative of the current flowing through the main transistor hits the compensation signal. Owing to the inherent delay of the control circuit, there exists a minimum on-time, wherein the main transistor can turn off only after its on-time reaches this minimum on time. 
     If the input voltage of the switching converter keeps increasing, the on-time of the main transistor will decrease until it reaches the minimum on-time. Hereafter, if the input voltage continues rising, the output voltage of the switching voltage will go up and a large ripple would arise accordingly. 
     SUMMARY 
     To solve the problem mentioned above, the present invention involves a time threshold which is larger than the minimum on-time. If the on-time of the main transistor becomes smaller than the time threshold, the frequency of the clock signal would be adjusted to regulate the on-time of the main transistor to be equal to the time threshold. By doing so, the on-time of the main transistor would not fall to reach the minimum on-time anymore, thus the voltage ripple potentially caused by the minimum on-time can be fundamentally avoided. 
     Embodiments of the present invention are directed to a switching converter, comprising: a switching circuit having a main transistor, wherein the switching circuit is configured to convert an input voltage into an output voltage; a feedback circuit coupled to the switching circuit, wherein the feedback circuit is configured to generate a feedback signal indicative of the output voltage; a current sensing circuit configured to sense the current flowing through the main transistor and generate a current sensing signal; a clock generator configured to generate a clock signal; an error amplifying circuit coupled to the feedback circuit, wherein based on the difference between a first reference voltage and the feedback signal, the error amplifying circuit generates a compensation signal; a comparing circuit coupled to the current sensing circuit and the error amplifying circuit, wherein the comparing circuit compares the current sensing signal with the compensation signal and generates a reset signal; and a control circuit coupled to the clock generator and the comparing circuit, wherein based on the clock signal and the reset signal, the control circuit generates a control signal to control the main transistor. The clock generator is coupled to the control circuit to receive the control signal and detect whether the on-time of the main transistor is smaller than a time threshold based on the control signal. If the on-time of the main transistor is smaller than the time threshold, the clock generator will adjust the frequency of the clock signal to regulate the on-time of the main transistor to be equal to the time threshold. 
     Embodiments of the present invention are also directed to a controller used in a switching converter, wherein the switching converter has a main transistor and is configured to provide an output signal. The controller comprises: a clock generator configured to generate a clock signal to determine the switching frequency of the main transistor; and a control circuit coupled to the clock generator, wherein based on the clock signal and a feedback signal indicative of the output signal of the switching converter, the control circuit generates a control signal to control the main transistor. If the on-time of the main transistor is smaller than a time threshold, the clock generator will adjust the frequency of the clock signal to regulate the on-time of the main transistor to be equal to the time threshold. 
     Embodiments of the present invention are further directed to a control method of a switching converter, wherein the switching converter has a main transistor and is configured to provide an output signal. The control method comprises: generating a feedback signal indicative of the output signal of the switching converter; generating a clock signal to determine the switching frequency of the main transistor; generating a control signal to control the main transistor based on the clock signal and the feedback signal; detecting whether the on-time of the main transistor is smaller than a time threshold based on the control signal; and if the on-time of the main transistor is smaller than the time threshold, adjusting the frequency of the clock signal to regulate the on-time of the main transistor to be equal to the time threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present invention can be further understood with reference to the following detailed description and the appended drawings, wherein like elements are provided with like reference numerals. 
         FIG. 1  is a block diagram of a switching converter  100  in accordance with an embodiment of the present invention. 
         FIG. 2A  schematically illustrates a clock generator  102 A in accordance with an embodiment of the present invention. 
         FIG. 2B  illustrates working waveforms of the clock generator  102 A shown in  FIG. 2A  in accordance with an embodiment of the present invention. 
         FIG. 3  schematically illustrates a clock generator  102 B in accordance with an embodiment of the present invention. 
         FIG. 4  schematically illustrates a clock generator  102 C in accordance with an embodiment of the present invention. 
         FIG. 5  schematically illustrates a clock generator  102 D in accordance with an embodiment of the present invention. 
         FIG. 6  schematically illustrates a clock generator  102 E in accordance with an embodiment of the present invention. 
         FIG. 7  schematically illustrates a switching converter  100 A in accordance with an embodiment of the present invention. 
         FIG. 8  is a flow chart of a control method for switching converters in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     To solve the problem mentioned in the background, the present invention sets a time threshold (e.g. 80 ns) larger than the minimum on-time (e.g. 50 ns), and adjusts the frequency of the clock signal when the on-time of the main transistor becomes smaller than the time threshold, so as to regulate the on-time of the main transistor to be equal to the time threshold. Therefore, the on-time of the main transistor would not fall to reach the minimum on-time, and the voltage ripple caused by the minimum on-time can be fundamentally eliminated. 
       FIG. 1  is a block diagram of a switching converter  100  in accordance with an embodiment of the present invention. The switching converter  100  includes a switching circuit  101 , a clock generator  102 , a control circuit  103 , a feedback circuit  104 , a comparing circuit  105 , an error amplifying circuit  106  and a current sensing circuit  107 . The switching circuit  101  has a main transistor, and is configured to convert an input voltage Vin into an output voltage Vout. The switching circuit  101  can be configured in any suitable topologies, such as BUCK, BOOST, BUCK-BOOST, FLYBACK, etc. The feedback circuit  104  is coupled to the switching circuit  101 . It senses an output signal of the switching circuit  101  and generates a feedback signal FB accordingly. The output signal can be the output voltage Vout shown in  FIG. 1 , or alternatively, the output current or output power of the switching circuit  101 . 
     The error amplifying circuit  106  is coupled to the feedback circuit  104 , wherein based on the difference between a reference voltage Vref 1  and the feedback signal FB, the error amplifying circuit  106  generates a compensation signal COMP. The current sensing circuit  107  is configured to sense the current flowing through the main transistor and generate a current sensing signal ISENSE. The comparing circuit  105  is coupled to the current sensing circuit  107  and the error amplifying circuit  106 , wherein the comparing circuit  105  compares the current sensing signal ISENSE with the compensation signal COMP and generates a reset signal RST. The clock generator  102  is configured to generate a clock signal CLK. The control circuit  103  is coupled to the clock generator  102  and the comparing circuit  105 , wherein based on the clock signal CLK and the reset signal RST, the control circuit  103  generates a control signal CTRL to control the main transistor in the switching circuit  101 . 
     As can be seen from  FIG. 1 , the clock generator  102  is coupled to the control circuit  103  to receive the control signal CTRL. It detects whether the on-time Ton of the main transistor is smaller than a time threshold Tth based on the control signal CTRL. The frequency of the clock signal CLK is normally constant. But if the on-time Ton is smaller than the time threshold Tth, the clock generator  102  will adjust the frequency of the clock signal CLK to regulate the on-time Ton to be equal to the time threshold Tth. Generally, the time threshold Tth is configured to be larger than the minimum on-time of the main transistor. 
     It is well-known that, under the same load condition, an increase of the clock frequency would cause the on-time Ton to decrease, and vice versa. When the on-time Ton reduces to be smaller than the time threshold Tth due to an increase of the input voltage Vin, the on-time Ton would be regulated to be equal to the time threshold Tth. The regulation of the energy provided to the load is now realized through adjusting the clock frequency, and the voltage ripple potentially caused by the minimum on-time is fundamentally eliminated. 
     In some embodiments, to prevent misjudgment, the clock generator  102  adjusts the frequency of the clock signal CLK only when the on-time Ton of the main transistor is smaller than the time threshold Tth in a plurality of successive switching cycles. 
       FIG. 2A  schematically illustrates a clock generator  102 A in accordance with an embodiment of the present invention. The clock generator  102 A comprises a first current control circuit  221 A, a controllable current source  222 A, a current mirror  223 A, a frequency setting circuit  224 A, a capacitor C 1 , a transistor T 4 , a comparator COM 1  and a one-shot circuit  225 A. The first current control circuit  221 A has an input terminal and an output terminal, wherein the input terminal is coupled to the control circuit to receive the control signal CTRL, and wherein based on the control signal CTRL, the first current control circuit  221 A detects whether the on-time Ton of the main transistor is smaller than the time threshold Tth, and generates a first current control signal CCS 1  at the output terminal. The controllable current source  222 A has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to a power supply voltage Vcc, the control terminal is coupled to the output terminal of the first current control circuit  221 A to receive the first current control signal CCS 1 , the second terminal is configured to provide a current I 1 . The frequency setting circuit  224 A is configured to provide a setting current Iset. In the embodiment shown in  FIG. 2A , the frequency setting circuit  224 A includes a transistor T 3 , a resistor R 1  and an operational amplifier AMP 1 . People skilled in the art can recognize, however, that the frequency setting circuit  224 A may be configured in other suitable structures, such as a current source controlled by an external clock signal. 
     As can be seen from  FIG. 2A , the current mirror  223 A includes transistors T 1  and T 2 . It has a power supply terminal, a first terminal and a second terminal, wherein the power supply terminal is coupled to the power supply voltage Vcc, the first terminal is coupled to the frequency setting circuit  224 A and the second terminal of the controllable current source  222 A. The current Ichg provided at the second terminal of the current mirror  223 A can be expressed as:
 
 I   chg   =I   set   −I   1   (1.1)
 
     The capacitor C 1  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the current mirror  223 A, the second terminal is coupled to a reference ground. The transistor T 4  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the first terminal of the capacitor C 1 , the second terminal is coupled to the reference ground. The comparator COM 1  has a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is coupled to the first terminal of the capacitor C 1 , the second input terminal is configured to receive a threshold voltage Vth. The comparator COM 1  compares the voltage Vc 1  across the capacitor C 1  with the threshold voltage Vth and generates a comparison signal CMPO at the output terminal. The one-shot circuit  225 A has an input terminal and an output terminal, wherein the input terminal is coupled to the output terminal of the comparator COM 1 , the output terminal is coupled to the control terminal of the transistor T 4  and is configured to provide the clock signal CLK. 
       FIG. 2B  illustrates working waveforms of the clock generator  102 A in accordance with an embodiment of the present invention. As shown in the figure, when the clock signal CLK is logical low, the transistor T 4  is off. The capacitor C 1  is charged by the current Ichg and the voltage Vc 1  across the capacitor C 1  gradually increases. Once the voltage Vc 1  increases to reach the threshold voltage Vth, the comparison signal CMPO changes from logical low into logical high. The one-shot circuit  225 A is triggered to generate a pulse at the clock signal CLK, so the transistor T 4  is turned on for a time period to discharge the capacitor C 1 . 
     The frequency fclk of the clock signal CLK can be expressed as: 
                     f   clk     =       1       t   chg     +     t   pulse         =     1         C   1     ×       V   th       I   chg         +     t   pulse                   (   1.2   )               
Wherein tchg represents the charge time of the capacitor C 1 , and tpulse represents the pulse width of the clock signal CLK.
 
     Combing the equations (1.2) and (1.1), we can get: 
                     f   clk     =     1         C   1     ×       V   th         I   set     -     I   1           +     t   pulse                 (   1.3   )               
It is apparent from the equation (1.3) that the clock frequency fclk would be affected by the current I 1  which is provided by the controllable current source  222 A. The clock frequency fclk decreases when the current I 1  increases, and vice versa.
 
     When the on-time Ton of the main transistor is smaller than the time threshold Tth, the current I 1  varies under the control of the first current control signal CCS 1 , so as to regulate the on-time Ton to be equal to the time threshold Tth. On the other side, when the on-time Ton is larger than the time threshold Tth, the current I 1  is zero and the clock frequency fclk is a constant value determined by the setting current Iset. 
       FIG. 3  schematically illustrates a clock generator  102 B in accordance with an embodiment of the present invention. As shown in  FIG. 3 , the first current control circuit  221 B comprises a one-shot circuit  3211 , a flip-flop FF 1 , a logic circuit  3212 , current source IS 1 , IS 2 , transistors T 5 , T 6  and a capacitor C 2 . The one-shot circuit  3211  has an input terminal and an output terminal, wherein the input terminal is coupled to the control circuit to receive the control signal CTRL. The flip-flop FF 1  has a clock input terminal, a data input terminal and an output terminal, wherein the clock input terminal is coupled to the output terminal of the one-shot circuit  3211 , the data input terminal is coupled to the control circuit to receive the control signal CTRL, the output terminal is configured to provide an on-time detection signal DEC. The logic circuit  3212  has an input terminal, a first output terminal and a second output terminal, wherein the input terminal is coupled to the output terminal of the flip-flop FF 1  to receive the on-time detection signal DEC, and wherein based on the on-time detection signal DEC, the logic circuit  3212  generates logic signals LOG 1  and LOG 2  respectively at the first output terminal and the second output terminal. The current source IS 1  has a first terminal and a second terminal, wherein the first terminal is coupled to the power supply voltage Vcc. The transistor T 5  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the second terminal of the current source IS 1 , and the control terminal is coupled to the first output terminal of the logic circuit  3212  to receive the logic signal LOG 1 . The transistor T 6  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the second terminal of the transistor T 5 , and the control terminal is coupled to the second output terminal of the logic circuit  3212  to receive the logic signal LOG 2 . The current source IS 2  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the transistor T 6 , the second terminal is coupled to the reference ground. The capacitor C 2  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the transistor T 5  and the first terminal of the transistor T 6 , and is configured to provide the first current control signal CCS 1 , the second terminal is coupled to the reference ground. 
     The controllable current source  222 B includes a current mirror  3213 , a transistor T 7 , a resistor R 2  and a current source IS 3 . The transistor T 7  has a first terminal, a second terminal and a control terminal, wherein the control terminal is coupled to the first current control circuit  221 B to receive the first current control signal CCS 1 . The resistor R 2  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the transistor T 7 . The current source IS 3  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the resistor R 2 , the second terminal is coupled to the reference ground. The current mirror  3212  includes transistors T 8  and T 9 . It has a power supply terminal, a first terminal and a second terminal, wherein the power supply terminal is coupled to the power supply voltage Vcc, the first terminal is coupled to the first terminal of the transistor T 7 , the second terminal is configured to provide the current I 1 . 
     When the control signal CTRL changes from logical low into logical high, the one-shot circuit  3211  is triggered to generate a pulse signal which has a pulse width equal to the time threshold Tth. At the falling edge of the pulse signal, the flip-flop FF 1  is triggered to provide the signal at its data input terminal to the output terminal. If the on-time Ton of the main transistor is smaller than the time threshold, the on-time detection signal DEC output by the flip-flop FF 1  would be logical low. Under the control of the logic circuit  3212 , the transistor T 6  turns off and the transistor T 5  turns on for a time period to let the current source IS 1  charge the capacitor C 2 . The first current control signal CCS 1  which is equal to the voltage across the capacitor C 2  goes up. When the first current control signal CCS 1  becomes larger than the threshold voltage of the transistor T 7 , the current I 1  generated by the current mirror  3123  would be larger than zero and vary along with the first current control signal CCS 1 . Specifically speaking, the maximum value of the current I 1  is determined by the current source IS 3 . 
     If the on-time Ton of the main transistor is larger than the time threshold, the on-time detection signal DEC output by the flip-flop FF 1  would be logical high. Under the control of the logic circuit  3212 , the transistor T 5  turns off and the transistor T 6  turns on for a time period to let the current source IS 2  discharge the capacitor C 2 . The current I 1  generated by the current mirror  3123  as well as the first current control signal CCS 1  goes down. When the first current control signal CCS 1  becomes smaller than the threshold voltage of the transistor T 7 , the current I 1  would be zero and the clock frequency fclk would resume to the constant value mentioned before. 
     In one embodiment, the first current control circuit  221 B further comprises a fast startup circuit including a current source IS 4 , transistors T 10 ˜T 12  and a multi detection circuit  3214 . The current source IS 4  has a first terminal and a second terminal, wherein the first terminal is coupled to the power supply voltage Vcc. The transistor T 10  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the second terminal is coupled to the first terminal of the capacitor C 2 , the control terminal is coupled to the second terminal of the current source IS 4 . The transistor T 11  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the control terminal of the transistor T 10 , the second terminal is coupled to the reference ground, the control terminal is coupled to the first terminal of the capacitor C 2 . The multi detection circuit  3214  is coupled to the output terminal of the flip-flop FF 1  to receive the on-time detection signal DEC, wherein based on the on-time detection signal DEC, the multi detection circuit  3214  determines whether the on-time Ton of the main transistor is smaller than the time threshold Tth in a plurality of successive switching cycles, and generates a multi detection signal MTD. The transistor T 12  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the control terminal of the transistor T 10 , the second terminal is coupled to the reference ground, and the control terminal is coupled to the multi detection circuit  3214  to receive the multi detection signal MTD. 
     In normal operation, the transistor T 12  is on and the transistor T 10  is off. The fast startup circuit does not work. When the multi detection circuit  3214  detects the on-time Ton of the main transistor is smaller than the time threshold in a plurality of successive switching cycles, the transistor T 12  turns off and the fast startup circuit starts to work. The capacitor C 2  is charged by the power supply voltage Vcc through the transistor T 10  until the current flowing through the transistor T 11  becomes equal to the current provided by the current source IS 4 . 
       FIG. 4  schematically illustrates a clock generator  102 C in accordance with an embodiment of the present invention, wherein the first current control circuit  221 C comprises transistors T 13 ˜T 15 , a current source IS 5  and a capacitor C 3 . The transistor T 13  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the control terminal is coupled to the control terminals of the transistors T 1  and T 2 . The transistor T 14  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the second terminal of the transistor T 13 . The transistor T 15  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the second terminal of the transistor T 14 , the control terminal is coupled to the control circuit to receive the control signal CTRL. The current source IS 5  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the transistor T 15 , the second terminal is coupled to the reference ground. The capacitor C 3  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the transistor T 14  and the first terminal of the transistor T 15 , and is configured to provide the first current control signal CCS 1 , the second terminal is coupled to the reference ground. 
     The transistor T 14  is maintained on in current continuous mode. The transistor T 15  turns on when the main transistor is on and turns off when the main transistor is off. Based on the configuration shown in  FIG. 4 , the time threshold Tth can be expressed as: 
     
       
         
           
             
               
                 
                   
                     T 
                     th 
                   
                   = 
                   
                     
                       I 
                       chg 
                     
                     
                       
                         I 
                         
                           s 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           5 
                         
                       
                       × 
                       
                         f 
                         clk 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1.4 
                   ) 
                 
               
             
           
         
       
     
     When the on-time Ton of the main transistor is larger than the time threshold Tth as shown in the equation (1.4), the first current control signal CCS 1  which is equal to the voltage across the capacitor C 3  would be lower than the threshold voltage of the transistor T 7 . Thus the transistor T 7  turns off and the current I 1  is zero. When the on-time Ton is smaller than the time threshold Tth, the first current control signal CCS 1  would increase to be higher than the threshold voltage of the transistor T 7 . Consequently, the transistor T 7  turns on and the current I 1  becomes larger than zero. 
       FIG. 5  schematically illustrates a clock generator  102 D in accordance with an embodiment of the present invention. Compared with the clock generator  102 A shown in  FIG. 2 , the clock generator  103 D of  FIG. 5  further comprises a second current control circuit  525  and a controllable current source  526 . The second current control circuit  525  is configured to detect whether the switching converter works under a light load condition and generate a second current control signal CCS 2 . The controllable current source  525  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the second terminal is coupled to the first terminal of the current mirror  223 A to provide a current I 2 , the control terminal is coupled to the second current control circuit  525  to receive the second current control signal CCS 2 . Therefore, the current Ichg generated by the current mirror  223 A can be expressed as:
 
 I   chg   =I   set   −I   1   −I   2   (1.5)
 
     If the second current control circuit  525  detects that the switching converter does not work under the light load condition, the current I 2  output by the controllable current source  526  will be zero. Otherwise, if the second current control circuit  525  detects that the switching converter works under the light load condition, which means the output current or output power of the switching converter is lower than a predetermined value, the current I 2  output by the controllable current source  526  will be larger than zero. The clock frequency fclk as well as the current Ichg will decrease, which definitely lowers the switching loss of the switching converter and improves the light load efficiency. 
     The function of the second current control circuit  525  and the controllable current source  526  can be realized by a circuit  627  shown in  FIG. 6 . The circuit  627  includes a controllable current source  6271 , transistors T 16 ˜T 19  and a current source IS 6 . The controllable current source  6271  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the control terminal is coupled to the error amplifying circuit to receive the compensation signal COMP. The controllable current source  6271  provides a current Icomp determined by the compensation signal COMP. The transistor T 16  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the second terminal is coupled to the reference ground. The transistor T 17  has a first terminal, a second terminal and a control terminal, wherein the first terminal and the control terminal are coupled to the second terminal of the controllable current source  6271 . The transistor T 18  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the first terminal of the current mirror  223 A, the second terminal is coupled to the frequency setting circuit  224 A, the control terminal is coupled to the control terminal of the transistor T 17 . The transistor T 19  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the power supply voltage Vcc, the second terminal is coupled to the frequency setting circuit  224 A, the control terminal is coupled to the control terminal of the transistor T 16 . The current source IS 6  has a first terminal and a second terminal, wherein the first terminal is coupled to the second terminals of the transistors T 16  and T 17 , the second terminal is coupled to the reference ground. 
     Based on the configuration described above, it can be derived that:
 
 I   16   +I   17   =I   s6   (1.6)
 
 I   18   +I   19   =I   set   (1.7)
 
 I   16   ×I   18   =I   17   ×I   19   (1.8)
 
 I   17   =I   comp   (1.9)
 
 I   chg   =I   18   −I   1   (1.10)
 
     Wherein I 16 ˜I 19  respectively represent the current flowing through the transistors T 16 ˜T 19 . 
     When the switching converter does not works under the light load condition, the current Icomp output by the controllable current source  6271  is larger than the current provided by the current source IS 6 . The current I 16  flowing through the transistor T 16  and the current I 19  flowing through the transistor T 19  are both zero. Then just as shown in the equation (1.1), the current Ichg generated by the current mirror  223 A would not be affected by the circuit  627 . 
     When the switching converter works under light load condition, the current Icomp becomes smaller than the current provided by the current source IS 6 . The current Ichg generated by the current mirror  223 A can be expressed as: 
     
       
         
           
             
               
                 
                   
                     I 
                     chg 
                   
                   = 
                   
                     
                       
                         
                           I 
                           comp 
                         
                         × 
                         
                           I 
                           set 
                         
                       
                       
                         I 
                         
                           s 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           6 
                         
                       
                     
                     - 
                     
                       I 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   1.11 
                   ) 
                 
               
             
           
         
       
     
     It can be seen from the equation (1.11) that, the lighter the load, the smaller the compensation signal COMP, thus the lower the current Ichg and the clock frequency fclk. 
       FIG. 7  schematically illustrates a switching converter  100 A in accordance with an embodiment of the present invention. The switching circuit  101 A is a synchronous BUCK circuit consisting of an input capacitor Cin, transistors S 1 , S 2 , an inductor L and an output capacitor Cout, connected as shown in  FIG. 7 . The feedback circuit  104 A comprises a resistor divider composed of resistors R 3  and R 4 . The error amplifying circuit  106 A contains an error amplifier AMP 2 . The comparing circuit  105 A involves a comparator COM 2  which compares a sum of the current sensing signal ISENSE and a ramp signal RAMP with the compensation signal COMP and generates the reset signal RST. The control circuit  103 A includes a flip-flop FF 2  having a set terminal, a reset terminal and an output terminal, wherein the set terminal is coupled to the clock generator  102  to receive the clock signal CLK, the reset terminal is coupled to the comparing circuit  105 A to receive the reset signal RST, the output terminal is configured to provide the control signal CTRL which controls the transistors S 1  and S 2  through a driving circuit  708 . 
     Although the embodiments shown in  FIG. 1  and  FIG. 7  both utilize the peak current control method, people of ordinary skill in the art can recognize that the error amplifying circuit, current sensing circuit and comparing circuit are not necessary, and the present invention can be applied to other suitable control schemes, e.g. single loop PWM control. Furthermore, in the embodiments described above, the output current of the current mirror is equal to the input current. But it can be understood that this does not intend to limit the present invention, and the output current of the current mirror can also be proportional to the input current. 
       FIG. 8  is a flow chart of a control method for switching converters in accordance with an embodiment of the present invention. It comprises steps S 801 ˜S 805 . 
     At step S 801 , a feedback signal indicative of the output signal of the switching converter is generated. 
     At step S 802 , a clock signal configured to determine the switching frequency of the main transistor is generated. 
     At step S 803 , a control signal is generated to control the main transistor based on the clock signal and the feedback signal. 
     At step S 804 , whether the on-time of the main transistor Ton is smaller than a time threshold Tth is detected based on the control signal. If the on-time Ton is smaller than the time threshold Tth, proceed to step S 805 , otherwise, keep detecting. 
     At step S 805 , the frequency of the clock signal is adjusted to regulate the on-time Ton to be equal to the time threshold Tth. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. It should be understood, of course, the foregoing disclosure relates only to a preferred embodiment (or embodiments) of the invention and that numerous modifications may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims. Various modifications are contemplated and they obviously will be resorted to by those skilled in the art without departing from the spirit and the scope of the invention as hereinafter defined by the appended claims as only a preferred embodiment(s) thereof has been disclosed.