Patent Publication Number: US-2023163687-A1

Title: Discontinuous current mode dc-dc converter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/282,738, filed Nov. 24, 2021, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a discontinuous current mode (DCM) DC-DC converter. 
     Description of the Related Art 
       FIG.  1    illustrates a conventional DCM DC-DC converter  100  which converts a direct current voltage PVDD into an output voltage Vo. When the output voltage Vo drops lower than the reference voltage Vref, a charging trigger signal pfm_cmp is asserted to activate a fixed charging duration Ton. The driver  102  uses a charging signal, u, to turn on a charging path (through a power transistor Mp and an inductor L) for the output voltage Vo till the fixed charging duration Ton is satisfied. After the fixed charging duration Ton, the driver  102  turns off the charging path and uses the discharging signal, l, to turn on the discharging path (through a power transistor Mn and the inductor L) for the output voltage Vo. When an inductor voltage LX is increased to the ground voltage PGND, the zero-crossing signal ZC is asserted. According to the asserted zero-crossing signal ZC, the driver  102  turns off the charging path and the discharging path both. 
       FIG.  2    shows the signal waveforms of the conventional DCM DC-DC converter  100 . Because of the fixed charging duration Ton, the load current ILoad (referring to the dashed line) is limited and incapable to drive a heavy load. 
     BRIEF SUMMARY OF THE INVENTION 
     A DCM DC-DC converter adaptive to the loading state is introduced. 
     A DCM DC-DC converter in accordance with an exemplary embodiment of the present invention has an inductor, power transistors, a driver, a load detector, and a dynamic driver controller. The power transistors provide a charging path and a discharging path for an output voltage of the DCM DC-DC converter through the inductor. The driver drives the power transistors to control the charging path and a discharging path. The load detector receives the output voltage to determine the loading state of the DCM DC-DC converter. The dynamic driver controller controls the driver to provide an enhanced charging capability or a normal charging capability through the charging path, depending on the loading state. 
     In an exemplary embodiment, the driver turns off both the charging path and the discharging path according to the zero-crossing signal. The load detector determines that the DCM DC-DC converter operates with a heavy load when detecting, according to the zero-crossing signal, that the output voltage is lower than the reference voltage. The dynamic driver controller controls the driver to turn on the charging path to provide the enhanced charging capability when the heavy load is detected by the load detector. 
     In an exemplary embodiment, when the load detector does not detect the heavy load, a normal criteria is applied to assert the zero-crossing signal. When the load detector detects the heavy load, a shifted criteria is applied to assert the zero-crossing signal. 
     In an exemplary embodiment, the zero-crossing signal is asserted based on a ground voltage and an inductor voltage, wherein the ground voltage is applied to the power transistors, and the inductor voltage is detected from a connection terminal that connects the inductor to the power transistors. The enhanced charging capability is achieved by adding a negative offset to the ground voltage for generation of the zero-crossing signal. 
     In an exemplary embodiment, the zero-crossing signal is asserted based on a ground voltage and an inductor voltage, wherein the ground voltage is applied to the power transistors, and the inductor voltage is detected from a connection terminal that connects the inductor to the power transistors. The enhanced charging capability is achieved by adding a positive offset to the inductor voltage for generation of the zero-crossing signal. 
     In an exemplary embodiment, the load detector determines that the discontinuous current mode DC-DC converter operates with a heavy load when detecting, according to a charging current upper threshold alert, that the output voltage is lower than the reference voltage. The dynamic driver controller controls the driver to turn on the charging path to provide the enhanced charging capability when the heavy load is detected by the load detector. 
     In an exemplary embodiment, the load detector determines that the discontinuous current mode DC-DC converter operates with a heavy load when detecting, according to a discharging current lower threshold alert, that the output voltage is lower than the reference voltage. The dynamic driver controller controls the driver to turn on the charging path to provide the enhanced charging capability when the heavy load is detected by the load detector. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG.  1    illustrates a conventional DCM DC-DC converter  100  which converts a direct current voltage PVDD into an output voltage Vo; 
         FIG.  2    shows the signal waveforms of the conventional DCM DC-DC converter  100 ; 
         FIG.  3    illustrates a DCM DC-DC converter  300  in accordance with an exemplary embodiment of the present invention; 
         FIG.  4    illustrates a DCM DC-DC converter  400  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by adding a negative offset to the ground voltage PGND for generation of the zero-crossing signal ZC; 
         FIG.  5    shows the signal waveforms of the DCM DC-DC converter  400 ; 
         FIG.  6    illustrates a DCM DC-DC converter  600  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by adding a positive offset to the inductor voltage LX for generation of the zero-crossing signal ZC; 
         FIG.  7    illustrates a DCM DC-DC converter  700  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by extending the turning on of the charging path; 
         FIG.  8    shows the waveform of the inductor current IL in the DCM DC-DC converter  700 , which is not limited by the fixed discharging duration Toff; 
         FIG.  9    illustrates a DCM DC-DC converter  900  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by shrinking the turning on of the discharging path; 
         FIG.  10    shows the signal waveforms of the DCM DC-DC converter  900 ; 
         FIG.  11    illustrates a DCM DC-DC converter  1100  in accordance with an exemplary embodiment of the present invention, which increases the turning on of the charging path to achieve the enhanced charging capability (Ton_en) in another manner, different from that of  FIG.  7   ; 
         FIG.  12    shows the signal waveforms of the DCM DC-DC converter  1100 ; 
         FIG.  13    illustrates a DCM DC-DC converter  1300  in accordance with an exemplary embodiment of the present invention, which increases the turning on of the charging path to achieve the enhanced charging capability (Ton_en) by extending the turning on of the charging path in another way; 
         FIG.  14    illustrates a DCM DC-DC converter  1400  in accordance with an exemplary embodiment of the present invention, which is a combination of the techniques taught in  FIG.  6    and  FIG.  11   ; 
         FIG.  15    shows the signal waveforms of the DCM DC-DC converter  1400 ; 
         FIG.  16    illustrates a DCM DC-DC converter  1600  in accordance with an exemplary embodiment of the present invention, which is a combination of the techniques taught in  FIG.  6    and  FIG.  13   ; 
         FIG.  17    shows the signal waveforms of the DCM DC-DC converter  1600 ; 
         FIG.  18    illustrates a DCM DC-DC converter  1800  in accordance with an exemplary embodiment of the present invention, which is modified from the DCM DC-DC converter  600  of  FIG.  6   ; 
         FIG.  19    shows the signal waveforms of the DCM DC-DC converter  1800 ; and 
         FIG.  20    and  FIG.  21    illustrate DCM DC-DC converters  2000  and  2100  in accordance with exemplary embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG.  3    illustrates a DCM DC-DC converter  300  in accordance with an exemplary embodiment of the present invention, which includes an inductor L, power transistors Mp and Mn, a driver  302 , a load detector  304 , and a dynamic driver controller  306 . The power transistor Mp provides a charging path for an output voltage Vo of the DCM DC-DC converter  300  through the inductor L. The power transistor Mn provides a discharging path for the output voltage Vo through the inductor L. The driver  302  drives the power transistors Mp and Mn to control the charging path and the discharging path. The load detector  304  receives the output voltage Vo to determine the loading state of the DCM DC-DC converter  300 . The dynamic driver controller  306  is coupled between the load detector  304  and the driver  302 . The dynamic driver controller  306  controls the driver  302  to provide an enhanced charging capability (presented by Ton_en hereinafter) or a normal charging capability through the charging path, depending on the loading state. 
     In an exemplary embodiment, the driver  302  turns off both the charging path and the discharging path according to a zero-crossing signal (ZC hereinafter). The load detector  304  determines that the DCM DC-DC converter  300  operates with a heavy load when detecting, according to the zero-crossing signal ZC, that the output voltage Vo is still lower than the reference voltage (Vref hereinafter). The dynamic driver controller  306  controls the driver  302  to turn on the charging path to provide the enhanced charging capability (Ton_en) when a heavy load is detected by the load detector  304 . 
     In such a case, the enhanced charging capability (Ton_en) may be achieved by shifting the criteria that is applied to assert the zero-crossing signal ZC. When the load detector  304  does not detect a heavy load, a normal criteria is applied to assert the zero-crossing signal ZC. When the load detector  304  detects a heavy load, a shifted criteria is applied to assert the zero-crossing signal ZC. 
     In an exemplary embodiment, the zero-crossing signal ZC is asserted based on a ground voltage PGND and an inductor voltage LX, wherein the ground voltage PGND is applied to the power transistors Mp and Mn, and the inductor voltage LX is detected from a connection terminal that connects the inductor L to the power transistors (Mp and Mn). The enhanced charging capability (Ton_en) is achieved by adding a negative offset to the ground voltage PGND for generation of the zero-crossing signal ZC. 
       FIG.  4    illustrates a DCM DC-DC converter  400  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by adding a negative offset to the ground voltage PGND for generation of the zero-crossing signal ZC. 
     The DCM DC-DC converter  400  uses a D-flip flop  402  to detect the need for an enhanced charging capability (Ton_en). The D-flip flop  402  has a D terminal, D, coupled to the power voltage VDD, a clock terminal clk receiving the zero-crossing signal ZC, a reset terminal, Reset, asserted according to the inverted charging trigger signal pfm_cmp (pfm_cmp is asserted when the output voltage Vo is lower than the reference voltage Vref), and a Q terminal, Q, outputting a criteria changing signal Pos_ZC. The criteria changing signal Pos_ZC is high when the zero-crossing signal ZC and the charging trigger signal pfm_cmp both are high, which indicates a heavy load. 
     The DCM DC-DC converter  400  further has a multiplexer  404 , a zero-crossing comparator  406 , and an AND gate  408 . The multiplexer  404  is controlled by the criteria changing signal Pos_ZC to output the ground voltage PGND or a shifted ground voltage that is the ground voltage PGND plus a negative offset. The zero-crossing comparator  406  has a positive terminal (+) receiving the inductor voltage LX, and a negative terminal (-) coupled to the output terminal of the multiplexer  404 . The AND gate  408  generates the zero-crossing signal ZC based on an output signal zc_cmp of the zero-crossing comparator  406  and a discharging signal, l, that controls the discharging path. According to this structure, the criteria applied to assert the zero-crossing signal ZC is adaptive to the loading state of the DCM DC-DC converter  400 . For a normal load, the criteria is based on the normal ground voltage PGND. For a heavy load, the criteria is based on the shifted ground voltage (PGND plus a negative offset). 
     In this case, the driver  410  of the DCM DC-DC converter  400  is triggered by the charging trigger signal pfm_cmp to turn on the charging path for a fixed charging duration Ton. Corresponding to the enhanced charging capability (Ton_en), the turn-on duration (also marked by Ton_en) equals the fixed charging duration Ton. 
       FIG.  5    shows the signal waveforms of the DCM DC-DC converter  400 . Because of the detected heavy load (pfm_cmp and ZC both are high), the criteria changing signal Pos_ZC is “1”, and the turn-on duration Ton_en (with the fixed length Ton) due to the enhanced charging capability is applied to turn on the charging path. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  400 . 
     In another exemplary embodiment, the enhanced charging capability (Ton_en) is achieved by adding a positive offset to the inductor voltage LX for generation of the zero-crossing signal ZC. 
       FIG.  6    illustrates a DCM DC-DC converter  600  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by adding a positive offset to the inductor voltage LX for generation of the zero-crossing signal ZC. 
     The DCM DC-DC converter  600  uses a D-flip flop  602  to detect the need for an enhanced charging capability (Ton_en). The criteria changing signal Pos_ZC is high when the zero-crossing signal ZC and the charging trigger signal pfm_cmp both are high. A heavy load is reflected on the criteria changing signal Pos_ZC. 
     The DCM DC-DC converter  600  further has a multiplexer  604 , a zero-crossing comparator  606 , and an AND gate  608 . The multiplexer  604  is controlled by the criteria changing signal Pos_ZC to output the inductor voltage LX or a shifted inductor voltage (LX plus a positive offset). The zero-crossing comparator  606  has a positive terminal “+” coupled to the output terminal of the multiplexer  604 , and a negative terminal “-” receiving the ground voltage PGND. The AND gate  608  generates the zero-crossing signal ZC based on the output signal zc_cmp of the zero-crossing comparator  606  and the discharging signal, l, that controls the discharging path. According to this structure, the criteria applied to assert the zero-crossing signal ZC is adaptive to the loading state of the DCM DC-DC converter  600 . For a normal load, the criteria is based on the normal inductor voltage LX. For a heavy load, the criteria is based on the shifted inductor voltage (LX plus a positive offset). The turn-on duration Ton_en corresponding to the enhanced charging capability is the fixed charging duration Ton, too. The waveforms of the DMC DC-DC converter  600  are similar to those shown in  FIG.  5   . 
     In another exemplary embodiment, the enhanced charging capability (Ton_en) is achieved by extending the turning on of the charging path. Referring back to  FIG.  3   , the load detector  304  may determine that the DCM DC-DC converter  300  operates with a heavy load when detecting, according to a charging current upper threshold alert, that the output voltage Vo is lower than the reference voltage Vref. The dynamic driver controller  306  controls the driver  302  to extend the turning-on of the charging path for the enhanced charging capability (Ton_en) when a heavy load is detected by the load detector. 
       FIG.  7    illustrates a DCM DC-DC converter  700  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by extending the turning on of the charging path. 
     The DCM DC-DC converter  700  uses a dynamic charging controller  702  to turning off the charging path later (by controlling the turn off signal u_off for the charging path) when a heavy load is detected. In this manner, the turning on of the charging path is extended to achieve the enhanced charging capability (Ton_en). The dynamic charging controller  702  controls the driver  704  to turn off the charging path when a charging current Ip (detected from the charging path) reaches a charging current upper threshold (Ip_upper hereinafter) to issue a charging current upper threshold alert. If the output voltage Vo is still lower than the reference voltage Vref when the charging current upper threshold alert occurs, the dynamic charging controller  702  uses a shifted charging current upper threshold (Ip_upper plus a positive offset) to issue the next charging current upper threshold alert. If the output voltage Vo is not lower than the reference voltage Vref when a charging current upper threshold alert occurs, the dynamic charging controller  702  uses a non-shifted charging current upper threshold Ip_upper to issue the next charging current upper threshold alert. In an exemplary embodiment, the non-shifted charging current upper threshold Ip_upper is 500mA, and the shifted charging current upper threshold (Ip_upper plus a positive offset) is 700mA. In an exemplary embodiment, after turning off the charging path (as indicated by u_off), the driver  704  turns on the discharging path for a fixed discharging duration Toff (optional). As shown, the fixed discharging duration Toff starts by the turn-off signal u_off of the charging path, and is ended by the turn-off signal l_off of the discharging path. 
       FIG.  8    shows the waveform of the inductor current IL in the DCM DC-DC converter  700 , which is not limited by the fixed discharging duration Toff. Because of the detected heavy load (pfm_cmp is still high when Ip reaches Ip_upper), an enhanced charging capability (Ton_en) is applied and the turning on of the charging path is extended from Ton to Ton_en. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  700 . 
     In another exemplary embodiment, the enhanced charging capability (Ton_en) is achieved by shrinking the turning on of the discharging path. Referring back to  FIG.  3   , the load detector  304  determines that the DCM DC-DC converter  300  operates with a heavy load when detecting, according to a discharging current lower threshold alert, that the output voltage Vo is lower than the reference voltage Vref. The dynamic driver controller  306  controls the driver  302  to turn on the charging path for the enhanced charging capability (Ton_en) when a heavy load is detected by the load detector  304 . 
       FIG.  9    illustrates a DCM DC-DC converter  900  in accordance with an exemplary embodiment of the present invention, which achieves the enhanced charging capability (Ton_en) by shrinking the turning on of the discharging path. 
     The DCM DC-DC converter  700  uses a dynamic discharging controller  902  to control the driver  904  to turn off the discharging path (by a turn-off signal l_off for the discharging path) when a discharging current, In. detected from the discharging path drops to a discharging current lower threshold In_lower to issue the discharging current lower threshold alert. If the output voltage Vo is still lower than the reference voltage Vref when a discharging current lower threshold alert occurs, the dynamic discharging controller  902  uses a shifted discharging current lower threshold (In_lower plus a positive offset) to issue the next discharging current lower threshold alert. If the output voltage Vo is not lower than the reference voltage Vref when a discharging current lower threshold alert occurs, the dynamic discharging controller  902  uses a non-shifted discharging current lower threshold In_lower to issue the next discharging current lower threshold alert. In an exemplary embodiment, the non-shifted discharging current lower threshold In_lower is 0mA, and the shifted discharging current lower threshold (In_lower plus a positive offset) is 200mA. The driver  904  is triggered by the charging trigger signal pfm_cmp to turn on the charging path for a fixed charging duration Ton. The turn-on duration due to the enhanced charging capability is also marked by Ton_en, whose length is the fixed charging duration Ton, too. 
       FIG.  10    shows the waveform of the inductor current IL in the DCM DC-DC converter  900 . Because of the detected heavy load (pfm_cmp is still high when In drops to In_lower), an enhanced charging capability (which results in a turn-on duration Ton_en with the same length with the fixed Ton) is applied to turn on the charging path. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  900 . 
       FIG.  11    illustrates a DCM DC-DC converter  1100  in accordance with an exemplary embodiment of the present invention, which increases the turning on of the charging path to achieve the enhanced charging capability (Ton_en) in another manner, different from that of  FIG.  7   . The pulse width modulation signal PWM controlling the driver to turn on the charging path (controlled by the charging signal u) or the discharging path (controlled by the discharging signal l) depends on the criteria changing signal Pos_ZC. When the DCM DC-DC converter  1100  is driving a normal load, the criteria changing signal Pos_ZC is 0, and a normal charging duration Ton is applied to generate the pulse width modulation signal PWM. When the DCM DC-DC converter  1100  is driving a heavy load, the criteria changing signal Pos_ZC is 1, and a longer charging duration (&gt;Ton) is applied to generate the pulse width modulation signal PWM, and the turning on of the charging path is increased (plus an additional charging duration Ton_add). 
       FIG.  12    shows the signal waveforms of the DCM DC-DC converter  1100 . Because of the detected heavy load (pfm_cmp and ZC both are high), the criteria changing signal Pos_ZC is “1”, the charging duration is increased (&gt;Ton, which increases the duty cycle of PWM), and an additional charging duration Ton_add is applied to turn on the charging path. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  1100 . Enhanced charging capability (e.g., Ton_en = normal Ton + Ton_add) is achieved 
       FIG.  13    illustrates a DCM DC-DC converter  1300  in accordance with an exemplary embodiment of the present invention, which increases the turning on of the charging path to achieve the enhanced charging capability (Ton_en) by extending the turning on of the charging path in another way. 
     The DCM DC-DC converter  1300  uses a multiplexer  1302  to output (according to the criteria changing signal Pos_ZC) a peak inductor current limit Ipeak_set_L or an increased peak inductor current limit Ipeak_set_H to be compared with the inductor current IL sensed by a current sensor  1304 . The comparator  1306  has a positive terminal receiving the sensed inductor current IL, and a negative terminal receiving the output of the multiplexer  1302 . The compared result  1308  is sent to an S terminal of an SR latch  1310  (whose R terminal receives the zero-crossing signal ZC). A Q terminal of the SR latch  1310  is coupled to an R terminal of another SR latch  1312  (whose S terminal receives the charging trigger signal pfm_cmp). The Q terminal of the SR latch  1312  is used to control the driver  1314  to turn on the charging path (controlled by the charging signal u) or the discharging path (controlled by the discharging signal l). When the DCM DC-DC converter  1300  is driving a normal load, the criteria changing signal Pos_ZC is 0, and the peak inductor current limit Ipeak_set_L is applied to deassert the charging signal, u. When the DCM DC-DC converter  1300  is driving a heavy load, the criteria changing signal Pos_ZC is 1, and the increased peak inductor current limit Ipeak_set_H is applied to deassert the charging signal, u, and the turning on of the charging path is increased (with the additional charging duration Ton_add). The signal waveforms of the DCM DC-DC converter  1300  are similar to those shown in  FIG.  12   . 
       FIG.  14    illustrates a DCM DC-DC converter  1400  in accordance with an exemplary embodiment of the present invention, which is a combination of the techniques taught in  FIG.  6    and  FIG.  11   . When a heavy load is detected, a positive offset to the inductor voltage LX for generation of the zero-crossing signal ZC is applied, and the turning on of the charging path is increased by increasing the duty cycle of PWM.  FIG.  15    shows the signal waveforms of the DCM DC-DC converter  1400 , and  FIG.  15    shows that the turn-on duration Ton_en due to the enhanced charging capability is longer than the normal charging duration Ton rather than equals to the normal charging duration Ton. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  1400 . 
       FIG.  16    illustrates a DCM DC-DC converter  1600  in accordance with an exemplary embodiment of the present invention, which is a combination of the techniques taught in  FIG.  6    and  FIG.  13   . When a heavy load is detected, a positive offset to the inductor voltage LX for generation of the zero-crossing signal ZC is applied, and the turning on of the charging path stopped according to the increased peak inductor current limit Ipeak_set_H.  FIG.  17    shows the signal waveforms of the DCM DC-DC converter  1600 , and  FIG.  17    shows that the enhanced charging capability Ton_en in response to the detected heavy load. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  1600 . 
       FIG.  18    illustrates a DCM DC-DC converter  1800  in accordance with an exemplary embodiment of the present invention, which is modified from the DCM DC-DC converter  600  of  FIG.  6   . In comparison with  FIG.  6   , in  FIG.  18   , the peak inductor current IL is limited by a peak inductor current limit Ipeak_set. The current sensor  1802  senses an inductor current IL through the inductor L. The comparator  1804  has a positive terminal ‘+’ receiving the inductor current IL sensed by the current sensor  1802 , and a negative terminal ‘-’ receiving a peak inductor current limit Ipeak_set. The SR latch  1806  has an S terminal, S, receiving an output terminal of the comparator  1804 , an R terminal, R, receiving the zero-crossing signal ZC. The SR latch  1808  has an S terminal, S, receiving a charging trigger signal pfm_cmp that is asserted when the output voltage Vo is lower than the reference voltage Vref, an R terminal coupled to a Q terminal of the SR latch  1806 , and a Q terminal coupled to the driver  1810  to turn on the charging path.  FIG.  19    shows the signal waveforms of the DCM DC-DC converter  1800 , and  FIG.  19    shows the enhanced charging capability in response to the detected heavy load. The load current ILoad (referring to the dashed line) is adaptive to the loading state of the DCM DC-DC converter  1800 . 
       FIG.  20    and  FIG.  21    illustrate DCM DC-DC converters  2000  and  2100  in accordance with exemplary embodiments of the present invention. The zero-crossing signal ZC is generated based on the sensed current ILO (through the inductor) or In (through the power transistor Mn). In  FIG.  20   , in response to a heavy load (Pos_ZC is 1), the sensed current ILO/In is shifted by a negative offset to be compared with a reference current Iref for the generation of the zero-crossing signal ZC. In  FIG.  21   , in response to a heavy load (Pos_ZC is 1), the reference current Iref is shifted by a positive offset to be compared with the sensed current ILO/In for the generation of the zero-crossing signal ZC. By shifting the criteria for asserting the zero-crossing signal ZC, an enhanced charging capability is provided in response to the heavy load. 
     The adaptive design of the present invention does not need a huge output capacitor nor suppress the inductance of the inductor L. The PCB cost is not increased, and the conversion efficiency is not affected 
     While the invention has been described by way of example and in terms of the preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.