Abstract:
A switching regulator for fixing a frequency which includes a power stage circuit, comprising an upper gate switch, a lower gate switch and an inductor; a reference voltage generator for generating a reference voltage; a comparator for outputting a comparing result according to the output voltage and the reference voltage; a constant frequency compensating circuit for a control signal according to the comparing result, a phase signal and a compensating signal. The constant frequency compensating circuit comprises a charging capacitor. The phase signal corresponding to the cross voltage of the lower gate bridge, and the compensating signal corresponding to the output voltage, and the constant frequency compensating circuit utilizes the phase signal to initialize a voltage of a terminal of the charging capacitor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a switching regulator and constant frequency compensating circuit, and more particularly, to a switching regulator and constant frequency compensating circuit for fixing an operating frequency according to an output voltage and a phase signal. 
         [0003]    2. Description of the Prior Art 
         [0004]    Power supply devices play an essential role in modern information technology. Among all the power supply devices, DC-DC (direct current to direct current) switching regulators have been widely used and are mainly utilized for providing stable DC power sources to electronic devices. Please refer to  FIG. 1 , which is a schematic diagram of a conventional DC-DC switching regulator  10 . The DC-DC switching regulator  10  is utilized for providing a stable voltage to a load Load 1 , and includes an upper gate switch  100 , a lower gate switch  102 , a constant time trigger circuit  104 , a comparator  106 , an inductor L 1 , a capacitor C 1 , a reference voltage Vref 1  and an inverter INV 1 . The constant time trigger circuit  104  outputs a pulse signal with constant active time to control operations of the upper gate switch  100  and the lower gate switch  102 . Whenever the output voltage Vout 1  is smaller than the reference voltage Vref 1 , the comparator  106  outputs a signal to the constant time trigger circuit  104 , so that the constant time trigger circuit  104  outputs the pulse signal to turn on the upper gate switch  100  and turn off the lower gate switch  102 . Thus, an external voltage source Vinl supplies electric energy to the inductor L 1  and then to the load Load 1  via the upper gate switch  100 . Since the pulse signal outputted by the constant time trigger circuit  104  has a constant active time, the upper gate switch  100  can be turned on for a constant interval when the output voltage Vout 1  is smaller than the reference voltage Vref 1 . If the output voltage Vout 1  is still higher than the reference voltage Vref 1  after the upper gate switch  100  is turned on, the upper gate switch  100  would remain turned off until the output voltage Vout 1  is smaller than the reference voltage Vref 1 . In other words, when the upper gate switch  100  is turned off, the output voltage of the DC-DC switching regulator  10  starts falling, and only when the output voltage Vout 1  is smaller than the reference voltage Vref 1 , the upper gate switch  100  will be turned on again. In other words, the DC-DC switching regulator  10  utilizes a pulse width modulation (PWM) method to turn the upper gate switch  100  on/off, so as to regulate the power delivered to the load Load 1  and thus to stabilize the output voltage Vout 1 . 
         [0005]    Meanwhile, since the operating period of the PWM signal is the summation of the turn-on and turn-off periods of the upper gate switch  100 , when resistance of the load Load 1  varies, the duty cycle of the PWM signal varies accordingly, to stabilize the output voltage. In other words, since the turn-on period of the constant time trigger circuit  104  is fixed, and only the turn-off period can be changed, it implies the operating period (or the operating frequency) of the PWM signal varies as the resistance of the output load varies. Also, operating characteristics of some components in the DC-DC switching regulator  10 , such as the inductor L and the capacitor C for enhancing energy efficiency and reducing ripples, are highly related to the operating frequency of the DC-DC switching regulator  10 . In other words, if the operating frequency varies within a wide range, these frequency-sensitive components cannot be optimized. In such a situation, some negative phenomena occur. For example, the ripples of the output voltage Vout 1  will become too large to meet requirements of some applications, because the operating frequency varies from the optimized frequency. 
         [0006]    Please refer to  FIG. 2 , which is a schematic diagram of a conventional DC-DC switching regulator  20  for fixing an operating frequency. The structure of the DC-DC switching regulator  20  is different from that of the DC-DC switching regulator  10  by some components added for fixing the operating frequency. The DC-DC switching regulator  20  includes an upper gate switch  200 , a lower gate switch  202 , a constant time trigger circuit  204 , a comparator  206 , an inductor L 2 , a capacitor C 2 , a reference voltage Vref 2   a  and an inverter INV 2 . Besides, the DC-DC switching regulator  20  further includes a frequency fixing circuit  250 . The frequency fixing circuit  250  includes an error amplifier  252 , a compensator  254 , a frequency-to-voltage converter  256  and a voltage reference Vref 2   b . Noticeably, the constant time trigger circuit  204  further includes a control input terminal  204   a  and thus is not the same with the constant time trigger circuit  104 . The control input terminal  204   a  is utilized for adjusting the turn-on period of the constant time trigger circuit  204 , to fix the operating frequency. In other words, the turn-on period of the constant time trigger circuit  204  is no longer constant. In the DC-DC switching regulator  20 , when the frequency tends to vary, the constant time trigger circuit  204  adjusts the length of the turn-on period according to the control signals received by the control input terminal  204   a . In addition, the constant time trigger circuit  204  integrates the frequency to voltage converter  256 , the error amplifier  252  and the compensator  254  into a closed control loop, such that the output voltage V 256  of the frequency to voltage converter  256  can follow and fix at the reference voltage Vref 2   b . As a result, the operating frequency (or the operating period) of the PWM signal outputted by the constant time trigger circuit  204  can be fixed. 
         [0007]    Since the operating frequency of the DC-DC switching regulator  20  can be fixed, the designer can optimize the designs of the frequency-sensitive components to reduce the ripples. However, in order to fix the operating frequency, the frequency to voltage converter  256 , the error amplifier  252  and the compensator  254  of the DC-DC switching regulator  20  need to be implemented by more complex circuitry, causing larger chip area and higher production cost. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an objective of the present invention to provide a switching regulator and constant frequency compensating circuit for fixing an operating frequency according to an output voltage and a phase signal. 
         [0009]    The present invention discloses a switching regulator for fixing an operating frequency. The switching regulator includes a power stage circuit, a reference voltage generator, a comparator and a constant frequency compensating circuit. The power stage circuit is utilized for receiving an input voltage, and providing an output voltage according to a control signal. The power stage circuit includes an upper gate switch, a lower gate switch, coupled to the upper gate switch, and an inductor, coupled to the upper gate switch and the lower gate switch. The reference voltage generator is utilized for generating a reference voltage. The comparator is utilized for outputting a comparison result according to the output voltage and the reference voltage. The constant frequency compensating circuit is utilized for outputting the control signal according to the comparison result, a phase signal and a compensating signal, comprising a charging capacitor having one terminal coupled to a ground, for charging. The phase signal is corresponding to a voltage across the lower gate switch of the power stage circuit. The compensating signal is corresponding to the output voltage of the power stage circuit. The constant frequency compensating circuit utilizes the phase signal to initialize a voltage of another terminal of the charging capacitor of the constant frequency compensating circuit. 
         [0010]    The present invention further discloses a constant frequency compensating circuit for a switching regulator. The constant frequency compensating circuit includes a charging capacitor, having one terminal coupled to a ground, for charging, a reference current source, coupled to another terminal of the charging capacitor, for providing a constant current related to an input voltage of the switching regulator to charge the charging capacitor, a comparator, including a positive terminal, for receiving a compensating signal, and a negative terminal, coupled to the another terminal of the charging capacitor, the comparator utilized for comparing magnitudes of signals of the positive terminal and the negative terminal, to output a control signal, and a switch, including a first terminal, coupled to the another terminal of the charging capacitor, and a second terminal, for receiving a phase signal, the switch utilized for conducting connection between the first and the second terminals according to the control signal. The phase signal is corresponding to a voltage across a lower gate switch of the switching regulator. The compensating signal is corresponding to an output voltage of the switching regulator. The constant frequency compensating circuit utilizes the phase signal to initialize a voltage of the another terminal of the charging capacitor of the constant frequency compensating circuit. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a conventional DC-DC switching regulator. 
           [0013]      FIG. 2  is a schematic diagram of a conventional DC-DC switching regulator for fixing an operating frequency. 
           [0014]      FIG. 3  is a schematic diagram of a DC-DC switching regulator according to an embodiment of the present invention. 
           [0015]      FIG. 4A  and  FIG. 4B  are schematic diagrams of the constant frequency compensating circuit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Please refer to  FIG. 3 , which is a schematic diagram of a DC-DC switching regulator  30  according to an embodiment of the present invention. The DC-DC switching regulator  30  is mainly utilized for providing an output voltage Vout 3  to a load Load 3 , and a current source Iout 3  denotes the current drained by the load Load 3 . The DC-DC switching regulator  30  includes a power stage circuit  32 , a comparator  306 , a reference voltage Vref 3 , an inverter INV 3 , and a constant frequency compensating circuit  404 . The power stage circuit  32  includes an upper gate switch  300 , a lower gate switch  302 , an inductor L 3 , and a capacitor C 3 . The most distinguished difference between the structure of the DC-DC switching regulator  30  and that of the DC-DC switching regulator  20  is: the DC-DC switching regulator  30  utilizes the constant frequency compensating circuit  404  to fix the operating frequency, and thus has a simpler structure. Besides, compared with the constant time trigger circuit  104  of the DC-DC switching regulator  10 , the constant frequency compensating circuit  404  further includes a control input terminal  404   a  and an input terminal  404   b , for receiving output voltage Vout 3  and a phase signal PSIG, respectively, to adjust turn-on period of the constant frequency compensating circuit, in order to fix the operating frequency. The operating principles of the constant frequency compensating circuit  404  are illustrated as follows. 
         [0017]    In the DC-DC switching regulator  30 , the output voltage Vout 3  can be derived by examining the voltage across the inductor L 3 , and is expressed by the following equation: 
         [0000]    
       
         
           
             
               Vout 
                
               
                   
               
                
               3 
             
             = 
             
               
                 
                   1 
                   Ts 
                 
                 · 
                 
                   
                     ∫ 
                     0 
                     Ton 
                   
                    
                   
                     
                       ( 
                       
                         
                           Vin 
                            
                           
                               
                           
                            
                           3 
                         
                         - 
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             Rds 
                           
                            
                           
                               
                           
                            
                           1 
                         
                       
                       ) 
                     
                     · 
                     
                        
                       t 
                     
                   
                 
               
               + 
               
                 
                   1 
                   Ts 
                 
                 · 
                 
                   
                     ∫ 
                     Ton 
                     Ts 
                   
                    
                   
                     
                       ( 
                       
                         0 
                         - 
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             Rds 
                           
                            
                           
                               
                           
                            
                           2 
                         
                       
                       ) 
                     
                     · 
                     
                        
                       t 
                     
                   
                 
               
             
           
         
       
     
         [0018]    The symbols Ts, Ton, IL 3 , Rds 1  and Rds 2  denote an operating period, a turn-on period of the upper gate switch  300 , an induction current, conduction resistances of the upper gate switch  300  and the lower gate switch  302 , respectively. The first term on the right side of the above equation denotes an input voltage Vin 3  charging the inductor L 3  when the upper gate switch  300  is turned on and the lower gate switch  302  is turned off. The second term on the right side denotes the inductor L 3  providing electrical energy to the load Load 3  when the upper gate switch  300  is turned off and the lower gate switch  302  is turned on. The above two terms determine the magnitude of the output voltage Vout 3 . Besides, if the input voltage Vin 3 , the output voltage Vout 3  and the inductor current IL 3  are relatively stable, the above equation can be rearranged as the following equation: 
         [0000]        Ts·V out3 =T on·( V in3 −IL 3 ·Rds 1)+[0−( Ts−T on)·( IL 3 ·Rds 2)]
 
         [0019]    By rearranging the above equation, the operating period Ts can be expressed as the following equation: 
         [0000]    
       
         
           
             
               
                 
                   Ts 
                   = 
                     
                    
                   
                     Ton 
                     · 
                     
                       
                         
                           Vin 
                            
                           
                               
                           
                            
                           3 
                         
                         + 
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             
                               ( 
                               
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 - 
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           Vout 
                            
                           
                               
                           
                            
                           3 
                         
                         + 
                         
                           ( 
                           
                             IL 
                              
                             
                                 
                             
                              
                             
                               3 
                               · 
                               Rds 
                             
                              
                             
                                 
                             
                              
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     Ton 
                     · 
                     
                       
                         Vin 
                          
                         
                             
                         
                          
                         3 
                       
                       
                         Vout 
                          
                         
                             
                         
                          
                         3 
                       
                     
                     · 
                     
                       
                         
                           1 
                           + 
                           
                             
                               IL 
                                
                               
                                   
                               
                                
                               
                                 3 
                                 · 
                                 
                                   ( 
                                   
                                     
                                       Rds 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     - 
                                     
                                       Rds 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               Vin 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                         
                         
                           1 
                           + 
                           
                             
                               ( 
                               
                                 IL 
                                  
                                 
                                     
                                 
                                  
                                 
                                   3 
                                   · 
                                   Rds 
                                 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               ) 
                             
                             
                               Vout 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                         
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
         [0020]    According to the principles of pulse width modulation (PWM), the turn-on period Ton is determined by the output voltage Vout 3  and the input voltage Vin 3 . In other words, the turn-on period Ton can be expressed as the following equation: 
         [0000]    
       
         
           
             Ton 
             = 
             
               K 
                
               
                   
               
                
               
                 1 
                 · 
                 
                   
                     Vout 
                      
                     
                         
                     
                      
                     3 
                   
                   
                     Vin 
                      
                     
                         
                     
                      
                     3 
                   
                 
               
             
           
         
       
     
         [0000]    where a parameter K1 denotes a constant parameter and is determined by related circuit parameters. Then, the equation for the operating period Ts can be rearranged as the following equation: 
         [0000]    
       
         
           
             Ts 
             = 
             
               K 
                
               
                   
               
                
               
                 1 
                 · 
                 
                   
                     
                       1 
                       + 
                       
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             
                               ( 
                               
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 - 
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         
                           Vin 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                     
                       1 
                       + 
                       
                         
                           ( 
                           
                             IL 
                              
                             
                                 
                             
                              
                             
                               3 
                               · 
                               Rds 
                             
                              
                             
                                 
                             
                              
                             2 
                           
                           ) 
                         
                         
                           Vout 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                   
                   . 
                 
               
             
           
         
       
     
         [0021]    Typically, the parameter K1 equals 2.5 micro-second when a switching regulator operates at 400 KHz. 
         [0022]    By closely examining the above equation, the stability and the operating frequency of the DC-DC switching regulator  30  varies as any of the inductor current IL 3 , the output voltage Vout 3 , the input voltage Vin 3 , the conduction resistance of the upper gate switch Rds 1  or the conduction resistance of the lower gate switch Rds 2  varies. According to detailed numerical simulation, the 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    term in the denominator of the above equation affects the stability of the operating frequency most. Therefore, according to the above equation for the operating period Ts, if the affection of the term in the denominator is removed, the operating frequency of the DC-DC switching regulator can become more stable. 
         [0023]    Therefore, in order to stabilize the operating frequency of the DC-DC switching regulator  30 , the present invention modifies the above equation of the turn-on period Ton as the following equation: 
         [0000]    
       
         
           
             Ton 
             = 
             
               K 
                
               
                   
               
                
               
                 1 
                 · 
                 
                   
                     
                       
                         Vout 
                          
                         
                             
                         
                          
                         3 
                       
                       + 
                       
                         IL 
                          
                         
                             
                         
                          
                         
                           3 
                           · 
                           Rds 
                         
                          
                         
                             
                         
                          
                         2 
                       
                     
                     
                       Vin 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   . 
                 
               
             
           
         
       
     
         [0024]    As can be seen from the above equation, a new term Vout 3 +IL 3 ·Rds 2  in the nominator is utilized for replacing the original term Vout 3  in the nominator to determine the turn-on period Ton, which is equivalent to increasing the turn-on period Ton by 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    times. As a result, the operating period Ts can be modified as the following equation according to the above equation: 
         [0000]    
       
         
           
             Ts 
             = 
             
               K 
                
               
                   
               
                
               
                 1 
                 · 
                 
                   
                     [ 
                     
                       1 
                       + 
                       
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             
                               ( 
                               
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 - 
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         
                           Vin 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                     ] 
                   
                   . 
                 
               
             
           
         
       
     
         [0025]    As can be seen from the above equation, the term 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    in the denominator has been removed, thus significantly enhancing the stability of the operating frequency of the DC-DC switching regulator  30 . 
         [0026]    In order to realize the above operating principles, the present invention includes the constant frequency compensating circuit  404  for regulating the turn-on period Ton according to the phase signal PSIG and the output voltage Vout 3 . In other words, the constant frequency compensating circuit  404  can increase the turn-on period Ton by 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    times according to variations of the phase signal PSIG and the output voltage Vout 3 , so as to stabilize the output voltage and fix the operating frequency. Noticeably, the phase signal PSIG is measured at one terminal of the inductor L 3  connected with the lower gate switch  302 , and equals a voltage IL 3 ·Rds 2 , which is also the voltage across the lower gate switch  302  when the lower gate switch  302  is turned on. 
         [0027]    Please refer to  FIG. 4A , which is a schematic diagram of the constant frequency compensating circuit  404  according to an embodiment of the present invention. The constant frequency compensating circuit  404  includes a reference current source I_ref, a charging capacitor C_ref, a comparator COMP 1 , and a switch SW 1 . The reference current source I_ref is utilized for providing a constant current related to the input voltage Vin 3  to charge the charging capacitor C_ref. When the upper gate switch  300  is turned off, and the lower gate switch  302  is turned on, the constant frequency compensating circuit  404  samples a voltage level of the phase signal PSIG at the moment that the switch SW 1  is turned on, and takes this voltage level as a start voltage of the charging capacitor C_ref. Since the phase signal PSIG is a negative value when the lower gate switch  302  is turned on, a voltage of a positive terminal of the charging capacitor C_ref is a negative value after the phase signal PSIG is sampled, and the charging capacitor C_ref is charged from the negative voltage value. Meanwhile, the greater an absolute value of the negative voltage value is, the longer the charging capacitor C_ref is charged to a specific positive voltage, i.e. the output voltage Vout 3 . In other words, when the lower gate switch  302  is turned on, the greater the absolute value of the negative value corresponding to the phase signal PSIG, the greater the load current Iout 3  is. Therefore, the constant frequency compensating circuit  404  needs to charge the charging capacitor C_ref longer, such that the voltage of the positive terminal of the charging capacitor C_ref can become greater than the output voltage Vout 3 . Based on the operating principles, the constant frequency compensating circuit  404  can increase the turn-on period Ton according to the load current Iout 3 . Besides, the constant frequency compensating circuit  404  utilizes the constant current provided by the reference current source I_ref to charge the charging capacitor C_ref, and utilizes the comparator COMP 1  to compare the voltage of the positive terminal of the charging capacitor C_ref with the output voltage Vout 3 . When the absolute value of the phase signal PSIG is greater than the output voltage Vout 3 , the output signal of the comparator COMP 1  changes a state, in order to turn the upper gate switch  300  and the lower gate switch  302  on/off, and adjust the turn-on period Ton. In addition, the switch SW 1  is open (cut-off) when the lower gate switch  302  is turned off, and the switch SW 1  is conducted when the lower gate switch  302  is turned on. As a result, the charging capacitor C_ref can sample the voltage level of the phase signal PSIG at the moment that the lower gate switch  302  is turned on, and starts charging. The charging capacitor C_ref can be utilized for maintaining the same voltage level with the phase signal PSIG when the lower gate switch  302  is turned on. 
         [0028]    In short, when the system utilizes the reference current source I_ref to provide a constant current to charge the charging capacitor C_ref, the voltage across the charging capacitor C_ref increases gradually. When the voltage across the charging capacitor C_ref reaches a predefined voltage, the system utilizes the comparator COMP 1  to trigger a control signal CS. The start voltage of the charging capacitor C_ref of the constant frequency compensating circuit  404  is a variable, i.e. the voltage level of the phase signal PSIG. For example, when the start voltage of the charging capacitor C_ref is smaller, i.e. a greater load current Iout 3 , it takes longer for the voltage of the positive terminal of the charging capacitor C_ref to reach the predefined voltage, i.e. the output voltage Vout 3 , causing longer charging time as well as the turn-on period Ton. On the other hand, if the start voltage of the charging capacitor C_ref value is higher, i.e. a smaller load current Iout 3 , the charging time as well as the turn-on period Ton is shorter. In the prior art, since the turn-on period Ton of the constant time trigger circuit  104  is a constant value, even if the resistance of the load varies, the turn-on period Ton is still constant. Thus, the operating period or the operating frequency varies as the load varies, deteriorating the system performance. In comparison, the constant frequency compensating circuit  404  of the present invention adjusts the turn-on period Ton according to the load current Iout 3  based on the above principles, so as to fix the operating period or the operating frequency substantially to a constant. 
         [0029]    Noticeably, any voltage signal proportional to Vout 3 +IL 3 ·Rds 2  can be utilized for removing the 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    term in the denominator and adjusting the length of the turn-on period Ton. Those with ordinary skill in the art can make modifications and alterations according to the spirit of the present invention. For example, the voltage K0·(Vout 3 +IL 3 ·Rds 2 ) can be utilized for removing the 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    term in the denominator. In order to realize the extended design, please refer to  FIG. 4B , which is a schematic diagram of the constant frequency compensating circuit  404  according to an alteration of the present invention. In  FIG. 4B , the constant frequency compensating circuit  404  includes a reference current source I_ref related to an input voltage Vin 3 , a charging capacitor C_ref, a comparator COMP 1 , a switch SW 1 , and voltage dividing resistances R 3 -R 6 . In this structure, the designer can generate voltages K0·Vout 3  and K0·PSIG by adjusting resistances of the voltage dividing resistances R 3 -R 6 , so as to adjust the turn-on period Ton. Noticeably, the voltage dividing resistances R 3 -R 6  of  FIG. 4B  are utilized for dividing the voltage, and need to meet the constraint condition of R 3 /R 4 =R 5 /R 6 =K0. Under such a situation, the operating period Ts can be expressed as the following equation: 
         [0000]    
       
         
           
             Ts 
             = 
             
               K 
                
               
                   
               
                
               
                 0 
                 · 
                 K 
               
                
               
                   
               
                
               
                 1 
                 · 
                 
                   
                     [ 
                     
                       1 
                       + 
                       
                         
                           IL 
                            
                           
                               
                           
                            
                           
                             3 
                             · 
                             
                               ( 
                               
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 - 
                                 
                                   Rds 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         
                           Vin 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                     ] 
                   
                   . 
                 
               
             
           
         
       
     
         [0030]      FIG. 4B  is an extended design of  FIG. 4A , except for the added voltage dividing resistances R 3 -R 6  for generating voltages K0·Vout 3  and K0·PSIG, other structures and operating principles of the circuit are the same with those in  FIG. 4A , and are not narrated hereinafter. Noticeably, by observing the above equation, the equation includes a constant parameter K0, and a parameter K1 related to internal circuit. The designer can adjust the above parameters (K0 or K1) according to requirements, in order to select a proper operating frequency (or period) for the regulator circuit. 
         [0031]    In addition, since the upper gate switch  300  and the lower gate switch  302  are often realized by power transistors of the same type in many applications, resistances of the conduction resistor Rds 1  of the upper gate switch  300  and the conduction resistor Rds 2  of the lower gate switch  302  are close. As a result, the 
         [0000]    
       
         
           
             [ 
             
               1 
               + 
               
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     
                       ( 
                       
                         
                           Rds 
                            
                           
                               
                           
                            
                           2 
                         
                         - 
                         
                           Rds 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       ) 
                     
                   
                 
                 
                   Vin 
                    
                   
                       
                   
                    
                   3 
                 
               
             
             ] 
           
         
       
     
         [0000]    term in the nominator of the above Ts equation approximates to 1, such that the operating period Ts approximates to a constant value K0·K1. Besides, even if the upper gate switch  300  and the lower gate switch  302  are not transistors of the same type, the present invention still can increase the turn-on period Ton by 
         [0000]    
       
         
           
             1 
             + 
             
               
                 ( 
                 
                   IL 
                    
                   
                       
                   
                    
                   
                     3 
                     · 
                     Rds 
                   
                    
                   
                       
                   
                    
                   2 
                 
                 ) 
               
               
                 Vout 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0000]    times according to variations of the output voltage Vout 3  and the load current Iout 3 , so as to stabilize the operating frequency of the system. In detail, the present invention dynamically adjusts the start voltage of the charging capacitor C_ref of the negative terminal of the comparator COMP 1  (related to the phase signal PSIG) by varying the load current Iout 3 , so as to equivalently dynamically adjust the voltage of the positive terminal of the comparator COMP 1  (related to the output voltage Vout 3 ). Therefore, the present invention can perform frequency compensation by varying the load current Iout 3 . 
         [0032]    Noticeably, except for utilizing the phase signal PSIG to obtain a voltage signal proportional to the output current, the present invention can utilize other methods to obtain signals proportional to the output current as well. For example, a sensing resistor can be added in the output current path, such that a voltage proportional to the current can be obtained by measuring a voltage across the sensing resistor. In other words, the present invention can utilize the frequency compensator  404  to receive any signal proportional to the output current for replacing the phase signal PSIG, which fixes the operating frequency as well. 
         [0033]    In addition, the architecture of the present invention is simpler than the conventional DC-DC switching regulator. As can be seen from the above, the DC-DC switching regulator  20  needs to sample and hold the PWM signal generated by the constant time trigger circuit  204 , and if the operating period exceeds the predefined value, the turn-on period Ton is adjusted to fix the operating frequency. In order to implement the architecture of the prior art, the circuit at least needs to include an operational transconductance amplifier (OTA) and a large on-chip capacitor to realize error integration function. In comparison, the present invention only needs a compact circuit to realize the constant frequency compensating circuit, for substantially fixing the operating frequency. The present invention only needs about 20% of the chip area of the prior art according to experimental results. Besides, if the load current Iout 3  is within 20 A, Vout 3 =1.5V, Rds 1 =9 mΩ, and Rds 2 =4 mΩ, the operating frequency can be fixed within less than 5% variation according to experiment results. 
         [0034]    The present invention regulates the operating frequency of the DC-DC switching regulator to a substantially fixed value, such that the design of the frequency-sensitive components of the DC-DC switching regulator becomes much simpler, and the output ripples are effectively reduced. To sum up, the present invention discloses a method for adjusting the turn-on period of the constant time trigger circuit according to the output voltage and the voltage across the lower gate switch (the phase signal). The present invention further discloses a compact circuit to alter charging time, such that the turn-on period can be effectively adjusted, so as to fix the operating frequency within a range. According to the experimental results, the present invention has excellent performance and effectively saves cost. 
         [0035]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.