Abstract:
Constant on-time control circuit includes a comparing circuit including a comparator including a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage from the output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the output end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal to control a switch set of the DC/DC converter according to the comparing signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bulk DC/DC converter, and more particularly, to a bulk DC/DC converter in constant on-time mode. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating a conventional bulk DC/DC converter  100 . The bulk DC/DC converter  100  converts an input voltage source V IN  to be an output voltage source V OUT , wherein the voltage V OUT  is lower than the voltage V IN . As shown in  FIG. 1 , the DC/DC converter  100  comprises a control circuit  110 , a switch set  120 , an inductor L, an output capacitor C OUT , and a voltage-dividing set  130 . The switch set  120  comprises two switches Q 1  and Q 2 . The voltage-dividing set  130  comprises two voltage-dividing resistors R B1  and R B2 . The control circuit  110  comprises a comparator CMP 1 , a pulse generator  111 , and a drive circuit  112 . The operation principles of the bulk DC/DC converter  100  are described as follows. 
     The control circuit  110  controls the operation of the DC/DC converter  100  by constant on-time manner. That is, when the control circuit  110  detects the output voltage V OUT  is lower than a predetermined value, the switch Q 1  is turned on for a constant period of time (constant on-time) by the control circuit  110  (while the switch Q 2  is turned off) for allowing the input voltage source V IN  conducting to the inductor L through the switch set  120 . 
     During the operation of the DC/DC converter  100 , the inductor L carries current I L , and the current I L  flows into the equivalent serial resistor R E  of the output capacitor C OUT  so that the resistor R E  carries voltage V L  reflecting the current I L . As shown in  FIG. 1 , the waveform of the voltage V L  is saw-toothed because the switch Q 1  is periodically turned on/off. The comparator CMP 1  receives the feedback voltage V FB  divided from the voltage V L  and the output voltage V OUT  by the resistors R B1  and R B2 , and compares with a reference voltage V REF1 , so as to determine when to turn on the switch Q 1 . More specifically, when the voltage (feedback voltage V FB ) on the negative input end of the comparator CMP 1  is lower than the voltage (reference voltage V REF1 ) on the positive input end of the comparator CMP 1 , which means the output voltage V OUT  is too low, and the switch Q 1  is needed to be turned on for allowing the input voltage source V IN  to charge the inductor L and the output capacitor C OUT , the comparator CMP 1  controls the pulse generator  111  to generate a pulse signal P ON . When the pulse generator  111  is triggered by the comparator CMP 1 , the pulse generator  111  generates a pulse signal P ON  with a predetermined duration T P  and predetermined logic. The drive circuit  112  controls the switch set  120  according to the pulse signal P ON . More particularly, when the drive circuit  112  receives the pulse signal P ON , the switch Q 1  is driven to turn on for the predetermined duration T P . In addition, except in the dead time both of the switches Q 1  and Q 2  are turned off, when the switch Q 1  is turned on, the switch Q 2  is turned off; when the switch Q 1  is turned off, the switch Q 2  is turned on. In this way, the control circuit  110  controls the DC/DC converter  100  to operate regularly in constant on-time mode. 
     However, not all kinds of capacitors definitely have equivalent serial resistors, and because of the improvement to the manufacture of capacitors, the equivalent serial resistances of the capacitors become smaller, or even do not exist. For example, the multi-layer ceramic capacitor (MLCC) is very similar to an ideal capacitor and therefore the equivalent serial resistor does not exist on the MLCC. Consequently, when the MLCC is utilized as the output capacitor C OUT , the resistor R E  does not exist, and thus the information of the current I L  cannot be informed to the control circuit  110 , causing the control circuit  110  unable to control the DC/DC converter  100  according to the voltage V L  effectively. The control circuit  110  is still able to operate by the feedback of the output voltage V OUT . However, the phase of the output voltage V OUT  is far behind the phase of the voltage V L  because of the output capacitor C OUT , which makes the control circuit  110  unable to react to the variation of the output voltage V OUT  in time. For this reason, the DC/DC converter  100  is not able to operate stably in constant on-time mode while utilizing the MLCC as the output capacitor, causing inconvenience. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control circuit in constant on-time mode for controlling a DC/DC converter, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage divided from an output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal for controlling a switch set of the DC/DC converter. 
     The present invention further provides a bulk DC/DC converter in constant on-time mode, comprising a switch set coupled between an input voltage source and ground; an inductor coupled to the switch set for receiving the input voltage source through the switch set; an output end coupled to the inductor for outputting an output voltage; an output capacitor coupled between the capacitor and the ground; a voltage-dividing set coupled between the output end and the ground for generating a feedback voltage according to the output voltage; and a control circuit, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end coupled to the voltage-dividing set for receiving the feedback voltage; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal so as to transmit the input voltage source to the inductor through the switch set. 
     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 
         FIG. 1  is a diagram illustrating a conventional bulk DC/DC converter. 
         FIG. 2  is a diagram illustrating a bulk DC/DC converter of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating a bulk DC/DC converter  200  of the present invention. The bulk DC/DC converter  200  converts an input voltage source V IN  to be an output voltage source V OUT , wherein the voltage V OUT  is lower than the voltage V IN . As shown in  FIG. 2 , the DC/DC converter  200  comprises a control circuit  210 , a switch set  220 , an inductor L, an output capacitor C OUT , and a voltage-dividing set  230 . The switch set  220  comprises two switches Q 1  and Q 2 . The voltage-dividing set  230  comprises two voltage-dividing resistors R B1  and R B2 . The control circuit  210  comprises a pulse generator  211 , a drive circuit  212 , a comparing circuit  213 , and an amplifying circuit  214 . The comparing circuit  213  comprises a comparator CMP 2 , and a voltage adjusting circuit  215 . The voltage adjusting circuit  215  comprises a voltage reference source V REF2 , a constant current source IS, a switch SW, a resistor R X , and a capacitor C X . The amplifying circuit  214  comprises an amplifier OP, a voltage reference source V REF3 , a resistor R 1 , and a capacitor C 1 . The switch SW comprises a first end  1 , a second end  2 , and a control end C. It is noticeable that the output capacitor C OUT  utilized in the DC/DC converter  200  is assumed to not have the equivalent serial resistor. The operation principles of the bulk DC/DC converter  200  are described as follows. 
     The control circuit  210  controls the operation of the DC/DC converter  200  by constant on-time manner. That is, when the control circuit  210  detects the output voltage V OUT  is lower than a predetermined value, the switch Q 1  is turned on for a constant period of time (constant on-time) by the control circuit  210  (while the switch Q 2  is turned off) for allowing the input voltage source V IN  conducting to the inductor L through the switch set  220 . 
     Because the output capacitor C OUT  used in the DC/DC converter  200  is assumed to have no equivalent serial resistor, the information about the current carried by the inductor L is not able to feed back to the control circuit  210 . Thus, the control circuit  210  is designed to control the DC/DC converter  200  to operate regularly without the equivalent serial resistor. 
     In the amplifying circuit  214 , the amplifier OP receives the feedback voltage V FB  through the voltage-dividing set  230  dividing the output voltage V OUT , and the received voltage V FB  is amplified through the resistor R 1  and the capacitor C 1  to be the amplified feedback voltage V FBO . In addition, the voltage reference source V REF3  provides a reference voltage V REF3 . 
     In the comparing circuit  213 , the comparator CMP 2  compares the control voltage (V C ) on its positive input end and the amplified feedback voltage V FBO  on its negative input end, and accordingly outputs a comparing signal S C . The voltage reference source V REF2  provides a reference voltage V REF2 . Thus, the control voltage V C  equals to the reference voltage V REF2  before being adjusted by the voltage adjusting circuit  215 , and is smaller than the reference voltage V REF2  after being adjusted by the voltage adjusting circuit  215 . More specifically, when the amplified feedback voltage V FBO  is lower than the control voltage V C , which means the output voltage V OUT  is too low, and the switch Q 1  is needed to be turned on for allowing the input voltage source V IN  to charge the inductor L and the output capacitor C OUT , the comparator CMP 2  generates the comparing signal S C  to trigger the pulse generator  211  to generate the pulse signal P ON . When the pulse generator  211  is triggered by the comparator CMP 1 , the pulse generator  211  generates a pulse signal P ON  with a predetermined duration T P  and predetermined logic. The drive circuit  212  controls the switch set  220  according to the pulse signal P ON . More particularly, when the drive circuit  212  receives the pulse signal P ON , the switch Q 1  is driven to turn on for the predetermined duration T P  and the input voltage source V IN  conducts through the turned-on switch Q 1  to charge the inductor L and the output capacitor C OUT  for raising the output voltage V OUT . Meanwhile, the pulse signal P ON  controls the switch SW to turn on for conducting the constant current source IS for lowering the control voltage V C . After the predetermined duration T P  of the pulse signal P ON , the switch SW is turned off so that the constant current source IS cannot discharge the resistor R X  and the capacitor C X . Instead, the voltage reference source V REF2  charges the resistor R X  and the capacitor C X  for raising the control voltage V C . By repeatedly executing the above actions, the control voltage V C  can be of the saw-toothed waveform. 
     In other words, the inductor L starts to be charged at the beginning of the pulse signal P ON  and the current I L  increases, and the inductor L starts to be discharged at the end of the pulse signal P ON  and the current I L  decreases. Simply speaking, in the present invention, the pulse signal P ON  is used to simulate the variation of the current I L  on the inductor L and is fed back to the comparing circuit  213  to correspondingly adjust the control voltage V C , i.e. the control voltage V C  varies in response to the variation of the current I L . In this way, the control circuit  210  is capable of stably controlling the DC/DC converter  200  without the equivalent serial resistor R E . 
     To sum up, the control circuit of the present invention, by utilizing the pulse signal to simulate the change of the current on the inductor, is still capable of stably controlling the bulk DC/DC converter when there is no equivalent serial resistor, providing great convenience. 
     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.