Patent Publication Number: US-2006012357-A1

Title: DC/DC converter

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to DC/DC converter, and more particularly, relates to a DC/DC converter capable of reduced power consumption without requiring additional reference voltages.  
      2. Description of the Related Art  
       FIG. 1  shows a conventional DC/DC converter  10 . In the converter  10 , the oscillator  11  enables the DC/DC conversion circuit  12  to provide a negative voltage to charge the storage capacitor C according to an input signal Sin. The DC/DC conversion circuit  12  then charges the storage capacitor C to a predetermined voltage and maintains the predetermined voltage. In this case, however, the DC/DC conversion circuit  12  is turned on and continuously consumes power.  FIG. 2  shows another conventional DC/DC converter  20 . In the converter  20 , the comparator  24  produces and outputs a driving signal to the oscillator  21  according to the voltage at node D, thereby controlling whether load is charged by the DC/DC conversion circuit  23 . The converter  20 , however, requires an additional reference voltage Vref.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a DC/DC converter, which is capable of reduced power consumption without requiring additional reference voltages.  
      In one aspect of the present invention, the circuit includes a DC/DC conversion circuit enabled to output an output voltage and a switch responsive to the output voltage to enable or disenable the DC/DC conversion circuit. In one embodiment of the present invention, a DC/DC conversion circuit provides an output voltage to a storage capacitor upon receiving an enable signal. First and second resistors are connected in series to produce a first voltage according to the output voltage. The switch includes a Schmitt trigger coupled to the first voltage to output a first control signal through an inverter when the first voltage is smaller than a second voltage and outputs a second control signal through the inverter when the first voltage is higher than a third voltage, wherein the second voltage is smaller than the third voltage. The switch also includes an oscillator, which is turned off upon receiving the first control signal such that the DC/DC conversion circuit stops providing the output voltage, and is turned on and outputs the enable signal upon receiving the second control signal such that the DC/DC conversion circuit provides the output voltage to the storage capacitor.  
      In another embodiment, the circuit includes an amplifier coupled between the detection circuit and the control circuit to amplify the first voltage, and the switching circuit can be an oscillator.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention can be more fully understood by the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:  
       FIG. 1  shows a conventional DC/DC converter;  
       FIG. 2  shows another conventional DC/DC converter;  
       FIG. 3  shows a DC/DC converter according to one embodiment of the present invention;  
       FIG. 4  shows an output diagram of the DC/DC converter shown in  FIG. 3 ;  
       FIG. 5  shows another embodiment of the DC/DC converter according to the present invention; and  
       FIG. 6  is a schematic diagram of an electronic device having a display system with a DC/DC converter of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The following description is of the best-contemplated mode of carrying out the invention. This 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  shows a DC/DC converter  30  according to one embodiment of the present invention. As shown in  FIG. 3 , the converter  30  has a DC/DC conversion circuit  34 , a detection circuit  36 , and a switch  37  including a control circuit  38  and an oscillator  32 .  
      The DC/DC conversion circuit  34  provides an output voltage Vout to a storage capacitor C upon receiving an enable signal EN, wherein resistor R 6  can be a load. The detection circuit  36  produces a first voltage V 1  according to the output voltage of the storage capacitor C. In this embodiment, the detection circuit  36  is voltage-divided circuit composed of the first and second resistors R 4  and R 5 . The first resistor R 4  has a first end coupled to the DC/DC conversion circuit  34 , and a second end coupled to the second resistor R 5 , and the first voltage V 1  is a divided voltage of the output voltage Vout.  
      The switch is coupled to the detection circuit and the DC/DC conversion circuit. The switch is responsive to the first voltage (i.e., the output voltage) to enable or disenable the DC/DC conversion circuit. In this embodiment, the control circuit  38  is coupled to the detection circuit  36  to receive the first voltage V 1 , and has a trigger, such as a Schmitt trigger ST, and an inverter INV 1 . The input of the Schmitt trigger ST is coupled to the first voltage V 1  output from the detection circuit  36 , and the input of the inverter INV 1  is coupled to the output of the Schmitt trigger ST. Typically, a Schmitt trigger has a first trigger level and a second trigger level, for example the first trigger level is higher then the second trigger level. The Schmitt trigger outputs an output signal of a first logic level, such as LOW, when the input signal thereof is higher than the first trigger level. The Schmitt trigger continues to output the first logic level signal when the input signal decreases to the first trigger level. The Schmitt trigger outputs the second logic level signal until the input signal is reduced to smaller than the second trigger level, such as HIGH. Therefore, in the present invention, the control circuit  38  outputs a first control signal S 1  to the oscillator  32  when the first voltage V 1  is smaller than a second voltage V 2 , and outputs a second control signal S 2  to the oscillator  32  when the first voltage is higher than a third voltage V 3 .  
      The oscillator  32  is coupled between the output of the inverter INV 1  and the DC/DC conversion circuit  34 . The oscillator  32  is turned off upon receiving the first control signal S 1 . The oscillator  32  does not output the enable signal. EN to the DC/DC conversion circuit  34 , such that the DC/DC conversion circuit  34  stops providing output voltage Vout to the storage capacitor C and load R 6 . Additionally, the oscillator  32  is turned on upon receiving the second control signal S 2 . The oscillator  32  then outputs the enable signal EN to the DC/DC conversion circuit  34 , such that the DC/DC conversion circuit  34  provides the output voltage Vout to the storage capacitor C 1  and the load R 6 .  
       FIG. 4  is an example of an output wave diagram of the DC/DC converter  30 . In this embodiment, the output voltage Vout provided by the DC/DC converter  30  is a negative voltage, the second voltage V 2  can be 3.8V, and the third voltage V 3  can be 4.4V.  
      At time t 0 , the output voltage is 0V, and the detection circuit  36  produces a first voltage V 1  to output to Schmitt trigger ST. For example, in this case, the first voltage V 1  is 5.6V when Vdd is 8V. At this time, the Schmitt trigger ST is triggered to output a low logic signal to the inverter INV 1  as the first voltage V 1  of 5.6V is higher than the third voltage V 3  of 4.4V. The inverter INV 1  then converts the low logic signal output from the Schmitt trigger ST to a high logic signal as the second control signal S 2 , and outputs to the oscillator  32 . Consequently, the oscillator  32  is turned on and outputs an enable signal EN upon receiving the second control signal S 2 , such that the DC/DC conversion circuit  34  provides the output voltage Vout to the storage capacitor C and the load R 6 . In this case, the voltage of the storage capacitor C and the load R 6  is increased to a negative value from 0V because the output voltage Vout is negative.  
      At time t 1 , the detection circuit  36  produces a first voltage V 1  smaller than the second voltage (3.8V) to output to the Schmitt trigger ST when the output voltage Vout exceeds −6V. The Schmitt trigger ST is then triggered and outputs a high logic signal to inverter INV 1 . The inverter INV 1  then converts the signal of high logic output from the Schmitt trigger ST to a low logic signal as the first control signal S 1 , and outputs to the oscillator  32 . Consequently, the oscillator  32  is turned off upon receiving the first control signal S 1 , such that the DC/DC conversion circuit  34  stops providing the output voltage Vout to the storage capacitor C and the load R 6 . At this time, the voltage of the storage capacitor C and the load R 6  starts to discharge.  
      At time t 2 , the detection circuit  36  produces a first voltage V 1  higher than the third voltage (4.4V) to output to the Schmitt trigger ST when the output voltage Vout is higher than −4V. The Schmitt trigger St is then triggered again and outputs a low logic signal to inverter INV 1 . The inverter INV 1  then converts the low logic signal output from the Schmitt trigger ST to a high logic signal as the second control signal S 2 , and outputs to the oscillator  32 . Consequently, the oscillator  32  is turned on upon receiving the second control signal S 2 , such that the DC/DC conversion circuit  34  provides the output voltage Vout to the storage capacitor C 1  and the load R 6  again. At this time, the voltage of the storage capacitor C and the load R 6  starts to discharge from −4V to −6V.  
      At time t 4 , the detection circuit  36  produces a first voltage V 1  smaller than the second voltage (3.8V) to output to the Schmitt trigger ST again when the output voltage Vout exceeds −6V. The Schmitt trigger St is then triggered and outputs a high logic signal to the inverter INV 1 . The inverter INV 1  then converts the signal of high logic output from the Schmitt trigger ST to a low logic signal as the first control signal S 1 , and outputs to the oscillator  32 . Consequently, the oscillator  32  is turned off upon receiving the first control signal S 1 , such that the DC/DC conversion circuit  34  stops providing the output voltage Vout to the storage capacitor C and the load R 6 . At this time, the voltage of the storage capacitor C and the load R 6  starts to discharge again. Therefore, the present invention can keep the output voltage within in a predetermined range, for example −4V-−6V.  
      In the DC/DC converter  30  of the present invention, the DC/DC conversion circuit  34  and the oscillator  32  can be controlled by the output voltage Vout from the storage capacitor C, and the load R 6  and is not continuously turned on. Therefore, the present invention reduces energy consumption caused by the DC/DC conversion circuit  34  and the oscillator  32 .  
      Additionally, as shown in  FIG. 5 , the DC/DC converter  30  further includes an amplifier coupled between the first and second resistors R 4  and R 5  and the Schmitt trigger ST to amplify the first voltage V 1 , such that the DC/DC converter does not malfunction because the voltage difference is too small.  
       FIG. 6  schematically shows an electronic device  100  deploying a power consuming device such as a display system  110 , and having a DC/DC converter  30  described above. The electronic device  100  may be a portable device such as a PDA, notebook computer, tablet computer, cellular phone, or a display monitor device, etc. Generally, then electronic device  100  includes a housing  120 , the display system  110  having the DC/DC converter  30  and a display element  130 , and a user interface  140 , etc. Further, the DC/DC converter  30  in accordance with the present invention may be deployed to provide an output voltage to power the display element  130  and user interface  140 , etc.  
      While the inventive DC/DC converter is described above in connection with an LCD display system, the present invention may be deployed in other types of display systems, such as systems deploying a plasma display element, or a cathode ray tube display element.  
      While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To 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.