Patent Publication Number: US-2007103130-A1

Title: DC-DC converter and organic light emitting display using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2005-0106170, filed on Nov. 7, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a DC-DC converter and an organic light-emitting display using the same, and more specifically to a DC-DC converter configured to output a voltage according to a comparison result obtained by comparing an input voltage with a reference voltage; and an organic light-emitting display using the same.  
      2. Description of the Related Technology  
       FIG. 1  is a circuit diagram showing a previous comparator. Referring to  FIG. 1 , the comparator includes an input unit and first, second and third inverters.  
      The input unit has a first switch SW 1  for switching transmission of the input voltage Vin; and a second switch SW 2  for switching transmission of a reference voltage Vref.  
      The first inverter has a first transistor M 1  as the P MOS transistor and a second transistor M 2  as the N MOS transistor. And the first power supply Vdd is connected to a source of the first transistor M 1  to supply a high level of voltage, and the second transistor M 2  has a source connected to a ground GND to supply a low level of voltage. And, the first capacitor C 1  and the third switch SW 3  are connected to the first node N 1 .  
      The second inverter has a third transistor M 3  as the P MOS transistor and a fourth transistor M 4  as the N MOS transistor. And the first power supply Vdd is connected to a source of the third transistor M 1  to supply a high level of voltage, and a ground is connected to a source of the fourth transistor M 4  to supply a low level of voltage. And, the second inverter is connected with the first inverter through the second capacitor C 2 , and terminals of the second capacitor C 2 , the fourth switch M 4 , and the third and fourth transistors M 3 ,M 4  are connected to the second node N 2 .  
      The third inverter has a fifth transistor M 5  as the P MOS transistor and a sixth transistor M 6  as the N MOS transistor. And the first power supply Vdd is connected to a source of the fifth transistor M 5  to supply a high level of voltage, and a ground is connected to a source of the sixth transistor M 6  to supply a low level of voltage.  
       FIG. 2  is a timing diagram showing input/output waveforms of the circuit shown in  FIG. 1 . Referring to  FIG. 2 , an input voltage Vin input at an input terminal of a comparator unit changes in a voltage level and is compared with the reference voltage Vref. The first to fifth switches SW 1  to SW 5  conduct a switching operation according to the first control signal P 1  and the second control signal P 2 , where the first, third and fourth switches SW 1 , SW 3 , SW 4  are operated according to the first control signal P 1  and the second and fifth switches SW 2 , SW 5  are operated by the second control signal P 2 .  
      Firstly, if the first, third and fourth switches SW 1 , SW 3 , SW 4  are turned on by the first control signal P 1  and the second and fifth switches SW 2 ,SW 5  are turned off by the second control signal P 2 , then the input voltage Vin is transmitted to the first capacitor C 1 , and the voltage corresponding to a threshold voltage difference between the first inverter and the second inverter is stored in the second capacitor C 2 .  
      And, if the first, third and fourth switches SW 1 , SW 3 , SW 4  are subsequently turned off by the first control signal P 1  and the second and fifth switches SW 2 , SW 5  are turned on by the second control signal P 2 , then the reference voltage Vref is transmitted to the first capacitor C 1  to compare the input voltage Vin with the reference voltage Vref.  
      At this time, if the input voltage Vin is higher than the reference voltage Vref, then an output port of the third inverter outputs a low level of voltage, and if the input voltage Vin is lower than the reference voltage Vref, then an output port of the third inverter outputs a high level of voltage.  
      In the comparator described above, the output voltage is determined according to a difference between the reference voltage Vref and the input voltage Vin in the first capacitor C 1 , and therefore the comparator has a problem in that it takes more time to change the output voltage into the high level or the low level if there is not a high difference between the reference voltage Vref and the input voltage Vin than if there is a high difference between the reference voltage Vref and the input voltage Vin.  
      In order to address the problem, the comparator as described above can have a large capacitance, and therefore it has a problem because its power consumption is increased due to a large consumption of the current.  
     SUMMARY OF CERTAIN INVENTIVE ASPECTS  
      Accordingly, certain embodiments solve such drawbacks of the device described above, and therefore can provide a DC-DC converter capable of improving a response time characteristic of the signal and also reducing power consumption.  
      One embodiment is a comparator configured to receive an input voltage and a reference voltage and to determine an output corresponding to a difference between the input voltage and the reference voltage. The comparator includes an input unit configured to transmit the input voltage to a first stage and to transmit the reference voltage to a second stage, and an amplification unit including a first capacitor configured to store the input voltage and the reference voltage, a second capacitor connected to the first capacitor and configured to receive and to store a feedback voltage, and at least one inverter configured to output signals corresponding to the voltage stored in the first capacitor and the second capacitor. The comparator also includes a feedback unit configured to receive a first voltage output from within the amplification unit to generate a first voltage when the input voltage is transmitted to the amplification unit, to receive a second voltage output from within the amplification unit to generate a second voltage when the reference voltage is transmitted to the amplification unit, to generate a feedback voltage corresponding to a difference between the first voltage and the second voltage, and to transmit the feedback voltage to the amplification unit, and an output unit configured to receive and to output a voltage corresponding to the output voltage of the amplification unit.  
      Another embodiment is a DC-DC converter including a charge pump including a voltage output terminal configured to vary and output an output voltage according to an input voltage, and a comparator configured to receive a comparator input voltage and a reference voltage, and to determine an output voltage corresponding to a difference between the comparator input voltage and the reference voltage. The comparator includes an input unit configured to supply the input voltage to a first stage and to supply the reference voltage to a second stage, a first capacitor configured to store the input voltage and the reference voltage, a second capacitor connected to the first capacitor and configured to receive and to store a feedback voltage, and at least one inverter configured to output signals corresponding to the voltage stored in the first capacitor and the second capacitor. The DC-DC converter also includes a feedback unit configured to receive a first voltage output from within the amplification unit to generate a first voltage when the input voltage is supplied to the amplification unit, to receive a second voltage output from within the amplification unit to generate a second voltage when the reference voltage is supplied to the amplification unit, to generate a feedback voltage corresponding to a difference between the first voltage and the second voltage, and to supply the feedback voltage to the amplification unit, and an output unit configured to receive and to output a voltage corresponding to the output voltage of the amplification unit.  
      Another embodiment is a organic light-emitting display including a pixel unit configured to display an image corresponding to data signals and scan signals, a data driving unit configured to supply the data signals to the pixel unit, a scan driving unit configured to supply the scan signals to the pixel unit, and a DC-DC converter configured to supply a power supply to the pixel unit, the data driving unit and the scan driving unit. The DC-DC converter includes a charge pump including a voltage output terminal configured to vary and output an output voltage according to an input voltage, and a comparator configured to receive a comparator input voltage and a reference voltage, and to determine an output voltage corresponding to a difference between the comparator input voltage and the reference voltage. The comparator includes an input unit configured to supply the input voltage to a first stage and to supply the reference voltage to a second stage, a first capacitor configured to store the input voltage and the reference voltage, a second capacitor connected to the first capacitor and configured to receive and to store a feedback voltage, and at least one inverter configured to output signals corresponding to the voltage stored in the first capacitor and the second capacitor. The DC-DC converter also includes a feedback unit configured to receive a first voltage output from within the amplification unit to generate a first voltage when the input voltage is supplied to the amplification unit, to receive a second voltage output from within the amplification unit to generate a second voltage when the reference voltage is supplied to the amplification unit, to generate a feedback voltage corresponding to a difference between the first voltage and the second voltage, and to supply the feedback voltage to the amplification unit, and an output unit configured to receive and to output a voltage corresponding to the output voltage of the amplification unit.  
      Another embodiment is a comparator configured to receive an input voltage and a reference voltage and to determine an output corresponding to a difference between the input voltage and the reference voltage. The comparator includes an input unit configured to transmit the input voltage to a first stage and to transmit the reference voltage to a second stage, an amplification unit, a feedback unit configured to receive a first voltage output from within the amplification unit to generate a first voltage when the input voltage is transmitted to the amplification unit, to receive a second voltage output from within the amplification unit to generate a second voltage when the reference voltage is transmitted to the amplification unit, to generate a feedback voltage corresponding to a difference between the first voltage and the second voltage, and to transmit the feedback voltage to the amplification unit, and an output unit configured to receive and to output a voltage corresponding to the output voltage of the amplification unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of certain embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a circuit diagram showing a previous comparator;  
       FIG. 2  is a timing diagram showing input/output waveforms of the circuit shown in  FIG. 1 ;  
       FIG. 3  is a schematic view showing a configuration of an organic light-emitting display;  
       FIG. 4  is a schematic view showing a DC-DC converter used in the organic light-emitting display shown in  FIG. 3 ;  
       FIG. 5  is a circuit diagram showing an embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ;  
       FIG. 6  is a circuit diagram showing another embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ;  
       FIG. 7  is a timing diagram showing the input/output waveforms of the comparator shown in  FIGS. 5 and 6 ;  
       FIG. 8  is a characteristic curve showing an output property of the comparator shown in  FIGS. 5 and 6 ;  
       FIG. 9  is a circuit diagram showing another embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ; and  
       FIG. 10  is a timing diagram showing the input/output waveform of the comparator shown in  FIG. 9 . 
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS  
       FIG. 3  is a schematic view showing a configuration of an organic light-emitting display according to some embodiments. Referring to  FIG. 3 , the organic light-emitting display has a pixel unit  100 , a data driving unit  200 , a scan driving unit  300  and a DC-DC converter  400 .  
      In the pixel unit  100 , a plurality of data lines D 1  to Dm and a plurality of scan lines S 1  to Sn cross each other, and pixels  110  are formed in region in which the data lines D 1  to Dm and the scan lines S 1  to Sn cross. The pixels  110  present an image by displaying a grey level corresponding to data signals transmitted through the data lines D 1  to Dm and scan signals transmitted through the scan lines S 1  to Sn.  
      The data driving unit  200  is connected with a plurality of the data lines D 1  to Dm to transmit data signals to a plurality of the data lines in parallel, and to simultaneously transmit data signals to a pixel row arranged in a latitudinal direction of the pixel unit  100 .  
      The scan driving unit is connected with a plurality of the scan lines S 1  to Sn to transmit scan signals to a specific pixel  110  by transmitting the scan signals to the pixel  110  to which the scan signals to be transmitted.  
      The DC-DC converter  400  converts a D.C. power supply level, transmitted from the outside, to a suitable D.C. power supply level for each electrical loads and transmits the D.C. power level to each of the electrical loads, and the D.C. power level generated in the DC-DC converter  400  is transmitted to the pixel unit  100 , the data driving unit  200  and the scan driving unit  300 , etc.  
       FIG. 4  is a schematic view showing an embodiment of a DC-DC converter used in the organic light-emitting display shown in  FIG. 3 . Referring to  FIG. 4 , the DC-DC converter includes a clock switch  430 , a charge pump  410 , a clock divider  440  and a comparator  420 .  
      The clock switch  430  receives clocks from a clock generation unit CLK, and adjusts the clocks generated in the clock generation unit CLK using the first clock CLK 1  and the second clock CLK 2  transmitted through the inverter  450 .  
      The charge pump  410  synchronizes with the first clock CLK 1  and the second clock CLK 2 , and charges a capacitor to generate a higher voltage or a lower voltage than the input voltage, and output the generated voltage to each of the driving units.  
      The clock divider  440  transmits the clocks CLK and CLKB from the clock generation unit CLK to the comparator unit  420  to operate the comparator unit  420 .  
      The comparator  420  is synchronized by the clocks CLK and CLKB, and compares a reference voltage Vref with an input voltage Vin by receiving the input voltage Vin from an output port of the charge pump  410  and receiving the reference voltage Vref through the reference voltage source, and allows the clock switch  430  to be operated by the first clock CLK 1  and the second clock CLK 2  by transmitting the compared signals to the clock switch  430  through the inverter  450 . This allows a charge pump to control an output voltage to correspond to the first clock CLK 1  and the second clock CLK 2 .  
       FIG. 5  is a circuit diagram showing an embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ; and  FIG. 7  is a timing diagram showing the input/output waveforms of the comparator shown in  FIG. 5 . Referring to  FIG. 5 , the comparator  420  has an input unit  421 , amplification unit  422 , output unit  424 , feedback unit  425 , and first, second and an third inverters.  
      Referring to  FIG. 5 , the input unit has an input voltage connected with the capacitor C 10  through the first switch SW 11 , and a reference voltage Vref connected with the capacitor C 10  through the second switch SW 12 . The capacitor C 10  is connected with the gates of the first and second transistors M 11  and M 12  of the first inverter.  
      The capacitor C 11  has a first electrode connected with the gates of the first and second transistors M 11  and M 12  of the first inverter, and a second electrode connected with a sixth switch SW 16  and a seventh switch SW 17 . Also, the first, second, and third inverters are connected in the same manner as in  FIG. 1 . The sixth switch SW 16  and the seventh switch SW 17  are connected to the output port of the second inverter, that is, between a fourth switch SW 14  and a fifth switch SW 15 , and therefore signals output through the output port of the second inverter are transmitted by means of switching operations of the sixth switch SW 16  and the seventh switch SW 17 .  
      The comparator can be operated according to the signals shown in  FIG. 7 , where the first switch SW 11 , the third switch SW 13 , the fourth switch SW 14  and the sixth switch SW 16  conduct the switching operation according to the first control signal P 1 , the second switch SW 12  conducts the switching operation according to the second control signal P 2 , and the fifth switch SW 15  and the seventh switch SW 17  conduct the switching operation according to the third control signal P 3  in the comparator.  
      Operation of the comparator will be described with reference to  FIGS. 5 and 7 . Firstly, the first switch SW 11 , the third switch SW 13 , the fourth switch SW 14  and the sixth switch SW 16  are turned on by the first control signal P 1 , and the second switch SW 12  and the fifth switch SW 15  are turned off by the second control signal P 2  and the third control signal P 3 . Accordingly, an input voltage Vin is transmitted to a capacitor C 10 , and the voltage corresponding to a threshold voltage difference between the first inverter and the second inverter is stored in the second capacitor C 12 . The third inverter is at a floating state since the fifth switch SW 15  remains turned off. At this time, if the sixth switch SW 16  is on, then the output signal of the second inverter is stored in the first capacitor C 11  via the sixth switch SW 16 . If the voltage stored in the first capacitor C 11  is transmitted to the first inverter via switch SW 13 , the output voltage of the first inverter is adjusted.  
      When the second switch SW 12  is turned on by the second control signal P 2 , the voltage transmitted to the capacitor C 10  is changed from the input voltage Vin to the reference voltage Vref. While the third switch SW 13  is open, the voltages at the input of the first inverter is changed. As a result, the voltage transmitted to the second inverter is also changed.  
      When the fifth and seventh switches SW 15 , SW 17  are turned on after a time t 1  while the second switch SW 12  is on, the voltage stored in the first capacitor C 11  is changed according to the switching operation of the seventh switch SW 17 . The first transistor M 11  of the first inverter and the gate voltage of the second transistor M 12  receive the voltage stored in the first capacitor C 11 . The feedback operation of the circuit increases the output swing and the switching speed of the first, second, and consequently the third inverters.  
       FIG. 8  is a characteristic curve showing an output property of the comparator shown in  FIG. 5 . Referring to  FIG. 8 , Vout represents a characteristic curve of the inverter, and Inverse represents a curve in which the characteristic curve of the inverter is at a reversed state.  
      The characteristic curve shows that the output changes significantly around an input of 2.5V, and the response characteristics of the signal is a high signal and a low signal corresponding to the difference between the input voltage and the reference voltage. The feedback procedure affects the voltage input to the amplification unit through the voltage stored in the first capacitor to improve switching characteristics.  
       FIG. 6  is a circuit diagram showing another embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 . The comparator shown in  FIG. 6  has a capacitor C 21  for storing a feedback voltage and is connected between the output port of the amplification unit and the sixth and seventh switches SW 26  and SW 27 .  
      The voltage stored in capacitor C 21  is stored according to the switching operation of the sixth and seventh switches SW 36  and SW 37 , which may occur as shown in  FIG. 7 . Accordingly, the comparators shown in  FIG. 5  and  6  have similar advantageous characteristics, because the voltage stored in the first capacitor C 21  is transmitted to the first inverter at a similar time point.  
       FIG. 9  is a circuit diagram showing another embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ; and  FIG. 10  is a timing diagram showing the input/output waveform of the comparator shown in  FIG. 9 . Referring to  FIGS. 9 and 10 , the comparator shown in  FIG. 9  has one switch to conduct a feedback operation, unlike the comparator shown in  FIGS. 5 and 6 , and the switch conducting the feedback operation conducts the switching operation according to the fourth control signal P 4  shown in  FIG. 10 .  
      The fourth control signal P 4  is “on” when the first control signal P 1  is turned on. And when the second control signal P 2  is “on” to input the input voltage Vin at a time (t 2 ) after the second control signal P 2  is turned on, a voltage is charged in the first capacitor C 31  according to the fourth control signal P 4 . As a result, the reference voltage Vin is input to the first inverter according to the second control signal P 2 .  
      Accordingly, the voltage is stored onto the first capacitor C 31  of  FIG. 9  similarly to the storing of voltage onto the first capacitor C 11  of the comparator shown in  FIG. 5 . Accordingly, the comparator shown in  FIG. 9  also has similar advantageous characteristics as shown in the characteristic curve of in  FIG. 8 .  
      The converter aspects described above, and the DC-DC converter and an organic light-emitting display using the converter have improved response rate because of varying the voltage input to the inverter to increase a changing level of the output voltage. Also, the DC-DC converter may reduce a power consumption by shutting off the inverter circuit to prevent flow of the current if the input/output unit is not operated.  
      Although certain embodiments have been shown and described in detail, the embodiments mentioned herein are examples for the purpose of illustrations only, and are not intended to limit the scope of the invention to the specific details described. Also, it would be appreciated by those skilled in the art that changes might be made in these embodiment without departing from the principles and spirit of the invention.