Patent Publication Number: US-2007103128-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. 2005-0106168, filed on Nov. 7, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
     BACKGROUND  
      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 comparator according to the prior art. 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 the input voltage (Vin); and a second switch (SW 2 ) for switching 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 output a high level of voltage, and the second transistor (M 2 ) has a source connected to a ground (GND) to output a low level of voltage. Also, the first capacitor (C 1 ) and the third switch (SW 3 ) are connected to the input of the first inverter.  
      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 output a high level of voltage, and a ground is connected to a source of the fourth transistor (M 4 ) to output 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 (SW 4 ), and the third and fourth transistors (M 3 ,M 4 ) are connected at the output of the second inverter.  
      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 output a high level of voltage, and a ground is connected to a source of the sixth transistor (M 6 ) to output a low level of voltage.  
       FIG. 2  is a waveform view showing input/output waveforms of the circuit shown in  FIG. 1 . Referring to  FIG. 2 , an input voltage (Vin) input to an input terminal of a comparator unit is compared with the reference voltage (Vref). The first to fifth switches (SW 1  to SW 5 ) perform 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 by 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 ), 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 ).  
      When the first, third and fourth switches (SW 1 ,SW 3 ,SW 4 ) are 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 ), 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 that it takes more time to change the output voltage into the high level or the low level if there is not a large difference between the reference voltage (Vref) and the input voltage (Vin) than if there is large difference between the reference voltage (Vref) and the input voltage (Vin).  
      In order to solve the problem, the comparator as described above should have a large capacitance, however, it then has a problem that its power consumption is increased because of the large capacitance.  
     SUMMARY OF CERTAIN INVENTIVE ASPECTS  
      Accordingly, the present invention is designed to solve such drawbacks of the prior art, and therefore an aspect of the present invention is to provide a low power comparator with high speed response, and an organic light-emitting display using the same.  
      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 and a feedback voltage to a second stage, an amplification unit including at least one inverter configured to operate according to the input voltage transmitted to the first stage, the feedback voltage transmitted to the second stage, and the reference voltage, a feedback unit configured to receive a voltage output in the amplification unit so as to generate a feedback voltage and to transmit the generated feedback voltage to the input unit so as to control the voltage transmitted to the amplification unit, and an output unit configured to receive and output the output voltage of the amplification unit.  
      Another embodiment is a DC-DC converter including a charge pump, and a comparator configured to receive an input voltage and a reference voltage and to determine an output voltage corresponding to a difference between the input voltage and the reference voltage, where the comparator includes an input unit configured to transmit the input voltage to a first stage and to transmit the reference voltage and a feedback voltage to a second stage, an amplification unit including at least one inverter configured to operate according to the input voltage transmitted to the first stage, the feedback voltage transmitted to the second stage, and the reference voltage, a feedback unit configured to receive a voltage output in the amplification unit so as to generate a feedback voltage and to transmit the generated feedback voltage to the input unit so as to control the voltage transmitted to the amplification unit, and an output unit configured to receive and output the output voltage of the amplification unit.  
      Another embodiment is an 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 transmit the data signals to the pixel unit, a scan driving unit configured to transmit the scan signals to the pixel unit, and a DC-DC converter configured to transmit a power supply to the pixel unit, the data driving unit and the scan driving unit. The DC-DC converter includes a charge pump, and a comparator configured to receive an input voltage and a reference voltage and to determine an output voltage corresponding to a difference between the input voltage and the reference voltage, where the comparator includes an input unit configured to transmit the input voltage to a first stage and to transmit the reference voltage and a feedback voltage to a second stage, an amplification unit including at least one inverter configured to operate according to the input voltage transmitted to the first stage, the feedback voltage transmitted to the second stage, and the reference voltage, a feedback unit configured to receive a voltage output in the amplification unit so as to generate a feedback voltage and to transmit the generated feedback voltage to the input unit so as to control the voltage transmitted to the amplification unit, and an output unit configured to receive and output the output voltage of the amplification unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a circuit diagram showing a comparator according to prior art;  
       FIG. 2  is a waveform view 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 according to one embodiment;;  
       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 one embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ;  
       FIG. 6  is a circuit diagram showing one embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ; and  
       FIG. 7  is a circuit diagram showing one embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS  
      Hereinafter, embodiments will be described with reference to the accompanying drawings.  
       FIG. 3  is a schematic view showing a configuration of an organic light-emitting display according to the present invention. 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 near regions 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 data signals to a specific pixel  110  by transmitting the data signals to the pixel  110  to which the scan signals are connected.  
      The DC-DC converter  400  converts a D.C. power supply voltage, input from the outside, to a suitable D.C. power supply voltage for the electrical loads and transmits the D.C. power supply to each of the electrical loads. The D.C. power supply 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 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 controls 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 so as to generate a higher voltage or lower voltage than the input voltage, and output the voltage generated by the charge pump  410 , and then transmit the voltage to each of the driving units. Hereinafter, the circuit and operation of the charge pump  410  will not be described herein since they have been widely known in the art.  
      The clock divider  440  transmits the clocks (CLK, CLKB) from the clock generation unit (CLK) to the comparator unit  420  to operate the comparator unit  420 .  
      The comparator  420  is synchronized with the clocks (CLK, CLKB), and compares a reference voltage (ref) 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 (ref) 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 , and therefore 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 one embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 . Referring to  FIG. 5 , the comparator has an input unit and first to third inverters. The first to third inverters will be not described herein since they have essentially the same function as the comparator shown in  FIG. 1 .  
      Referring to  FIG. 5 , the input unit has an input voltage (Vin) connected with a first capacitor (C 11 ) through the first switch (SW 11 ) and a reference voltage (Vref) connected with a first capacitor (C 11 ) through second and sixth switches (SW 12 , SW 16 ). Also, the reference voltage (Vref) is charged into a third capacitor (C 13 ) by the switching operations of the second switch (SW 12 ) and the sixth switch (SW 16 ). And, the third capacitor (C 13 ) has a first electrode connected to the second switch (SW 12 ) and a second electrode connected to the output port of the second inverter, and receives the voltage transmitted through the output port of the second inverter to control the voltage input to the first inverter.  
      When the signals as shown in  FIG. 2  are input, the first switch (SW 11 ), the second switch (SW 12 ), the third switch (SW 13 ) and the fourth switch (SW 14 ) conduct the switching operation according to the first control signal (P 1 ), and the fifth and sixth switches (SW 15 , SW 16 ) conduct the switching operation according to the second control signal (P 2 ).  
      When the signals are input as shown in  FIG. 2 , if the first switch (SW 11 ), the second switch (SW 12 ), the third switch (SW 13 ) and the fourth switch (SW 14 ) are firstly turned on by the first control signal (P 1 ) and the fifth and sixth switches (SW 15 , SW 16 ) are turned off by the second control signal (P 2 ), then the input voltage (Vin) is input to a first capacitor (C 11 ), and the reference voltage (Vref) is transmitted to a third capacitor (C 13 ). And then, when the first switch (SW 11 ), the second switch (SW 12 ), the third switch (SW 13 ) and the fourth switch (SW 14 ) are turned off by the first control signal (P 1 ), and the fifth and sixth switches (SW 15 , SW 16 ) are turned on by the second control signal (P 2 ), then the first capacitor (C 11 ) transmits the voltage stored in the third capacitor (C 13 ). At this time, because the third capacitor (C 13 ) is connected with the output port of the second inverter, the voltage stored in the third capacitor (C 13 ) corresponds to the reference voltage and the voltage output in the output port of the second inverter. That is, the voltage at the output port of the second inverter is fed back by the third capacitor (C 13 ) to control the voltage input to the first inverter.  
      Accordingly, an amplified difference between the input voltage (Vin) and the reference voltage (Vref) by means of the first capacitor (C 11 ) is fed back to the input so as to improve the response characteristics of the signal since the range of the output voltage of the third inverter is further increased.  
       FIG. 6  is a circuit diagram showing another embodiment of the comparator used in the DC-DC converter shown in  FIG. 4 ; and  FIG. 7  is a circuit diagram showing the comparator used in the DC-DC converter shown in  FIG. 4 . Referring to  FIG. 6 , the comparator shown in  FIG. 5  has a difference that it has a fourth capacitor (C 24 ) between the first switch SW 21  and the first capacitor (C 21 ), as shown in  FIG. 6 . The fourth capacitor (C 24 ) is configured to stabilize an early negative feedback operation.  
      Referring to  FIG. 7 , a fourth capacitor (C 34 ) is connected to the gate of the first transistor (M 31 ) and the gate of the second transistor (M 32 ), as shown in  FIG. 7 , the same operation is achieved as in the fourth capacitor (C 24 ) shown in  FIG. 6  to stabilize a negative feedback operation.  
      According to the present invention, the DC-DC converter and the organic light-emitting display using the same may be useful to increase a response rate by varying the voltage input to the inverter to increase a changing level of the output voltage. Also, the DC-DC converter of the present invention may reduce 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 just examples for the purpose of illustration only, and are not intended to limit the scope of the invention. Also, it would be appreciated by those skilled in the art that changes might be made to these embodiments without departing from the principles and spirit of the invention.