Abstract:
A DC-DC converter having low power consumption by reducing an input current and an organic light emitting display using the same. In one embodiment, a DC-DC converter includes a buck-boost circuit for generating and outputting a second power of a second power source by receiving an input voltage. The buck-boost circuit adjusts and outputs the voltage level of the second power of the second power source in response to the voltage level of the input voltage. A controller controls the buck-boost circuit. The buck-boost circuit includes a first coil coupled between a first node and a ground; a first capacitor having first and second electrodes, the first electrode being coupled to the first node, a first switch coupled between the first node and an input terminal; and a second switch coupled between the second electrode of the capacitor and ground.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0110783, filed on Nov. 17, 2009, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One or more aspects of the present invention relate to a DC-DC converter and an organic light emitting display device using the same. 
     2. Description of Related Art 
     Various types of flat panel display devices which may have less weight and volume than cathode ray tubes have recently been developed. The types of flat panel display devices include a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, and the like. 
     Among the flat panel display devices, the organic light emitting display device displays images using an organic light emitting diode (OLED) which produces light with a luminance corresponding to an amount of current inputted (or supplied) to the OLED. 
     The organic light emitting display device has various characteristics such as excellent color reproduction, thin and lightweight devices, and the like. Accordingly, the organic light emitting device has been used in fields such as mobile phones, PDAs, MP3 players, and the like. 
     SUMMARY 
     Aspects of embodiments of the present invention are directed toward a DC-DC converter which has low power requirements and a small size by reducing the amplitude of input current and an organic light emitting display device using the DC-DC converter. 
     According to an embodiment of the present invention, there is provided a DC-DC converter including a buck-boost circuit for generating and outputting a second power of a second power source using an input voltage, the buck-boost circuit being configured to adjust and output the voltage level of the second power of the second power source in accordance with the voltage level of the input voltage; and a controller for controlling the buck-boost circuit, wherein the buck-boost circuit includes a first coil coupled between a first node and a ground; a first capacitor having first and second electrodes, the first electrode being coupled to the first node; a first switch coupled between the first node and an input terminal; a second switch coupled between the second electrode of the first capacitor and the ground; and a third switch coupled between the second electrode of the first capacitor and a first output terminal. 
     Another embodiment of the present invention provides an organic light emitting display device including a display unit for displaying an image in response to a data signal, a scan signal, a first power of a first power source, and a second power of a second power source; a data driver for generating and outputting the data signal; a scan driver for generating and outputting the scan signal; a DC-DC converter for generating and outputting the first power of the first power source and the second power of the second power source; and a controller for outputting a control signal to control the DC-DC converter, wherein the DC-DC converter includes: a buck-boost circuit for generating and outputting the second power of the second power source using an input voltage, the buck-boost circuit configured to adjust and output the voltage level of the second power source in accordance with a voltage level of the input voltage; and a controller for controlling the boost circuit and the buck-boost circuit, wherein the buck-boost circuit includes a first coil coupled between a first node and a ground; a first capacitor having first and second electrodes, the first electrode being coupled to the first node; a first switch coupled between the first node and an input terminal; a second switch coupled between the second electrode of the first capacitor and the ground; and a third switch coupled between the second electrode of the first capacitor and a first output terminal. 
     In a DC-DC converter and an organic light emitting display device using the same according to embodiments of the present invention, the peak value of current is decreased, thereby increasing the efficiency of a buck-boost circuit. Further, a coil with low inductance is used in the buck-boost circuit, thereby reducing the size of the DC-DC converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1  is a circuit diagram of a pixel used in an organic light emitting display device. 
         FIG. 2  is a block diagram illustrating the structure of an organic light emitting display device according to an embodiment of the present invention. 
         FIG. 3  is a circuit diagram illustrating a structure of a DC-DC converter, according to one embodiment of the present invention, which may be used in the organic light emitting display device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, certain exemplary embodiments according to the present invention will be described with reference to the accompanying drawings. Here, when a first element is described as being coupled to a second element, the first element may be not only directly coupled to the second element but may also be indirectly coupled to the second element via a third element. Further, some of the elements that are not essential to the complete understanding of the invention are omitted for clarity. Also, like reference numerals refer to like elements throughout. 
       FIG. 1  is a circuit diagram of a pixel used in a general organic light emitting display device. 
     Referring to  FIG. 1 , the pixel includes a first transistor M 1 , a second transistor M 2 , a capacitor Cst, and an organic light emitting diode (OLED). 
     The first transistor M 1  has a source electrode coupled to a first power source ELVDD, a drain electrode coupled to an anode electrode of the OLED, and a gate electrode coupled to a first node N 1 . The second transistor M 2  has a source electrode coupled to a data line Dm, a drain electrode coupled to the first node N 1 , and a gate electrode coupled to a scan line Sn. The capacitor Cst has a first electrode coupled to the source electrode of the first transistor M 1  and a second electrode coupled to the first node N 1 . The OLED has an anode electrode coupled to the drain electrode of the first transistor M 1  and a cathode electrode coupled to a second power source ELVSS. 
     In the pixel configured as described above, the voltage at the first node N 1  is determined in response to the data signal supplied through the data line Dm. The first transistor M 1  allows (or controls) current to flow from the first power source ELVDD through the OLED to the second power source ELVSS based on the voltage at the first node N 1 . Through such an operation, the OLED emits light with a luminance corresponding to the current flowing through it. 
     The first power source ELVDD and the second power source ELVSS are generated by a boost circuit and a buck-boost circuit, respectively. 
     The amount of current inputted to the buck-boost circuit is much larger than the current inputted to the boost circuit. Therefore, the buck-boost circuit is less efficient than the boost circuit. Since the current inputted to the buck-boost circuit is greater than that inputted to the boost circuit, the inductance of a coil provided to the buck-boost circuit is necessarily greater than that provided to the boost circuit. Therefore, there is a limitation in reducing the size of the buck-boost circuit. 
       FIG. 2  is a block diagram illustrating the structure of an organic light emitting display device according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the organic light emitting display device of this embodiment includes a display unit (or pixel unit or display region)  100 , a data driver  110 , a scan driver  120 , a DC-DC converter  130  and a controller  140 . 
     The display unit  100  includes a plurality of pixels  101  and each of the pixels  101  includes an organic light emitting diode (OLED) that emits light in response to the flow of current. The display unit  100  further includes a plurality of scan lines S 1 , S 2 , . . . , Sn- 1 , and Sn arranged in rows to supply scan signals and a plurality of data lines D 1 , D 2 , . . . , Dm- 1 , and Dm arranged in columns to supply data signals. The display unit  100  receives a first power of a first power source ELVDD and a second power of a second power source ELVSS supplied from the exterior thereof (or an external source). 
     The data driver  110  receives an image signal having components of red, green and blue to generate a data signal. The data driver  110  is coupled to the data lines D 1 , D 2 , . . . , Dm- 1 , and Dm of the display unit  100  to supply the generated data signal to the display unit  100 . 
     The scan driver  120  supplies scan signals to specific rows of the display unit  100 . The scan driver  120  is coupled to the scan lines S 1 , S 2 , . . . , Sn- 1 , and Sn to supply the generated scan signal to the display unit  100 . The data signals outputted from data driver  110  are supplied to pixels  101  to which the scan signals are supplied from the scan driver  120 , so that a driving current generated in each of the pixels  101  flows into the corresponding organic light emitting diode. 
     The DC-DC converter  130  supplies the first power of the first power source ELVDD and the second power of the second power source ELVSS to the display unit  100 . The DC-DC converter  130  generates the first power of the first power source ELVDD and the second power of the second power source ELVSS by boosting or inverting a voltage inputted from the exterior thereof (or an external source). In embodiments of the present invention, the peak value of current inputted to the DC-DC converter  130  is reduced, thereby reducing power consumption. Accordingly, the efficiency of the DC-DC converter is increased (or improved). 
     The controller  140  generates a control signal CS to control the DC-DC converter  130  such that the efficiency of the DC-DC converter  130  is increased. Particularly, it is possible to decrease the peak value of current inputted when the second power source ELVSS is generated. 
       FIG. 3  is a circuit diagram illustrating the structure of the DC-DC converter used in the organic light emitting display device of  FIG. 2 . 
     Referring to  FIG. 3 , the DC-DC converter  130  includes a buck-boost circuit  131  and a boost circuit  132 . 
     The buck-boost circuit  131  of one embodiment of the present invention includes a first coil L 1 , a first capacitor C 1 , a first switch S 1 , a second switch S 2  and a third switch S 3 . The buck-boost circuit  131  generates a second power of a second power source ELVSS by inverting an input voltage Vbat. Generally, a buck-boost circuit generates the second power of the second power source ELVSS by applying an electromotive force generated by current flowing through the first coil L 1 . However, if the current flowing through the first coil L 1  is increased by the input voltage Vbat, the efficiency of the buck-boost circuit is decreased due to the increase of power consumption (or increased current). In order to mitigate such a problem, the buck-boost circuit  131  according to one embodiment further includes the first capacitor C 1  coupled in parallel with the first coil L 1 . 
     The operation of the buck-boost circuit  131  configured as described above will be described. When the first and second switches S 1  and S 2  are in an ‘on’ state and the third switch S 3  is in an ‘off’ state, current flows through the first coil L 1 . At this time, the first capacitor C 1  is coupled in parallel with the first coil L 1 , and hence current is inputted (or flows) to the first coil L 1  and the first capacitor C 1 . Here, the current flows through the first coil to thereby generate an electromotive force. 
     When the first and second switches S 1  and S 2  are in an ‘off’ state and the third switch S 3  is in an ‘on’ state, the current flowing from an output terminal and the current charged in the first capacitor C 1  flow through the first coil L 1  to ground. 
     Thus, the first capacitor C 1  slows (or reduces) the current flowing through the first coil L 1 , thereby decreasing the peak value of the current flowing through the first coil L 1 . Accordingly, power consumption is reduced, thereby increasing the efficiency of the buck-boost circuit  131 . 
     The boost circuit  132  includes a second coil L 2 , a fourth switch S 4 , a fifth switch S 5  and a sixth switch S 6 . The boost circuit  132  generates a first power at a first power source ELVDD by boosting an input voltage Vbat. In the boost circuit  132 , when the fourth and fifth switches S 4  and S 5  are in an ‘on’ state and the sixth switch S 6  is in an ‘off’ state, current flows through the second coil L 2 . When the fourth switch S 4  remains in the ‘on’ state and the fifth switch is in an ‘off’ state while the sixth switch S 6  remains in the ‘off’ state, an electromotive force is generated at the second coil L 2  so as to maintain the state in which current can flow through the second coil L 2 . Then, when the sixth switch S 6  is in an ‘on’ state, the electromotive force generated at the second coil L 2  is applied to an output terminal, so that the first power of the first power source ELVDD is outputted to the output terminal. 
     The first to third switches S 1  to S 3  of the buck-boost circuit  131  and the fourth to sixth switches S 4  to S 6  receive a control signal CS supplied from the controller  140  and perform switching operations, so that the second power of the second power source ELVSS and the first power of the first power source ELVDD can be generated by the buck-boost circuit  131  and the boost circuit  132 , respectively. 
     A second capacitor C 2  and a third capacitor C 3  are respectively coupled to the output terminals of the buck-boost circuit  131  and the boost circuit  132  so that the second power source ELVSS and the first power source ELVDD can be stably outputted from the buck-boost circuit  131  and the boost circuit  132 , respectively. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.