The present invention relates to a drive circuit for driving a display panel such as a liquid crystal display panel, and a display device including the display panel.
In a conventional liquid crystal display device of an active matrix type, a liquid crystal display panel generally has a plurality of display pixels arranged in a matrix pattern. Each of the display pixels includes a liquid crystal layer sandwiched between two glass plates and an active element such as TFT (Thin Film Transistor) for controlling a voltage applied to the liquid crystal layer.
Further, the conventional liquid crystal display device includes a drive circuit for driving the liquid crystal display panel. The drive circuit includes a gate driver and a source driver. The gate driver is provided for supplying a control signal through a scanning line (a gate line) for tuning on or off each of the active elements. The source driver is provided for supplying a gradation voltage through a data line (a source line) to opposite electrodes sandwiching the liquid crystal layer of each of the display pixels.
When an electrical field is applied to the liquid crystal layers with a direct current with the same polarity for a prolonged period of time, the liquid crystal layers tend to deteriorate. In order to prevent the deterioration, a specific drive method has been widely adopted. In the drive method, the graduation voltage with a positive polarity and the graduation voltage with a negative polarity are alternately supplied to each of the display pixels. The graduation voltage with the positive polarity is greater than a reference voltage, and the graduation voltage with the negative polarity is smaller than the reference voltage.
For example, when a dot inversion method is adopted as the drive method, the graduation voltage with the positive polarity and the graduation voltage with the negative polarity are alternately supplied per dot (for example, per display pixel). When a line inversion method is adopted as the drive method, the graduation voltage with the positive polarity and the graduation voltage with the negative polarity are alternately supplied per line.
When the liquid crystal display device adopts the drive method described above, the source driver generally includes an impedance conversion circuit. The impedance conversion circuit includes an operation amplifier (referred to as a high voltage side operation amplifier) for outputting an analog voltage with the positive polarity and an operation amplifier (referred to as a low voltage side operation amplifier) for outputting an analog voltage with the negative polarity.
Patent Reference 1 has disclosed a conventional liquid crystal display device of an active matrix type. According to Patent Reference 1, a source driver includes a high voltage side operation amplifier (or a positive amplifier) of a voltage follower type and a low voltage side operation amplifier (or a negative amplifier) of the voltage follower type.
In the source driver disclosed in Patent Reference 1, the high voltage side operation amplifier is operated with a group of a power source voltage AVDD and a power source voltage AGNDP (AVDD>AGNDP), and the low voltage side operation amplifier is operated with a group of a power source voltage AVDDN and a power source voltage AGND (AVDDN>AGND). Accordingly, four power source voltages AVDD, AGNDP, AVDDN, and AGND are used for driving the high voltage side operation amplifier and the low voltage side operation amplifier.    Patent Reference 1: Japanese Patent Publication No. 2006-292807
Patent Reference 2 has disclosed another conventional liquid crystal display device. According to Patent Reference 2, a source driver includes a high voltage side operation amplifier operated with a group of a power source voltage VLCD and a power source voltage VMM (VMM=1/2 VLCD) and a low voltage side operation amplifier operated with a group of the power source voltage VMM and a ground GND (GND=0 V). Accordingly, three power source voltages VCLD, AMM, and GND are used for driving the high voltage side operation amplifier and the low voltage side operation amplifier.    Patent Reference 2: Japanese Patent Publication No. 10-062744
According to Patent Reference 1 and Patent Reference 2, the source driver further includes a switch circuit for connecting each of the data lines alternately to an output terminal of the high voltage side operation amplifier and an output terminal of the low voltage side operation amplifier. Accordingly, the analog voltage with the positive polarity and the analog voltage with the negative polarity are alternately supplied to the data lines.
More specifically, the switch circuit is configured to connect the data line at an i-th position to the high voltage side operation amplifier and the data line at an (i+1)-th position adjacent to the i-th position to the low voltage side operation amplifier. Accordingly, the analog voltage with the positive polarity is supplied to the data line at the i-th position, and the analog voltage with the negative polarity is supplied to the data line at the (i+1)-th position. At this moment, a potential of the data line at the i-th position is lower than a reference voltage, and a potential of the data line at the (i+1)-th position is higher than the reference voltage.
At the next timing, the switch circuit switches the data line at the i-th position from the high voltage side operation amplifier to the low voltage side operation amplifier, and the data line at the (i+1)-th position from the low voltage side operation amplifier to the high voltage side operation amplifier. As a result, the analog voltage with the negative polarity is supplied to the data line at the i-th position, and the analog voltage with the positive polarity is supplied to the data line at the (i+1)-th position. Accordingly, the potential of the data line at the i-th position transits from a high potential to a low potential, and the potential of the data line at the (i+1)-th position transits from a low potential to a high potential.
In the conventional liquid crystal display device, when the switch circuit switches the data line, the output terminal of the high voltage side operation amplifier is switched from the data line at the i-th to the data line at the (i+1)-th, and the output terminal of the low voltage side operation amplifier is switched from the data line at the (i+1)-th to the data line at the i-th. Accordingly, a potential of the output terminal of the high voltage side operation amplifier may abruptly decrease, and a potential of the output terminal of the low voltage side operation amplifier may abruptly increase temporarily.
When the potential of the output terminal of the high voltage side operation amplifier abruptly decreases, or the potential of the output terminal of the low voltage side operation amplifier abruptly increases, a bias in a forward direction is applied to a parasite bipolar transistor inside the high voltage side operation amplifier or a parasite bipolar transistor inside the low voltage side operation amplifier. As a result, an excess current may flow in the parasite bipolar transistor, thereby damaging the high voltage side operation amplifier or the low voltage side operation amplifier.
In view of the problems described above, an object of the present invention is to provide a drive circuit for driving a display panel, and a display device including the display panel capable of solving the problems of the conventional display device. In the present invention, it is possible to prevent an excess current from flowing in a high voltage side operation amplifier and a low voltage side operation amplifier.
Further objects and advantages of the invention will be apparent from the following description of the invention.