A display adopting OLED (Organic Light-Emitting Diode) is a newly developing flat panel display device. Due to the advantages of the display adopting OLED, such as simple preparation process, low cost, fast response speed, easy to achieve color display and a large display screen, low power consumption, easy to match a integrated circuit driver, high luminance, wide range of operating temperature, thin and light structure, easy to achieve flexible display, and the like, so the display adopting OLED has a wide range of applications.
According to the driving manners, OLED can be divided into two different types: Passive Matrix Organic Light Emission Display (PMOLED) and Active Matrix Organic Light Emission Display (AMOLED). The Passive Matrix Organic Light Emission Display has a simple preparation process and a low cost, but has the disadvantages of crosstalk, high power consumption, and short life-span, etc., and thus does not meet the requirements of display with high resolution and large size. On the contrary, Active Matrix Organic Light Emission Display allows a pixel unit to emit light during the period of a frame by incorporating Thin Film Transistors (TFT) in the panel, and thus has the advantages of low driving current being required, low power consumption and long life-span and is capable of satisfying the requirements of display with high resolution, multiple grey levels and large size.
However, TFT has a threshold voltage, and the drift of the threshold voltage will cause non-uniformity of the luminance of OLED. Various pixel compensation circuits are proposed to solve the above problem, and can be divided, according to driving signals, into two different types: Voltage Programmed Pixel Circuit (VPPC) and Current Programmed Pixel Circuit (CPPC). CPPC is capable of compensating the effects of threshold voltage of TFT, carrier mobility and temperature. Meanwhile, the luminance of OLED can be controlled more accurately by adopting CCPC, since OLED is a current-driving device, the luminance of which is proportion to the current flowing through OLED.
The configuration of a current driving pixel unit of current mirror type in the prior art is shown in FIG. 1, and the timing sequence for controlling the pixel unit shown in FIG. 1 is illustrated in FIG. 2. In FIGS. 1, A2 and A4 are controlled to be tuned on alternatively, and OLED is driven by A1. Such a configuration is capable of compensating the variation of the output current caused by such factors as the parameters of devices in a pixel current array and the temperature. Nevertheless, the main defect of the pixel circuit shown in FIG. 1 lies in the parasitic capacitances generated by the switching transistors A2 and A3 and the overlap capacitances between signal lines, wherein the overlap capacitances causes the Current Programmed Pixel Circuit to take a long time to achieve a stable current in the condition of low grey level and low current, which in turn severely constrains the application of the pixel unit of current driving type in a display of large dimension and high resolution.