Study On A Digital Pixel Driving Circuit For OLEDoS Micro-display

Organic silicon micro-display(Organic Light-emitting Device on Silicon, OLEDo S) is a kind of OLED light emitting cells formed on a substrate of single crystal silicon driving circuit chip micro-display system. OLED microdisplay not only has the advantages of small size eyeshot, high information content, light weight and portable, but also combine the advantages of OLED: such as self luminous, low power consumption, wide vision,etc. In addition, the driving circuit is also compatible with the standard CMOS process, the production process is easy to achieve and the cost is low. Because the pixel area is small and the drive current is very small, so small pixel area to the wafer occupancy area is larger in small current driving circuit of OLED micro display pixel driving circuit design of a particularly important problem.OLED pixel drive circuit can be divided into analog driving circuits and digital driver circuit. Analog drive method provides an analog voltage or current signal driving OLED, the luminance and the drive current or voltage proportional to the OLED. However, the presence of some difficulty analog driving mode: the smaller the driving current required for the light emitting element OLED, in order to achieve a high grayscale, then the gray scale voltage required between adjacent or separated by a small current, and then the analog input the accuracy of the signal is very high, in addition, the connection impedance of the parasitic capacitance and other parameters also have some influence on the analog signal. Digital driving mode analog driving mode problem does not exist, only the light-emitting unit and the lighting is not bright two states, when the current flowing through the OLED pixel emits light emitting element is constant, the input signal into a digital signal, which is lit by gray time to achieve different, digital driving mode can effectively alleviate the problem of small current difficult to achieve, because each pixel using a constant current drive, the drive can be taken to achieve the highest current OLED brightness. Of course, the digital driving mode, there are some problems, such as due to the pixel area is small(such as a 15μm × 15μm), and drive more digital circuit device, the circuit structure is complicated circuit, resulting to be placed in a small pixel area.In this paper, in order to achieve a resolution of 800×600, a single pixel area of 15μm×15μm of 16 gray OLED microdisplays for example, the proposed digital pixel driver circuit for a silicon-based OLED microdisplay, and a tube device parameters and timing design. When not considering the power supply voltage fluctuations and other factors, the maximum brightness of the OLED light emitting unit is 288.04cd/m2, this brightness can meet the brightness range of micro-display. However, this time serving as a constant current source transistor Q3 tube saturation voltage is extremely small, almost critical position in the saturation region, susceptible to supply voltage fluctuations. The presence of 5% of the power supply voltage fluctuations, the maximum brightness OLED light emitting unit corresponding changes 1740.34 cd /m2, great impact.To solve this problem, we propose a common drive pixel technology, namely N pixels share one drive circuit, and a time-frame light. At this time, achieve the same brightness, the drive current is greatly changed N times, in addition, a common set of multi-pixel drive circuit to the driving circuit provides a corresponding increase of the effective area of N times the area of the constant current source transistor can also be increased its constant effect of greatly improved. When the three pixel common drive circuit, OLED light emitting unit corresponding to the brightness of 184.58cd/m2, in line with this bright luminance range of micro-display. Further, the pixel area of the pixel driving circuit(about 100.7 μm2), it is possible to meet the pixel size of 15μm×15μm when required to achieve a color display(in this case, area of is 15μm×5μm, when pixel size is 15μm×15μm). In addition to six pixels common drive circuit, for example, simulation, and further illustrates the multi-pixel sharing the driving circuit can effectively solve the difficult problem of small current to achieve, for the color display provides a feasible method.