A Current-controlled Oscillator Applied In AMOLED Driver Chip

A new flat panel display technology—OLED(Organic Light Emitting Diode), has emerged in recent years. As a new technology, AMOLED(Active Matrix Organic Light Emitting Diode) is recognized as the display technology of next generation with the development of OLED technology. AM refers to the pixel addressing technology and OLED refers to the light emitting diode which works based on the injection and compound of carriers in the electric field of light-emitting materials and organic semiconductor materials. C urrently, this technology is applied to more and more electronic products, such as computers, TVs and cellphones, which leads to a huge demand for AMO LED driving chips.The clock generation system plays a vital role in the AMOLED driving chip. An oscillator system was designed according to the requirements of the driving chip on the clock generation system. Based on the analysis and comparison of pros and cons of LC oscillator and ring oscillator, a three-stage ring oscillator structure was selected. A temperature compensation block was designed to prevent the output frequency of ring oscillator from decreasing with the rise of temperature. The oscillator was controlled by current to realize the temperature compensation and output frequency adjustment conveniently.The differential positive feedback structure was used in the delay cell of the oscillator. The power supply and substrate noise rejection ratio can be enhanced by the differential structure while the oscillating frequency can be increased through the positive feedback. The output frequency is adjusted by changing the tail current of the delay cell to change the load charging and discharging time of each delay cell. A PATA current was added to the bias current of the delay ce ll. The PATA current increases with the rise of temperature, resulting in an increase of the oscillating frequency which cancels the decrease of output frequency of conventional oscillator.The circuit function and working principle were analyzed firstly a nd then the oscillator system was designed and verified in a 90 nm high voltage CMOS process. The simulation results indicate that, under tt corner, when T=25 ℃, AVDD=2.8V, DVDD=1.2V and the PTAT current is a constant, the output frequency can be increased from 235.28 MHz to 664.56 MHz under the control of a 7-bit code RADJ(27 steps in total). The change rate of each step is approximately 0.7%. When the bias current of the delay cell is set to the default value(RADJ=1000000), the output frequency variation rate with temperature decreases from 140 ppm to 10 ppm as the PTAT current control code changes from 00000 to 11111. The output frequency variation with temperature is suppressed significantly. The simulation results also indicate that the performance of the oscillator under all corners achieves the system requirements.