Pixel Circuit Driving Method for Organic EL Displays
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Solution Overview
Problem
Current pixel circuit arrangements for organic EL displays face challenges in shortening the selection period per pixel while compensating for variations in threshold voltage, leading to increased display time and reduced pixel density due to the need for prolonged periods to charge floating capacitance and compensate threshold voltages.
Innovation Solution
The proposed display device incorporates a current-driven electro-optic element, a driving transistor, a first switching transistor, a second switching transistor, and a capacitor, where the driving transistor, first switching transistor, and electro-optic element are serially connected, with the second switching transistor placed between the current control and output terminals, allowing for adjustment of output current by changing the potential of the capacitor's terminals, thereby reducing the selection period and compensating threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional pixel circuit arrangements are used to compensate threshold voltage, then threshold voltage compensation is achieved, but the selection period per pixel is prolonged
Solution Approach 1:
The patent performs threshold voltage compensation in advance during a dedicated compensation period before the selection period begins. By completing the compensation action preliminarily, the subsequent selection period is shortened because the pixel circuit is already in its compensated state and does not need to wait for compensation during the selection phase.
Solution Approach 2:
The patent divides the operating cycle into distinct segments: a compensation period for threshold voltage compensation and a selection period for data input. This segmentation allows each function to be performed independently and efficiently in its own time window, preventing the selection period from being extended by compensation requirements.
2Reliability
If prolonged periods are used to charge floating capacitance and compensate threshold voltage, then proper current control is achieved, but pixel density is reduced
Solution Approach 1:
The patent performs both floating capacitance charging and threshold voltage compensation during a preliminary compensation period before the selection period. This preliminary action ensures that all necessary preparatory operations are completed in advance, allowing the selection period to proceed without delay and enabling higher pixel density.
Solution Approach 2:
The patent uses dynamic control of switching transistors to rapidly charge the floating capacitance and perform compensation operations during the compensation period. By dynamically managing the timing and duration of these operations, the system achieves proper current control accuracy while minimizing the time required, thus supporting higher pixel density.
3Reliability
If the pixel occupies the data wire during threshold voltage compensation, then compensation is completed, but the number of pixels that can be displayed is reduced
Solution Approach 1:
The patent segments the data wire usage into distinct time windows: a compensation period dedicated to threshold voltage compensation and a selection period dedicated to data input for pixel selection. By segmenting the timing, the data wire is fully utilized during the selection period without being occupied by compensation operations, thereby increasing the number of pixels that can be displayed.
Solution Approach 2:
The patent implements periodic operation where the compensation period and selection period alternate in a regular cycle. During the selection period, the data wire is exclusively available for pixel data input, while compensation occurs periodically in the compensation period. This periodic action ensures complete compensation while maximizing data wire utilization and pixel display capacity.
Data Source
AI summary
A display device which makes it possible to shorten a selection period per pixel while compensating variations in a threshold voltage of the driving transistor, and a method for driving the same are achieved. In a pixel circuit Aij, a potential wire Ui is set to a potential Vcc, a voltage of a gate wire Gi becomes Low, a voltage of a control wire Ri becomes High, and a voltage of a control wire Pi becomes High, so that a gate terminal of a driving TFT: Q1 has a potential of a data wire Dj. Moreover, a voltage of the gate wire Gi becomes High so as to compensate a threshold voltage of the driving TFT: Q1. Thereafter, a voltage of the control wire Pi becomes Low, and the potential wire Ui is set to a potential Vc, so that a voltage of a capacitor C1, i.e., a gate-source voltage of the driving TFT is changed. This causes a voltage of the control wire Ri to be Low, so that a driving current is flown into an organic EL: EL1.