Electroluminescent Display Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
In electroluminescent display devices, achieving desired luminance is hindered by threshold voltage variation of driving transistors, which affects pixel current, and maintaining gate voltage during light emission is challenging.
Innovation Solution
The electroluminescent display device incorporates a compensation circuit with capacitors and switching transistors to initialize nodes with data voltage and maintain gate voltage, using oxide transistors for stable operation, and a refresh transistor to unify node potentials, thereby compensating for threshold voltage variations and ensuring continuous voltage maintenance during emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional pixel circuit is used without sufficient initialization, then the circuit structure remains simple, but the node voltage cannot be properly initialized before data writing, leading to incorrect pixel current and luminance
Solution Approach 1:
The patent applies preliminary action by introducing a refresh transistor and refresh period that operates before the initialization period. The refresh transistor pre-initializes the second node voltage to the high-level source voltage ELVDD before the main initialization occurs, ensuring proper voltage levels are established in advance for accurate data writing and pixel current generation.
2Manufacturing precision
If threshold voltage variation of driving transistor is not compensated, then the compensation circuit structure remains simple, but pixel current cannot be accurately controlled, resulting in poor luminance uniformity
Solution Approach 1:
The patent implements feedback through the internal compensator consisting of the first capacitor Cst1 and second capacitor Cst2, along with switching transistors T1-T5. The compensator continuously monitors and adjusts the first node voltage to compensate for threshold voltage variations of the driving transistor, ensuring accurate pixel current control and luminance uniformity across different pixels and over time.
Solution Approach 2:
The compensation circuit performs preliminary compensation actions during the initialization period and refresh period, adjusting the node voltages before data writing and emission phases. This preliminary adjustment ensures that threshold voltage variations are accounted for before they affect pixel current, improving luminance uniformity without requiring complex real-time adjustment mechanisms.
3Reliability
If gate voltage is not continuously maintained during light emission, then the voltage control circuit remains simple, but the gate voltage drops during emission, causing incorrect pixel current and luminance
Solution Approach 1:
The patent ensures continuity of useful action by maintaining the gate voltage at the first node throughout the emission period. The internal compensator and switching transistors work continuously during emission to prevent voltage drops, ensuring the driving transistor receives stable gate voltage and produces accurate pixel current for consistent luminance throughout the light emission phase.
Solution Approach 2:
The circuit performs preliminary voltage setup during the initialization period before emission begins, establishing the correct gate voltage level. The refresh transistor also performs preliminary voltage restoration in the refresh period preceding initialization, ensuring the node is properly prepared and maintained for continuous operation during emission without voltage degradation.
Data Source
AI summary
An electroluminescent display device having a plurality of pixels is disclosed. Each pixel includes a driving transistor having a gate connected to a first node, a source connected to a third node, and a drain connected to a fourth node, the driving transistor generating pixel current corresponding to a data voltage when a high-level source voltage is applied to the third node, a light emitting element connected between the fourth node and an input terminal for a low-level source voltage, an internal compensator controlling voltages of the first to fourth nodes in accordance with operations of a plurality of switching transistors in an initialization period, a data writing period and an emission period, and a refresh transistor configured to apply the high-level source voltage to the second node in accordance with a scan signal in a refresh period preceding an initialization period.