OLED Pixel Circuit Initialization and Compensation Timing
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Solution Overview
Problem
In organic electroluminescent displays, the variation in threshold voltages of driving transistors leads to non-uniform image brightness, and existing solutions concurrently perform initialization and compensation, causing unwanted emission and degrading contrast ratio, especially in larger displays.
Innovation Solution
A pixel circuit design where initialization and compensation are performed during separate time periods, using a specific configuration of NMOS transistors and a storage capacitor to isolate the driving current from threshold voltage variations, with additional NMOS transistors managing reference voltages and emission control signals to maintain uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If initialization and compensation are performed concurrently, then the circuit operation is simplified, but unwanted emission occurs and contrast ratio deteriorates
Solution Approach 1:
The patent divides the initialization and compensation operations into separate time periods using distinct scan signals. The initialization signal (first scan signal) and compensation signal (second scan signal) are applied at different times, separating functions that were previously concurrent. This temporal segmentation eliminates unwanted emission during initialization while maintaining simplified circuit operation.
2Loss of time
If initialization and compensation are performed concurrently, then the processing time is reduced, but image brightness uniformity deteriorates
Solution Approach 1:
The patent implements periodic action by using separate scan periods for initialization and compensation. The first scan signal activates during an initialization period, while the second scan signal activates during a compensation period. This periodic separation ensures that initialization completes fully before compensation begins, guaranteeing uniform image brightness across the display while maintaining efficient processing timing.
3Device complexity
If threshold voltage compensation is not performed, then the circuit operation is simpler, but image brightness uniformity deteriorates
Solution Approach 1:
The patent applies preliminary action by performing threshold voltage compensation during a dedicated compensation period before the emission period begins. The second scan signal activates the compensation circuitry in advance, ensuring that threshold voltage variations are corrected before image display. This preliminary compensation maintains simple circuit operation while achieving uniform image brightness.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach ensures uniform image brightness, reduces unwanted emission, and improves contrast ratio by decoupling initialization from compensation, effectively addressing issues in larger displays and preventing motion blur and cross-talk.
Implementation Method 1
organic light emitting displays display images using organic light emitting diodes (OLEDs), which generate light via recombination of electrons and holes
Implementation Method 2
a storage capacitor coupled between the first node and a second node
Data Source
AI summary
A pixel circuit for an organic light emitting display includes: a fourth NMOS transistor including a gate electrode coupled to a first scan line, and a first electrode coupled to a first node; a storage capacitor coupled between the first node and a second node; a third NMOS transistor including a gate electrode coupled to a second scan line, and a first electrode coupled to a data line; a second NMOS transistor including a first electrode coupled to a second electrode of the third NMOS transistor, and a gate electrode and a second electrode coupled to the first node; a fifth NMOS transistor including a gate electrode coupled to an emission control line, and a first electrode coupled to a first power source; an organic light emitting diode (OLED) comprising an anode coupled to the second node; and a first NMOS transistor for providing a driving current to the OLED.