Current-Driven Display Device Sub-Frame Write Control
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Solution Overview
Problem
Existing display devices face issues with screen burn-in, uneven luminance, and insufficient write time due to voltage rise and current fluctuations in organic electroluminescence (EL) elements, particularly in time-division gradation display modes.
Innovation Solution
A display device using a current-driven type pixel drive circuit with two write currents in a 1:1/2N ratio, generated by two current sources, to control lighting time in sub-frame periods, ensuring uniformity and high-order gradation display while reducing write time and circuit complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If voltage-driven type pixel circuit is used with simple 2T-1C configuration, then circuit complexity is reduced, but luminance uniformity deteriorates due to threshold voltage variations and mobility differences among pixels
Solution Approach 1:
The patent replaces voltage-driven operation with current-driven operation. Instead of controlling luminance through voltage levels that are sensitive to transistor parameters, the invention uses current sources to directly control the drive current through the organic EL element. This substitution of control mechanism eliminates the dependence on threshold voltage and mobility variations, achieving uniform luminance across all pixels while maintaining simple circuit configuration.
2Stability of the object's composition
If current-driven type pixel circuit is used to improve luminance uniformity, then luminance uniformity is improved, but write time becomes insufficient and circuit complexity increases due to multi-bit current source driver
Solution Approach 1:
The patent divides the frame period into multiple sub-frame periods, with each sub-frame dedicated to writing data for specific bit positions. This temporal segmentation allows the use of simple current sources that only need to generate a limited number of current levels (e.g., 1.05 times reference current and 1.2 times reference current), rather than requiring complex multi-bit current sources. The segmentation of writing operations across sub-frames ensures sufficient write time while maintaining circuit simplicity.
Solution Approach 2:
The patent changes the operating parameters of the current sources by using slightly elevated current levels (1.05 times and 1.2 times the reference current) instead of requiring precise multi-bit current control. This parameter adjustment, combined with time-division multiplexing across sub-frames, achieves high-order gradation display without requiring complex current source circuits or excessive write time.
3Manufacturing precision
If number of display gradations is increased to improve image quality, then image quality is improved, but sub-frame period becomes too short ensuring sufficient write time
Solution Approach 1:
The patent segments the writing process into multiple sub-frames, each handling specific bit positions or current combinations. This allows high-order gradation (e.g., 10-bit or 12-bit) to be achieved by combining data written in different sub-frames, rather than requiring all gradation levels to be written within a single sub-frame. The segmentation distributes the writing load across time, ensuring sufficient write time for each operation while achieving high precision display.
Solution Approach 2:
The patent performs preliminary writing of data in earlier sub-frames before the final display frame is rendered. By writing data incrementally across multiple sub-frames and maintaining it in storage capacitors, the system prepares the pixel states in advance, allowing high-order gradation to be achieved without requiring all data to be written simultaneously within a single sub-frame period.
4Use of energy by moving object
If maximum drive current is reduced to 1 mA or less to improve luminous efficiency, then luminous efficiency is improved, but accuracy in minimum gradation deteriorates
Solution Approach 1:
The patent changes the current ratio parameters between different current sources to maintain adequate signal levels even when maximum current is reduced. By using current sources with ratios such as 1.05 times and 1.2 times the reference current, the system ensures that even minimum gradation steps produce sufficient current signals for accurate detection and display, maintaining gradation accuracy while operating at low current levels for high luminous efficiency.
Data Source
AI summary
Display unevenness is suppressed. A display device includes pixels (22) arranged in matrix, each including a current-driven type light emitting element (EL) and a drive transistor (Tr3) for supplying a current to the current-driven type light emitting element (EL). The current-driven type light emitting element (EL) is driven by dividing each frame period into a plurality of sub-frame periods for lighting time. The drive transistor is controlled under current write driving using two write currents having a ratio of 1:1/2N and a sum of the two write currents.