TFT Pixel Threshold Voltage Compensation Circuit with Source Follower
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Solution Overview
Problem
Existing OLED pixel circuits face challenges in achieving uniform brightness due to variations in threshold voltage and carrier mobility of drive transistors, leading to non-uniform display output and increased bezel size from additional control signal lines, while also struggling with drive transistor hysteresis and power supply voltage variations.
Innovation Solution
The proposed pixel circuit separates threshold compensation and data programming phases, uses a scan signal from another row to eliminate dedicated data signal lines, and incorporates an on-bias stress transistor to reduce hysteresis effects, with a source follower transistor isolating the drive transistor from power supply variations, allowing for ultra-short one horizontal time and reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If data programming and threshold compensation are performed simultaneously in the same operational phase, then the circuit operation is simplified, but the one horizontal time cannot be reduced further due to compensation accuracy requirements
Solution Approach 1:
The patent divides the operational phases by using different scan signals for different functions: one scan signal phase is dedicated to data programming while another scan signal phase is dedicated to threshold compensation. This segmentation allows both operations to occur in separate time windows, enabling the one horizontal time to be reduced to primarily cover only the data programming phase while compensation occurs independently in another phase.
2Measurement precision
If a dedicated data signal line is used for data programming, then data programming accuracy is improved, but the bezel size increases
Solution Approach 1:
The patent makes the scan signal line multi-functional by using it for both data programming and threshold compensation operations at different time phases. The same scan signal line that normally carries scan signals for row selection is reused to carry compensation signals during compensation phases, eliminating the need for separate dedicated data signal lines and reducing bezel size while maintaining programming accuracy.
3Measurement precision
If additional control signal lines are added for threshold compensation, then compensation accuracy is improved, but the bezel size increases
Solution Approach 1:
The patent reuses existing scan signal lines and control infrastructure for threshold compensation operations. By controlling the timing and phase of signal application, the same scan signal lines that are used for row selection and data programming are also used for compensation, eliminating the need for additional dedicated compensation signal lines and avoiding increased bezel size.
4Speed
If the one horizontal time is reduced for faster display responsiveness, then display responsiveness is improved, but threshold compensation accuracy deteriorates
Solution Approach 1:
The patent segments the timing so that data programming occurs in one phase with a short time window (determining display responsiveness) while threshold compensation occurs in a separate, independent phase. This allows the one horizontal time to be defined by the shorter data programming phase rather than being constrained by the longer compensation requirements, achieving both fast responsiveness and accurate compensation.
Data Source
AI summary
An enhanced pixel circuit for a display device provides separate compensation and data programming phases to permit minimization of the programming time. Variations in voltage supplies are accounted for by isolating the drive transistor from such power supply by using a second drive transistor configured as source follower relative to the first drive transistor. An on bias stress transistor is incorporated that can electrically connect the first drive transistor to the power supply during an on bias stress operation. During such operation, a voltage stress is applied to eliminate hysteresis effects associated with the drive transistor. The on bias stress operation may be performed as part of a refresh operation during which a data voltage is programmed to the pixel circuit, or as part of a low frequency operation during which a previously programming data voltage is maintained which reduces power consumption.