TFT Pixel Threshold Voltage Compensation Circuit With Source Follower
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Solution Overview
Problem
Existing pixel circuits for OLED displays face challenges in achieving uniform brightness due to variations in threshold voltage and carrier mobility of drive transistors, leading to non-uniform luminance and longer one horizontal time, which affects the display's responsiveness and resolution.
Innovation Solution
The proposed pixel circuit separates threshold compensation and data programming phases, using a second drive transistor as a source follower to isolate the drive transistor from power supply variations, allowing for ultra-short one horizontal time and maintaining accurate compensation of threshold voltages, thereby minimizing the impact of IR drops on OLED current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If threshold compensation and data programming are performed simultaneously in the same operational phase, then the circuit can be simpler, but the one horizontal time cannot be reduced further due to compensation accuracy requirements
Solution Approach 1:
The patent divides the previously combined threshold compensation and data programming operations into separate operational phases. The threshold compensation is performed in a first operational phase where the drive transistor is diode-connected, and data programming is performed in a second operational phase. This segmentation allows each phase to be optimized independently, reducing the one horizontal time while maintaining compensation accuracy.
2Measurement precision
If the drive transistor is diode-connected during programming period, then threshold voltage compensation accuracy is improved, but the programming time and one horizontal time are extended
Solution Approach 1:
The patent performs threshold voltage compensation in advance during a dedicated first operational phase before data programming begins. By completing the compensation measurement beforehand while the drive transistor is diode-connected, the system stores the compensated threshold voltage in a storage capacitor. This preliminary action allows data programming to proceed independently in a second phase without being constrained by compensation time requirements.
3Speed
If data programming is performed quickly to reduce one horizontal time, then display responsiveness is improved, but threshold compensation accuracy may be compromised
Solution Approach 1:
The patent segments the operational timeline into distinct phases: a first phase dedicated to threshold compensation and a second phase for data programming. This temporal segmentation allows the compensation phase to proceed at its required pace for accuracy, while the programming phase can be optimized for speed, thereby achieving both high compensation accuracy and fast display responsiveness.
4Device complexity
If power supply voltage variations are not isolated, then the circuit is simpler, but uniform luminance is degraded due to IR drop differences
Solution Approach 1:
The patent introduces a compensation capacitor as an intermediary element that decouples the drive transistor from power supply voltage variations. The capacitor stores the compensated threshold voltage and isolates it from IR drop fluctuations on the power supply line. This intermediary allows the circuit to maintain simple configuration while achieving uniform luminance by blocking the propagation of power supply variations to the OLED current.
Data Source
AI summary
A pixel circuit for a display device operable in an initialization phase, a compensation phase, a data programming phase, and an emission phase, whereby the one horizontal time is minimized while maintaining accurate compensation of the threshold voltages of the drive transistors, and further accounting for any variations in the voltage supplies. The pixel circuit includes a first drive transistor configured to control an amount of current to a light-emitting device during an emission phase depending upon voltages applied to a gate and a first terminal of the first drive transistor; and a second drive transistor that is configured as a source follower, wherein a first terminal of the second drive transistor is connected to a first power supply line and a second terminal of the second drive transistor is connected to a first terminal of the first drive transistor. The first drive transistor is one of a p-type or n-type transistor and the second drive transistor is the other of a p-type or n-type transistor. A light-emitting device is electrically connected at a first terminal to a second terminal of the first drive transistor during the emission phase and at a second terminal to a second power supply line.