CBVP Pixel Circuit for AMOLED Settling Time Reduction
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Solution Overview
Problem
Active-matrix organic light-emitting diode (AMOLED) displays face challenges in accurately providing a constant drive current due to small current requirements and large parasitic capacitance, making it difficult to design external drivers and manage current scaling in current-programmed pixel circuits.
Innovation Solution
A pixel circuit with a driving transistor, storage capacitor, and switch transistors is implemented, using a current-biased voltage-programmed (CBVP) driving scheme that provides programming voltage and bias current to accelerate programming and compensate for time-dependent parameters like threshold voltage shifts, ensuring stable current delivery to the OLED.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current programming is used to drive AMOLED pixels, then the OLED can be driven with accurate current, but the settling time increases due to small current and large parasitic capacitance
Solution Approach 1:
A bias current source is introduced as an intermediary element that injects additional current into the programming node. This bias current acts as a mediator that accelerates the charging of the parasitic capacitance during the programming phase, thereby reducing settling time without affecting the final accurate current level that is programmed into the OLED.
Solution Approach 2:
The bias current is applied periodically only during the programming phase and is turned off during the emission phase. This periodic application of bias current allows fast programming when needed while maintaining accurate current delivery during the emission phase, effectively managing the time-current accuracy tradeoff.
2Measurement precision
If current scaling is used to manage small OLED current requirements, then current levels can be adjusted, but mismatch effects increase when resizing mirror transistors
Solution Approach 1:
Instead of changing the physical size of transistors to scale current, the invention changes the electrical parameter (bias current magnitude) to achieve current scaling. This allows current level adjustment without modifying transistor dimensions, thereby avoiding the mismatch problems that arise from resizing transistors while maintaining all transistors at optimal sizes.
3Measurement precision
If external drivers are designed to supply small OLED current, then accurate current delivery can be achieved, but the driver design becomes difficult and complex
Solution Approach 1:
The pixel circuit itself generates and manages the bias current internally through the bias current source, eliminating the need for complex external driver circuits to precisely control small currents. The pixel circuit serves itself by incorporating the current scaling and acceleration functions within the pixel, thereby simplifying external driver design while maintaining accurate current delivery.
Data Source
AI summary
A light emitting device display, its pixel circuit and its driving technique is provided. The pixel includes a light emitting device and a plurality of transistors. A bias current and programming voltage data are provided to the pixel circuit in accordance with a driving scheme so that the current through the driving transistor to the light emitting device is adjusted.