OLED PWM Brightness Control via Emission Enable Signals
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Solution Overview
Problem
Accurately controlling the brightness and color of organic light-emitting diode displays, particularly at low gray levels, is challenging due to variations in efficiency and response of emissive organic materials, making it difficult to calibrate display performance effectively.
Innovation Solution
The implementation of a display system with an array of pixels including light-emitting diodes, drive transistors, emission enable transistors, and switching transistors, utilizing pulse-width modulation and two-phase clock signals to control emission enable signals, allowing for precise control of light-emitting diodes through emission control gate driver circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional brightness control methods are used, then the display can operate with simple control circuitry, but the brightness and color control accuracy deteriorates at low gray levels due to efficiency variations in organic light-emitting diodes
Solution Approach 1:
The patent applies pulse-width modulation (PWM) to control the emission enable signal, creating periodic on/off cycles that allow precise brightness control at low gray levels. The emission enable signal transitions between active and inactive states in periodic cycles, with the duty cycle determining the perceived brightness level, thereby achieving high precision brightness control without requiring complex analog control circuitry.
Solution Approach 2:
The patent implements dynamic control of the emission enable signal through PWM, where the signal transitions from static continuous control to dynamic periodic switching. This dynamic approach allows the display to adapt brightness levels precisely by varying the proportion of active time in each PWM cycle, resolving the contradiction between control accuracy and circuit simplicity.
2Manufacturing precision
If simple drive schemes are used, then the device complexity is reduced, but the ability to compensate for threshold voltage variations and control low gray levels accurately deteriorates
Solution Approach 1:
The patent incorporates threshold voltage compensation operations that are performed in advance during the non-emission phase. The emission enable signal is controlled to remain inactive during threshold voltage compensation, allowing the pixel circuit to pre-calculate and store compensation values before the emission phase begins. This preliminary action ensures accurate brightness control without adding complex real-time control circuitry.
Solution Approach 2:
The patent segments the emission enable signal control into distinct phases: a non-emission phase for threshold voltage compensation and data loading, and an emission phase for light output. This temporal segmentation allows complex operations to be performed sequentially rather than simultaneously, reducing the instantaneous circuit complexity while maintaining high calibration accuracy.
3Measurement precision
If continuous emission control is used, then the control circuitry is simpler, but the precision of brightness modulation at low gray levels deteriorates due to efficiency dependencies on drive current
Solution Approach 1:
The patent uses pulse-width modulation to convert continuous emission control into periodic switching control. The emission enable signal switches between on and off states in regular cycles, with the brightness determined by the ratio of on-time to total cycle time. This periodic approach provides precise brightness modulation at low gray levels by avoiding the efficiency variations that plague continuous analog control.
Solution Approach 2:
The patent replaces analog continuous control mechanisms with digital periodic switching control. Instead of using continuous analog voltage or current control that is sensitive to efficiency variations, the system uses digital PWM switching that is inherently more precise and less susceptible to material efficiency variations in organic light-emitting diodes.
Data Source
AI summary
A display may have an array of pixels arranged in rows and columns. Display driver circuitry may load data into the pixels via data lines that extend along the columns. The display driver circuitry may include gate driver circuitry that supplies horizontal control signals to rows of the pixels. The horizontal control signals may include emission enable signals for controlling emission enable transistors and scan signals for controlling switching transistors. During an emission phase of operation for the display, the emission enable signal may be pulse-width modulated by the emission control gate driver circuits in the gate driver circuitry to control the output of the light-emitting diodes. The emission control gate driver circuits may be controlled using an emission start signal and a pair of two-phase clocks.