Pixel Circuit PWM Emission Control for Micro LED Uniformity
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Solution Overview
Problem
Micro LEDs face challenges in accurately implementing desired luminance due to peak wavelength shifts with current density, especially when using Pulse Amplitude Modulation (PAM) pixel driving methods, and require internal compensation circuits to address driving characteristic differences between pixels.
Innovation Solution
A pixel circuit employing Pulse Width Modulation (PWM) driving with an emission time controller and compensation operation controller to adjust luminance by controlling the emission time of a light-emitting element within one image frame period, utilizing transistors and capacitors to compensate for threshold voltage and mobility variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If Pulse Amplitude Modulation (PAM) pixel driving method is used for micro LEDs, then the driving circuit can be simplified, but the peak wavelength shifts with current density making it difficult to accurately implement desired luminance
Solution Approach 1:
The patent changes the control parameter from current magnitude (PAM) to emission time duration (PWM). The emission time controller adjusts the luminance by controlling how long the light-emitting element is activated within each frame period, rather than varying the current strength. This resolves the wavelength shift issue while maintaining driving circuit simplicity.
Solution Approach 2:
The patent employs periodic pulse-width modulation where the light-emitting element is turned on and off in periodic cycles. By varying the duty cycle (ratio of on-time to total period), the average luminance is controlled without changing the peak current, thus avoiding wavelength shifts while maintaining simple driving circuitry.
2Reliability
If internal compensation circuit is added to micro LED display, then driving characteristic differences between pixels can be compensated, but the device complexity increases
Solution Approach 1:
The emission time controller serves multiple functions: it controls the emission time for PWM driving, performs threshold voltage compensation, and manages mobility compensation all through a single integrated circuit structure. This multi-functionality achieves pixel characteristic consistency without proportionally increasing device complexity.
Solution Approach 2:
The patent merges the compensation functions (threshold voltage compensation and mobility compensation) into the same emission time controller that handles PWM driving. By combining these functions in one circuit rather than adding separate compensation circuits, the overall device complexity is minimized while achieving reliable pixel uniformity.
3Measurement precision
If Pulse Width Modulation (PWM) driving method is used to control emission time, then luminance accuracy is improved, but additional control circuits are required increasing device complexity
Solution Approach 1:
The emission time controller automatically adjusts the emission duration based on the PWM data voltage input without requiring external intervention or complex additional control circuits. The controller self-regulates the turn-off timing of the driving transistor, achieving accurate luminance control through intrinsic circuit operation.
Solution Approach 2:
The emission time controller incorporates feedback mechanisms where the PWM data voltage is monitored and used to dynamically adjust the emission time. The controller continuously adapts the drive signal based on the input voltage level, ensuring accurate luminance control while maintaining a relatively simple circuit architecture through intelligent feedback-based timing control.
Data Source
AI summary
A pixel circuit including a constant current driver including a first driving transistor for controlling a current through a light-emitting element, a first electrode thereof being connected to a first power source, and a second electrode thereof being connected to an anode electrode of the light-emitting element, and an emission time controller including a second driving transistor between a second power source and the constant current driver, and configured to control the emission time by turning off the first driving transistor after a time at which a Pulse Width Modulation data voltage corresponding to an input image is applied to a gate electrode of the second driving transistor, and based on a magnitude of the PWM data voltage, wherein the second driving transistor is turned on based on the magnitude of the PWM data voltage, to transfer a driving voltage from the second power source to the constant current driver.