MicroLED Backplane Architecture Using Row PWM and Current Mirrors
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Solution Overview
Problem
Existing MicroLED displays face challenges in brightness, image uniformity, and luminance control due to issues such as dynamic false contouring, current mismatch, and parasitic capacitance, which affect image fidelity and visibility in bright natural light conditions.
Innovation Solution
Implementing a sub-pixel digital comparator with a shared pulse width row counter, single pulse width modulation with row-based precise control, and a dynamic current mirror to enhance brightness, accuracy, and uniformity, along with techniques to manage parasitic capacitance and current mismatch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional binary PWM techniques are used to control LED brightness, then transistor count is reduced, but image artefacts such as dynamic false contouring occur at high illumination duty cycles
Solution Approach 1:
The patent divides the PWM signal into multiple sequential pulses (e.g., 4 pulses per frame) with different duty cycles corresponding to different bit planes. Each pulse drives a subset of LEDs, allowing precise brightness control while avoiding the artifacts of traditional single-pulse or continuous PWM approaches. This segmentation enables accurate grey level reproduction without requiring excessive transistors per pixel.
2Measurement precision
If current sources are used to drive LEDs, then luminance control is improved, but current mismatch occurs between LEDs due to FET threshold variation
Solution Approach 1:
The patent implements a calibration process that measures the forward voltage of each LED before operation. These measured values are stored and used to pre-adjust the drive current for each LED, compensating for manufacturing variations in FET threshold voltages. This preliminary characterization allows the system to achieve uniform luminance across all pixels despite inherent component variations.
Solution Approach 2:
The system uses the measured forward voltage of each LED to dynamically adjust the drive current through feedback control. By continuously monitoring and adjusting current based on individual LED characteristics, the system maintains consistent luminance output across all pixels, eliminating the uniformity problems caused by FET threshold variation.
3Use of energy by moving object
If small pulse width modulation and low driving current are used for low luminance applications, then energy consumption is reduced, but parasitic capacitance in mLED cannot be overcome
Solution Approach 1:
The patent employs periodic pulsed driving with optimized duty cycles for different bit planes. By using multiple short pulses distributed throughout the frame period rather than continuous low-level driving, the system successfully charges the parasitic capacitance of mLEDs while maintaining low average power consumption. The periodic structure allows sufficient peak current to overcome capacitance without sustained high power draw.
4Device complexity
If voltage driving is used for LEDs, then circuit simplicity is improved, but luminance control accuracy deteriorates due to sharp current/voltage characteristics
Solution Approach 1:
The patent transitions from voltage-driven to current-driven LED control, recognizing that LEDs are inherently current-driven devices with sharp I-V characteristics. By changing the control parameter from voltage to current through dedicated current source circuits, the system achieves precise luminance control. The multiple-pulse PWM approach further refines this control by enabling fine-grained adjustment of average current while maintaining circuit feasibility.
Data Source
AI summary
Methods, apparatuses, and systems are described to display image data to a sub-pixel within a micro-LED (mLED) display. A row-based illumination period counter is used to operate sub-pixels within a row of the mLED display. An ON-state signal line and an illumination period signal line are coupled to the sub-pixels in the row. A first counter is incremented by an input clock signal and generates a first output. A second counter receives the first output and the second counter outputs, on the illumination period signal line, a second output that corresponds to an ON time for a sub-pixel. A sub-pixel mLED turns from the ON state to an OFF state when a stored sub-pixel data value is equal to or greater than the second output.