AM OLED Pixel Circuit PWM Dimming Color Balance
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Solution Overview
Problem
Existing AM OLED displays face challenges in achieving wide dynamic range dimming without affecting color balance and luminance uniformity, particularly in applications requiring high dimming ratios like cockpit and avionics displays, due to limitations in current driving methods and transistor performance at low luminance levels.
Innovation Solution
The implementation of Pulse Width Modulation (PWM) of the OLED pixel current, either externally by modulating the common cathode or power supply voltage, or internally through additional transistors, to achieve desired luminance levels while maintaining color balance and uniformity across the display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the grayscale driving conditions or VDD/VK voltages of AM OLED displays are changed to achieve lower display luminance levels for night-time viewing, then the display luminance is reduced, but luminance and color non-uniformities appear across the display surface
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the driving voltage parameters (VDD and VK) based on the desired luminance level. Instead of using fixed optimized driving conditions, the system modifies voltage parameters in real-time to achieve different luminance levels while compensating for the resulting non-uniformities through controlled parameter variation across the display matrix.
Solution Approach 2:
The invention implements dynamics by transitioning from static optimized driving conditions to dynamic voltage adjustment. The system continuously adapts the VDD and VK voltages based on ambient lighting conditions and desired luminance levels, enabling the display to maintain uniformity across different operating states rather than being fixed at optimal daytime conditions.
2Illumination intensity
If existing AM OLED displays are dimmed by changing grayscale data voltage or VDD/VK voltage, then the desired luminance level is achieved, but color balance is changed due to different transfer characteristics of R, G, B OLED materials
Solution Approach 1:
The patent addresses color balance by independently controlling the VDD and VK voltage parameters for each color sub-pixel (Red, Green, Blue). By adjusting these voltage parameters differently for each color channel, the system compensates for the different transfer characteristics of R, G, B OLED materials, maintaining accurate color balance across the full luminance range from daytime to night-time viewing conditions.
3Adaptability or versatility
If AM LCDs use backlighting to provide wide dynamic range dimming capability, then the dimming function is achieved, but weight and volume of the display increase significantly
Solution Approach 1:
The invention extracts and eliminates the backlighting unit from the display structure, retaining only the essential OLED emission layers and thin-film transistor circuitry. By removing the heavy backlight assembly while implementing voltage-controlled luminance adjustment, the system achieves wide dynamic range dimming capability without the weight and volume penalty of traditional LCD backlighting systems.
Solution Approach 2:
Instead of using physical backlighting components to achieve dimming, the patent employs electrical parameter changes (VDD and VK voltage adjustment) to control luminance output. This electrical control method replaces the need for physical backlight hardware, enabling compact, lightweight display design with full dimming functionality.
4Adaptability or versatility
If AM LCDs use backlighting to achieve wide dynamic range dimming, then the dimming function is achieved, but the volume of the display increases
Solution Approach 1:
The patent removes the bulky backlighting unit from the display architecture, keeping only the thin OLED active matrix structure. This extraction of unnecessary components dramatically reduces display volume while preserving wide dynamic range dimming capability through voltage control of the OLED emission layers.
Solution Approach 2:
The invention utilizes the inherently thin structure of OLED films and flexible substrate technology to create a compact display with minimal volume. By relying on thin-film transistor control and voltage-adjustable luminance rather than thick backlight assemblies, the display achieves both compact form factor and full dimming range capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables AM OLED displays to achieve wide dynamic range dimming (>2000:1) with improved uniformity and color balance, addressing the limitations of existing methods and meeting stringent requirements for avionics, cockpit, and hand-held military device applications.
Implementation Method 1
Active matrix organic light emitting diode (AM OLED) display
Data Source
AI summary
An improved AM OLED pixel circuit and method of wide dynamic range dimming for AM OLED displays are disclosed that maintain color balance throughout the dimming range, and also maintain the uniformity of the luminance and chromaticity of the display at low gray-levels as the display is dimmed to lower luminance values. As such, AM OLED displays can meet the stringent color/dimming specifications required for existing and future avionics, cockpit, and hand-held military device display applications. Essentially, the OLED pixel circuit and method of dimming that are disclosed use Pulse Width Modulation (PWM) of the OLED pixel current to achieve the desired display luminance. Two example circuits are disclosed that externally PW modulate the common cathode voltage or common power supply voltage to modulate the OLED current in order to achieve the desired display luminance. Three example circuits are disclosed that incorporate additional transistor switches in the pixel circuit to modulate the OLED current during the frame time. By PWM of the OLED current, in combination with data voltage (or current) modulation, wide dynamic range dimming can be achieved while maintaining the color balance and the luminance and chromaticity uniformity required over the surface of the display involved.