Pixel Circuit With CCG-PWM Driving for Color-Stable Brightness
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Solution Overview
Problem
The use of pulse amplitude modulation (PAM) in driving light emitting elements like micro LEDs and OLEDs can cause color shift phenomena and image distortion due to the wavelength shift with varying driving currents.
Innovation Solution
A pixel circuit incorporating a constant current generation (CCG) circuit and a pulse width modulation (PWM) circuit to provide a fixed driving current with controlled generation time, using a combination of transistors and capacitors to stabilize voltage and compensate for threshold voltages, thereby maintaining a constant current amplitude.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pulse amplitude modulation (PAM) is used to drive light emitting elements, then brightness control is achieved by adjusting current amplitude, but wavelength shift occurs causing color distortion
Solution Approach 1:
The patent changes the control parameter from current amplitude (PAM) to pulse width (PWM). By maintaining constant current amplitude and varying only the duration of current pulses, the system achieves brightness control without the wavelength shift that occurs when current amplitude varies. This parameter transformation resolves the contradiction between brightness control capability and color stability.
2Ease of manufacture
If constant current generation circuit is added to maintain fixed current amplitude, then color stability improves, but device complexity increases
Solution Approach 1:
The patent merges the constant current generation function with the existing pixel circuit structure. The CCG circuit is integrated into the pixel circuit, sharing transistors and nodes with the PWM circuit. By combining these functions rather than adding a completely separate constant current source, the patent achieves color stability while minimizing the increase in device complexity.
Solution Approach 2:
The patent designs the CCG circuit to serve multiple functions: generating constant current for the light emitting element, providing compensation signals, and integrating with the PWM timing control. This multi-functionality reduces the need for separate dedicated circuits, thereby limiting the complexity increase while achieving color stability.
3Ease of manufacture
If pulse width modulation (PWM) is used to control brightness by adjusting pulse duration, then color stability improves, but power consumption increases
Solution Approach 1:
The patent implements PWM by periodically switching the current pulses on and off with varying duty cycles. The light emitting element receives current in periodic pulses rather than continuous DC, which reduces average power consumption while maintaining color stability. The periodic nature of PWM allows brightness control through pulse width variation without the energy waste of continuously varying current amplitude.
Data Source
AI summary
A pixel circuit includes a light emitting element, a CCG circuit, and a PWM circuit. The CCG circuit includes a seventh transistor including a gate electrode connected to a fourth node, a first electrode configured to receive a second high power supply voltage, and a second electrode connected to a fifth node, an eighth transistor including a gate electrode configured to receive a second write gate signal, a first electrode receiving a data voltage, and a second electrode connected to a sixth node, and a second capacitor including a first electrode connected to the sixth node and a second electrode connected to the fourth node.