CMOS Image Sensor Pixel Reset Circuit for Lag Reduction
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Solution Overview
Problem
CMOS image sensors experience a lag phenomenon and reset level variation due to reduced pixel size, which affects dynamic range and image quality, and existing soft reset methods lead to substantial reset level variations during read-out operations.
Innovation Solution
Implementing a method that uses a hard reset with a boosted floating diffusion node by activating the transfer signal before the reset signal and switching to a higher power voltage for hard reset after a predetermined time, thereby maintaining a stable reset voltage level and reducing lag phenomenon.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a hard reset is used in reduced pixel size sensors, then the reset level is stable, but a lag phenomenon occurs due to insufficient potential difference between photodiode and floating diffusion node
Solution Approach 1:
The transfer signal is activated before the reset signal to preliminarily boost the floating diffusion node potential. This preliminary action creates sufficient potential difference between the photodiode and floating diffusion node before the hard reset occurs, preventing the lag phenomenon while maintaining reset stability.
Solution Approach 2:
The patent dynamically controls the timing sequence of transfer and reset signals. By making the reset process dynamic (activating transfer signal first, then reset signal after predetermined time), the system adapts to the reduced pixel size conditions and prevents lag phenomenon while maintaining stable reset level.
2Reliability
If a soft reset is used to prevent lag phenomenon, then the floating diffusion node is boosted, but substantial reset level variation occurs during read-out operation
Solution Approach 1:
The reset process is segmented into two distinct phases: first activating the transfer signal to boost the floating diffusion node, then activating the reset signal after a predetermined time. This segmentation allows the system to achieve both lag phenomenon prevention and reset level stability that a single soft reset cannot provide.
Solution Approach 2:
The patent changes the timing parameter by introducing a predetermined delay between transfer signal activation and reset signal activation. This parameter change transforms the soft reset behavior into a controlled two-stage process that eliminates reset level variation while preventing lag phenomenon.
3Area of moving object
If the pixel size is reduced, then the sensor resolution is improved, but the dynamic range characteristic deteriorates requiring higher pinning voltage
Solution Approach 1:
By preliminarily activating the transfer signal before the reset signal, the floating diffusion node is boosted in advance. This preliminary action compensates for the reduced pixel area effects, maintaining adequate dynamic range characteristics even in reduced pixel size sensors.
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 effectively removes the lag phenomenon and reset level variation, improving image quality by maintaining a stable reset voltage and preventing deterioration of the image quality caused by these issues.
Implementation Method 1
photo charges generated by the photodiode are transferred through the transfer transistor to a floating diffusion node
Data Source
AI summary
A method of driving a unit pixel may include activating a transfer signal prior to an activation of a reset signal to boost a floating diffusion node of the unit pixel, during a first section of a photodiode reset period; and activating a reset signal using a hard reset, during a second section of the photodiode reset period.