Variable Drive Booster for CMOS Imaging Sensor Dark Current Reduction
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Solution Overview
Problem
Conventional boosters used in CMOS imaging sensors are insufficient in speed for rolling shutter and global shutter modes, often introduce row-to-row variation in voltage levels, and struggle with large load capacities, leading to issues like dark current and lag.
Innovation Solution
A booster system with a vertical output driver, pre-charger, and variable drive capability, including a voltage-controlled oscillator, charge pumps, resistor divider, comparator, and AND gate, that adjusts voltage levels and enables precise control over the booster's operation to enhance speed and uniformity across the pixel array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional boosters are used to boost voltages for pixel readout, then voltage levels can be increased above Vdd and below Ground, but the boosters lack sufficient speed for rolling shutter and global shutter modes and introduce row-to-row variation
Solution Approach 1:
The booster is divided into multiple parallel charge pump circuits, each capable of independently charging the boosted node. This segmentation allows the system to achieve higher effective charging speed by activating multiple pumps simultaneously, resolving the speed deficiency while maintaining voltage uniformity across all rows through parallel operation.
Solution Approach 2:
The patent implements dynamic control of the charge pump circuits through a control circuit that activates specific pumps based on operational mode (rolling shutter or global shutter) and row position. This dynamic activation strategy optimizes charging speed for each row while ensuring uniform voltage levels, resolving the contradiction between speed and reliability.
2Power
If conventional boosters are used, then voltage boosting is achieved, but they fail to drive large loads encountered in global shutter mode
Solution Approach 1:
The booster is segmented into multiple parallel charge pump circuits, each contributing to the total charging current available to drive the boosted node. When global shutter mode requires driving large loads, multiple pumps can operate in parallel to provide sufficient combined current capability, overcoming the limitation of single-pump conventional boosters.
Solution Approach 2:
The control circuit dynamically adjusts the number of active charge pumps based on the operational mode and load requirements. In global shutter mode with large loads, the control circuit activates additional pumps to increase the effective charging current, thereby changing the system's power delivery parameter to match the high demand of large load driving.
3Speed
If faster voltage transitions are implemented to reduce dark current and lag, then image capture speed is enhanced, but power consumption and noise may increase
Solution Approach 1:
The charge pump circuits operate periodically rather than continuously, with the control circuit activating pumps only when voltage boosting is required for each row during rolling shutter mode or at appropriate timing during global shutter mode. This periodic operation achieves fast voltage transitions when needed while minimizing power consumption during idle periods.
Solution Approach 2:
Different charge pump circuits can be optimized with different characteristics tailored to specific operational requirements. Some pumps may be designed for faster response to reduce dark current and lag, while others may prioritize efficiency, allowing the system to achieve fast transitions with reduced overall power consumption by assigning appropriate pump characteristics to appropriate operational contexts.
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
The solution significantly reduces dark current and lag by enabling faster and more uniform voltage boosting across the pixel array, improving the reliability and performance of CMOS imaging sensors in both rolling shutter and global shutter modes.
Implementation Method 1
each pixel cell in the array can include a photodetector (e.g., photogate, photoconductor, photodiode, . . . ) that overlays a substrate for yielding a photo-generated charge
Implementation Method 2
charge generated by the photodetector can be sent to the floating diffusion region
Data Source
AI summary
The claimed subject matter provides systems and/or methods that facilitate reducing dark current and lag in a CMOS imaging System-on-Chip (iSoC) sensor. For instance, a vertical output driver can output a signal upon a node connected to gates of reset transistors and/or gates of transfer transistors of pixels in the pixel array while operating in rolling shutter mode and/or global shutter mode. Further, a pre-charger can transition a voltage of the node to a first voltage level. Moreover, a booster can further adjust the voltage of the node from the first voltage level to a second voltage level. The booster can have variable drive capability that enables varying operation thereof according to at least one degree of freedom (e.g., speed of the booster proceeding to the second voltage level, frequency of yielding charge to the node, the second voltage level, or timing of the booster and the pre-charger, . . . ).