Voltage Settling Circuitry for Image Sensor Column Lines
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Solution Overview
Problem
In imaging devices, there is a significant delay in column line voltage settling due to the limited drive strength of pixel source follower transistors, leading to image artifacts when sampling unsettled voltages, which necessitates delayed image sensor operations.
Innovation Solution
The implementation of voltage settling circuitry, including pre-charge, reset voltage slew boosting, and image signal slew boosting capabilities, accelerates the settling process by coupling column lines to reference voltages through transistors and current mirroring circuits, ensuring rapid voltage stabilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the drive strength of pixel source follower transistors is used to drive column lines, then the circuit complexity is kept low, but the voltage settling time becomes excessively long
Solution Approach 1:
The patent applies preliminary action by introducing a dedicated voltage settling circuit that performs the voltage settling function before the main readout operation. The settling circuit includes a settling transistor and settling resistor that are activated in advance to pre-establish the correct voltage levels on column lines, eliminating the need for the slow source follower transistors to perform this function during the readout sequence.
Solution Approach 2:
The patent uses an intermediary approach by inserting a dedicated voltage settling circuit between the pixel source follower transistors and the sampling circuitry. This intermediary settling circuit acts as a buffer that rapidly establishes voltages without requiring the main signal transistors to provide drive strength, thereby decoupling the slow transistor operation from the fast voltage settling requirement.
2Reliability
If voltage settling is delayed to ensure accurate sampling, then image artifacts are avoided, but the overall image capture time is significantly increased
Solution Approach 1:
The voltage settling circuit performs the settling action in advance before the sampling operation begins. By using a dedicated settling transistor and resistor network that is activated prior to readout, the system ensures voltages are fully settled before sampling occurs, eliminating artifacts while minimizing total time through parallel execution of settling and readout operations.
Solution Approach 2:
The patent implements continuous useful action by making the voltage settling process overlap with other readout operations. The settling circuit operates continuously during different phases of the readout sequence, and multiple column lines can be settled simultaneously in parallel, ensuring that voltage settling does not become a sequential bottleneck that delays the entire image capture process.
3Speed
If stronger drive circuits are used for column lines, then voltage settling speed increases, but the device complexity and power consumption increase
Solution Approach 1:
The patent applies segmentation by dividing the column line drive function into separate functional blocks: a weak source follower transistor for signal output and a separate dedicated settling circuit for voltage establishment. This segmentation allows each component to be optimized independently - the settling circuit uses strong drive transistors only when needed for voltage establishment, while the signal transistor remains simple and low-power.
Solution Approach 2:
The voltage settling circuit acts as an intermediary that handles the high-power voltage establishment function separately from the low-power signal transmission function. The settling circuit includes dedicated settling transistors and resistors that provide strong drive capability only during the voltage settling phase, without requiring continuous high-power operation, thus managing complexity and power consumption effectively.
Data Source
AI summary
An image sensor pixel may include a photodiode, a floating diffusion, and a transfer gate. Column readout circuitry coupled to the image sensor pixel via a column line. Voltage settling circuitry may be coupled to the column line. Voltage settling circuitry may include a pre-charging circuit, a reset voltage slew boosting circuit, and an image signal voltage slew boosting circuit. The pre-charging circuit may pull down the column line voltage to a grounding voltage. The reset voltage slew boosting circuit may pull up the column line voltage to a reference voltage near a reset level voltage. The image signal voltage slew boosting circuit may pull down the column line voltage to an additional reference voltage near an image signal voltage. With the use of the voltage settling circuitry, a faster pre-charge and clamping of the column line can be achieved.