Global Shutter Control Signal Generator Reduces N-Pump Drive Requirements
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Solution Overview
Problem
Global shutter image sensors face challenges in achieving simultaneous pixel cell activation for fast-moving objects, as existing technologies require a large driving capability for the N-pump to turn off global shutter switches in all pixel cells, leading to increased power consumption and potential image distortion.
Innovation Solution
A global shutter control signal generator that transitions the NMOS gate voltage from AVDD to NVDD, then to GND, and finally to NVDD, reducing the drive requirements for the N-pump and enabling efficient simultaneous activation of all pixel cells with reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a global shutter switch is used to reset all pixel cells simultaneously, then image capture accuracy for fast-moving objects is improved, but the driving capability requirement for the N-pump increases significantly
Solution Approach 1:
The patent segments the voltage transition process into two distinct phases: first transitioning from AVDD to GND (ground), then from GND to NVDD (negative voltage). This segmentation reduces the instantaneous drive requirement by handling voltage changes in steps rather than directly from AVDD to NVDD, resolving the contradiction between maintaining global shutter functionality and reducing power requirements.
Solution Approach 2:
The patent applies preliminary action by first setting the gate voltage to GND (ground) before transitioning to the final NVDD state. This intermediate preliminary step prepares the circuit by discharging capacitance and reducing voltage stress, thereby lowering the subsequent drive requirement when transitioning to NVDD, while still achieving the desired global shutter reset functionality.
2Use of energy by moving object
If the N-pump drive capability is reduced to lower power consumption, then power efficiency is improved, but the ability to simultaneously turn off all global shutter switches deteriorates
Solution Approach 1:
The patent introduces GND (ground) as an intermediary voltage state between AVDD and NVDD. This intermediary serves as a buffer that allows the N-pump to operate with reduced drive capability while still achieving reliable simultaneous switch activation. The intermediary ground state facilitates controlled voltage transitions that maintain reliability without requiring high instantaneous power.
Solution Approach 2:
The patent employs periodic action by implementing a two-stage voltage transition sequence: first AVDD to GND, then GND to NVDD. This periodic, staged approach allows the system to achieve reliable global shutter activation with reduced power consumption, as each stage operates within lower voltage differential limits that the reduced-capability N-pump can handle.
3Device complexity
If a direct voltage transition from AVDD to NVDD is used, then the number of voltage transition steps is reduced, but image distortion increases due to insufficient drive capability
Solution Approach 1:
The patent segments the voltage transition into controlled stages (AVDD→GND→NVDD) to prevent image distortion. This segmentation ensures that each transition step remains within the drive capability limits of the reduced-power N-pump, thereby maintaining image quality while managing device complexity through systematic voltage management rather than direct transitions.
Solution Approach 2:
The patent applies parameter changes by modifying the voltage transition profile from a direct single-step change to a two-step process with an intermediate ground state. This parameter modification (adding an intermediate voltage level) ensures that the N-pump operates within its reduced drive capability while still achieving reliable simultaneous switch activation without image distortion.
Data Source
AI summary
A pixel cell includes a photodiode disposed in a semiconductor material to accumulate image charge in response to light. A global shutter transistor is disposed in the semiconductor material and is selectively resets the image charge in the photodiode in response to a global shutter control signal. A global shutter control signal generator circuit is coupled to generate the global shutter control signal to have a first value, a second value, and a third value. The first value of the global shutter control signal is coupled to turn on the global shutter transistor to reset the photodiode. The third value of the global shutter control signal is coupled to control the global shutter transistor to be in a low leakage off mode. The second value of the global shutter control signal is between the first and third values and is turns off the global shutter transistor.