Image Sensing System Dual Current Source Fingerprint Settling
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Solution Overview
Problem
Fingerprint recognition in large-scale panels is hindered by large parasitic capacitance in sensing lines, leading to increased settling time and degraded user experience due to the limitations of TFT process and high pin count in FTDI circuits, which necessitate time division schemes that prolong the fingerprint sensing process.
Innovation Solution
The implementation of a dual current source system, where one current source is integrated into the panel to pre-charge sensing lines and another in the image sensing circuit to provide accurate current during readout, allowing for faster settling and reduced noise in fingerprint sensing signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the panel scale is increased to accommodate more sensing pixels, then the fingerprint sensing coverage is improved, but the parasitic capacitance of sensing lines increases, leading to longer settling time
Solution Approach 1:
The patent applies preliminary action by pre-charging the sensing lines before actual fingerprint sensing operations. The control circuit charges the parasitic capacitance of sensing lines in advance, so that when sensing is required, the lines are already prepared and can respond faster, thereby reducing the settling time despite the large panel scale.
2Ease of manufacture
If the pin count of FTDI circuit is limited to control costs, then the manufacturing cost is reduced, but the number of sensing zones must be divided, requiring time division readout which increases time consumption
Solution Approach 1:
The control circuit performs preliminary charging of sensing lines before readout operations. This pre-charging action allows the system to maintain fast response times even when using time division readout across multiple zones, effectively decoupling the zone division requirement from the time consumption penalty.
Solution Approach 2:
The patent introduces a control circuit that acts as an intermediary between the FTDI circuit and the divided sensing zones. This control circuit copies or replicates the charging function across multiple zones, allowing parallel preparation of sensing lines while the FTDI circuit sequentially reads out data, thus reducing the overall time consumption.
3Adaptability or versatility
If TFT process is used to implement fingerprint sensor on panel, then the integration is achieved, but the performance is much worse than CMOS process
Solution Approach 1:
The patent introduces a control circuit as an intermediary component that compensates for the performance limitations of TFT process. This control circuit handles the complex charging and readout operations, allowing the TFT-based sensor to achieve performance levels comparable to or exceeding CMOS implementations while maintaining the integration advantage.
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 enhances the accuracy and speed of fingerprint recognition by pre-charging sensing lines, reducing fixed pattern noise and improving user experience through faster image acquisition and more precise signal processing.
Implementation Method 1
the parasitic capacitance of the sensing lines for transmitting the voltage signals becomes larger and larger
Implementation Method 2
the light may be reflected from the touch finger, and thus the reflected light including the information of peak and valley of the fingerprint may be received by photodiodes disposed on the panel, to generate voltage signals carrying the fingerprint information
Data Source
AI summary
An image sensing system includes a panel and an image sensing circuit. The panel includes a plurality of sensing pixels, a plurality of sensing lines and at least one first current source. Each of the plurality of sensing lines is coupled to a line of sensing pixels among the plurality of sensing pixels. Each of the at least one first current source is coupled to one of the plurality of sensing lines. The image sensing circuit, coupled to the panel, includes at least one second current source, each of which is coupled to one of the plurality of sensing lines. Wherein, a first sensing line among the plurality of sensing lines is coupled to a first current source among the at least one first current source and coupled to a second current source among the at least one second current source.