CMOS Image Sensor Ramp Generator Using Current Subtraction
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Solution Overview
Problem
Conventional integrator type ramp signal generators for CMOS image sensors require large feedback capacitors, occupying significant chip area and consuming high power, which is challenging for achieving high frame rates and high gain while maintaining resolution.
Innovation Solution
A ramp signal generator with a reduced size feedback capacitor is achieved through current subtraction, utilizing current supply and current subtraction units with PMOS and NMOS transistors to adjust current ratios and generate a ramp signal, thereby minimizing the capacitance value.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional integrator type ramp signal generator is used, then the ramp signal can be generated for ADC conversion, but the feedback capacitor size becomes large and occupies significant chip area
Solution Approach 1:
The patent changes the fundamental parameter of how the ramp signal is generated - transitioning from a capacitor-based integration approach to a current-based approach. By using current sources and current mirrors to directly generate the ramp signal without requiring large feedback capacitors, the chip area is significantly reduced while maintaining the necessary ramp signal generation capability for ADC conversion
Solution Approach 2:
The patent extracts and removes the large feedback capacitor from the traditional integrator circuit. By taking out the capacitor-based integration function and replacing it with a current-based ramp generation mechanism using current sources and mirrors, the design eliminates the area-consuming component while preserving the essential functionality
2Area of stationary object
If the feedback capacitor size is reduced, then chip area is saved, but the power consumption and current control become more challenging
Solution Approach 1:
The patent employs feedback mechanisms through current mirrors that precisely replicate and control the current flowing through the ramp generation circuit. The current mirror structure provides automatic feedback control, ensuring stable current operation and efficient power usage without requiring large capacitors, thereby reducing both area and power consumption simultaneously
Solution Approach 2:
The current-based ramp generation circuit is designed to be self-regulating through the inherent properties of current mirrors and current sources. The circuit automatically maintains proper current levels and operational parameters without requiring external power management components, achieving efficient power consumption with reduced capacitor size
Data Source
AI summary
Disclosed are a ramp signal generator capable of reducing the size of a feedback capacitor by using a current subtraction and a CMOS image sensor using the same. The ramp signal generator may include a current supply unit suitable for supplying a first current; a current subtraction unit suitable for subtracting the first current from a second current, or the second current from the first current; and a ramp signal generation unit suitable for generating a ramp signal according to a third current and a reference voltage. The third current is a result of the subtraction.