Image Sensor Ramp Bias Boosting for Fast Analog Gain Switching
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Solution Overview
Problem
Image sensing devices face challenges in stabilizing and efficiently generating ramp signals when analog gain changes, leading to increased transition times for bias voltages, which affects the reliability and performance of image capture.
Innovation Solution
The implementation of a boost circuit that adds a boost current to the output terminal of the cascode bias voltage during an initial period when the analog gain is changed, along with a ramp signal generation circuit that adjusts the slope of the ramp signal based on main and cascode bias voltages, to minimize transition times for both main and cascode bias voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If analog gain is changed, then the image sensing device can adapt to different lighting conditions, but the transition time for bias voltages increases
Solution Approach 1:
The patent applies preliminary action by pre-charging the cascode bias voltage node through a dedicated boost circuit before the actual gain switching occurs. This preliminary charging action reduces the voltage transition time when gain changes, as the node is already partially charged and requires less time to reach its target voltage level. The boost circuit is activated in advance to prepare the voltage node for the upcoming gain transition.
Solution Approach 2:
The patent implements periodic action through the boost circuit that is activated only during specific gain transition periods rather than continuously. The boost circuit operates periodically when gain switching is detected, providing temporary current enhancement to accelerate voltage transitions. This periodic activation reduces overall power consumption while maintaining fast transition capability when needed.
2Speed
If a boost circuit is added to reduce transition time, then the speed of bias voltage transition improves, but the device complexity increases
Solution Approach 1:
The patent uses an intermediary approach by introducing a boost circuit that acts as a mediator between the existing bias voltage generation circuit and the cascode bias voltage node. This intermediary circuit provides temporary current enhancement only when needed during transitions, rather than permanently modifying the main bias circuit. The boost circuit includes a transistor that couples to the cascode bias voltage node and provides additional charging current during transition periods.
Solution Approach 2:
The patent applies dynamics by making the boost circuit dynamically controllable through a control signal that activates it only during gain transition periods. The circuit transitions from a static bias voltage generation system to a dynamic system where the boost circuit is selectively enabled based on switching signals. This dynamic operation allows the circuit to adapt its behavior - providing fast transitions when switching occurs and normal operation during steady states.
Data Source
AI summary
An image sensing device and an operating method thereof. The image sensing device includes a ramp signal generation circuit suitable for generating a ramp signal which corresponds to an analog gain, based on a main bias voltage, a cascode bias voltage and a plurality of ramp code signals, a bias voltage generation circuit suitable for generating the main bias voltage and the cascode bias voltage according to the analog gain, and a boost circuit suitable for boosting an output terminal of the cascode bias voltage according to the analog gain.