SPAD Shutoff Voltage Clamp Circuit for Safe Off-State Bias
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Solution Overview
Problem
Fully depleted single photon avalanche diodes (SPADs) face challenges in maintaining the anode voltage at a safe level when turned off to prevent damage to transistors, as the required voltage is high enough to cause damage.
Innovation Solution
A turn-off circuit is introduced, including a regulation circuit that sinks current from a turn-off voltage node to ground based on feedback voltage, maintaining a constant voltage level regardless of photocurrent, using an amplifier and transistor configuration with a resistive divider and additional diodes to manage the photocurrent and keep the SPAD in a non-avalanchable condition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the anode voltage is kept at 5 to 7 volts to keep the SPAD off, then the SPAD remains non-avalanchable, but the transistors T1 and T2 are damaged
Solution Approach 1:
A clamp diode is introduced as an intermediary component between the anode and the turn-off voltage node. The diode's cathode is coupled to a turn-off voltage node that is maintained at a safe voltage level (below transistor damage threshold) by a regulation circuit. This intermediary structure allows the anode to be held at the required 5-7V off-state voltage while protecting the transistors from damage through the diode's voltage clamping action.
Solution Approach 2:
A regulation circuit with feedback control is implemented to maintain the turn-off voltage node at a constant safe voltage level. The circuit includes a sense circuit coupled between the turn-off voltage node and ground that generates a feedback voltage, and a regulation circuit that sinks current from the turn-off voltage node based on the feedback voltage. This feedback mechanism ensures the voltage at the turn-off voltage node remains constant and safe, preventing transistor damage while maintaining SPAD off-state stability.
2Object-affected harmful factors
If a clamp diode is added to protect transistors, then transistor damage is prevented, but the device complexity increases
Solution Approach 1:
The regulation circuit is designed to automatically maintain the turn-off voltage node at the correct voltage level without requiring external intervention. The sense circuit continuously monitors the voltage and the regulation circuit automatically adjusts the current sinking to maintain constant voltage, making the protection mechanism self-regulating and reducing the need for additional complex control circuitry.
3Reliability
If a regulation circuit is added to maintain constant turn-off voltage, then transistor protection is improved, but power consumption and area increase
Solution Approach 1:
The regulation circuit operates autonomously using feedback from the sense circuit to automatically maintain the turn-off voltage node at the correct voltage level. This self-regulating mechanism eliminates the need for additional power-consuming control logic or manual adjustment, minimizing the power overhead of the protection circuitry.
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 solution effectively maintains a constant turn-off voltage, reducing the risk of transistor damage and improving efficiency by minimizing area and power consumption, suitable for applications like time-of-flight ranging and LiDAR.
Implementation Method 1
a sense circuit coupled between the turn-off voltage node and ground and configured to generate a feedback voltage
Implementation Method 2
A turn-off circuit is introduced, including a regulation circuit that sinks current from a turn-off voltage node to ground based on feedback voltage, maintaining a constant voltage level
Implementation Method 3
The regulation circuit may include an amplifier having a first input coupled to a reference voltage and a second input coupled to receive the feedback voltage
Implementation Method 4
a transistor having a gate coupled to an output of the amplifier, a first conduction terminal coupled to the turn-off voltage node, and a second conduction terminal coupled to ground
Implementation Method 5
When the SPAD has its anode disconnected from ground by the enable circuit and is exposed to light, the SPAD may generate a photocurrent
Data Source
AI summary
Described herein is an electronic device, including a pixel and a turn-off circuit. The pixel includes a single photon avalanche diode (SPAD) having a cathode coupled to a high voltage node and an anode selectively coupled to ground through an enable circuit, and a clamp diode having an anode coupled to the anode of the SPAD and a cathode coupled to a turn-off voltage node. The turn-off circuit includes a sense circuit coupled between the turn-off voltage node and ground and configured to generate a feedback voltage, and a regulation circuit configured to sink current from the turn-off voltage node to ground based upon the feedback voltage such that a voltage at the turn-off voltage node maintains generally constant.