SPAD Turn-Off Voltage Feedback Circuit for Safe Off-State Biasing
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Solution Overview
Problem
Fully depleted single photon avalanche diode (SPAD) pixels face challenges in maintaining the SPAD in an off state without damaging the transistors, as the required anode voltage is too high, leading to potential transistor damage.
Innovation Solution
A turn-off circuit is introduced, comprising a sense circuit, an amplifier, and a resistive divider, which generates a feedback voltage to maintain a constant turn-off voltage, ensuring the SPAD remains non-avalanchable by sinking sufficient current proportional to the photocurrent, and utilizing a charge pump to regulate the voltage and reduce power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the anode voltage is kept high (5-7 volts) to maintain the SPAD in an off state, then the SPAD remains non-avalanchable, but the transistors T1 and T2 may be damaged
Solution Approach 1:
The patent introduces a clamp diode D3 coupled between the anode of the SPAD and a turn-off voltage node VSPADOFF. This clamp diode acts as an intermediary element that actively regulates the anode voltage to a safe level when the SPAD is intended to be off, preventing the high voltage from damaging transistors T1 and T2 while still maintaining the SPAD in a non-avalanchable state.
2Object-affected harmful factors
If a clamp diode is used to regulate the turn-off voltage, then the transistors are protected from damage, but the circuit complexity increases
Solution Approach 1:
The patent employs a feedback mechanism where a sense circuit monitors the voltage at the turn-off voltage node VSPADOFF and generates a feedback signal. This feedback signal is processed by an amplifier that adjusts the control signal to a transistor, thereby dynamically regulating the clamp diode's operation to maintain the anode voltage at a safe level, balancing protection with controlled complexity.
3Manufacturing precision
If a turn-off circuit with feedback regulation is implemented, then the anode voltage is precisely controlled, but the area and power consumption increase
Solution Approach 1:
The patent utilizes parameter changes by dynamically adjusting the clamp diode's clamping voltage based on the SPAD's operational state. The sense circuit detects voltage conditions and the feedback mechanism modifies the turn-off voltage parameter in real-time, achieving precise voltage control without requiring overly complex circuitry that would increase area and power consumption significantly.
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
The solution effectively maintains the SPAD in a non-avalanchable condition while reducing area and power consumption, enhancing efficiency and suitability for applications like time-of-flight ranging and LiDAR.
Implementation Method 1
a sense circuit configured to generate a feedback voltage based upon a voltage at the turn-off voltage node; an amplifier having a first input coupled to a reference voltage, a second input coupled to receive the feedback voltage
Implementation Method 2
a transistor having a first conduction terminal coupled to the turn-off voltage node, a second conduction terminal coupled to ground
Implementation Method 3
A readout circuit is coupled to the SPAD by a decoupling capacitor
Implementation Method 4
a quench element coupled between a high voltage node and the first node
Data Source
AI summary
A sensing pixel includes a single photon avalanche diode (SPAD) coupled between a first node and a second node, with a clamp diode being coupled between a turn-off voltage node and the second node. A turn-off circuit includes a sense circuit configured to generate a feedback voltage based upon a voltage at the turn-off voltage node, a transistor having a first conduction terminal coupled to the turn-off voltage node, a second conduction terminal coupled to ground, and a control terminal, and an amplifier having a first input coupled to a reference voltage, a second input coupled to receive the feedback voltage, and an output coupled to the control terminal of the transistor. A readout circuit is coupled to the SPAD by a decoupling capacitor.