VCSEL Drive Circuit Stabilizes Current for TOF Sensors

Solution Overview

Problem

Existing light-emitting element drive circuits for time of flight (TOF) proximity sensors and distance measurement sensors face challenges in maintaining a stable drive current, especially when the forward voltage of the power supply fluctuates, affecting the accuracy and speed of autofocus and distance measurement in low-light conditions.

Innovation Solution

A light-emitting element drive circuit that adjusts the drain voltages of PMOS and NMOS transistors to be the same voltage before the light emission period, using a drive adjustment unit with an operational amplifier and switch to stabilize the current flowing through the VCSEL, ensuring consistent drive current during pulsed light emission.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the drain voltages of the transistors are not adjusted before light emission, then the circuit structure remains simple, but the drive current fluctuates due to forward voltage variations

Engineering Contradiction:

Improvedrive current stabilityVSAvoidcircuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent applies preliminary action by adjusting the drain voltages of the transistors in a pre-light emission period before the actual light emission. This preliminary voltage adjustment ensures that the drive current remains stable during light emission by compensating for forward voltage variations, without requiring continuous complex control circuitry.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent uses an operational amplifier as an intermediary element to adjust and equalize the drain voltages of the transistors. The operational amplifier acts as a mediator that senses voltage differences and provides the necessary adjustment to maintain stable drive current, simplifying the overall control mechanism while ensuring reliability.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If the drive current is not stabilized before light emission, then the response speed is faster, but the measurement precision decreases due to current fluctuations

Engineering Contradiction:

Improvedistance measurement accuracyVSAvoidresponse time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent performs the voltage adjustment in a pre-light emission period, preparing the transistor drain voltages before the actual measurement begins. This preliminary preparation ensures that when light emission starts, the drive current is already stabilized, providing both high measurement precision and fast response without the need for continuous adjustment during measurement.

Inventive Principle:

Principle #10Preliminary action

3Reliability

If the drain voltages are adjusted dynamically during operation, then the drive current stability is maintained, but the device complexity increases

Engineering Contradiction:

Improvedrive current stabilityVSAvoidcontrol circuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent performs the voltage adjustment in a pre-light emission period, preparing the transistor drain voltages before the actual measurement begins. This preliminary preparation ensures that when light emission starts, the drive current is already stabilized, providing both high measurement precision and fast response without the need for continuous adjustment during measurement.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent uses an operational amplifier as an intermediary element to adjust and equalize the drain voltages of the transistors. The operational amplifier acts as a mediator that senses voltage differences and provides the necessary adjustment to maintain stable drive current, simplifying the overall control mechanism while ensuring reliability.

Inventive Principle:

Principle #24Intermediary (Mediator)

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 suppresses fluctuations in the drive current, enhancing the accuracy and speed of distance measurement and autofocus operations, even in low-light conditions, by maintaining a stable current through the VCSEL.

Implementation Method 1

A light-emitting element drive circuit that adjusts the drain voltages of PMOS and NMOS transistors to be the same voltage before the light emission period, using a drive adjustment unit with an operational amplifier and switch to stabilize the current flowing through the VCSEL

Methodology Applied

Scientific EffectOperational amplifier voltage regulation:

Data Source

PatentUS10555387B2Light-emitting element drive circuit and portable electronic instrument

Publication Date: 2020.02.04 SHARP KK

AI summary

There is provided a VCSEL drive circuit that causes a VCSEL to emit pulsed light in a light emission period, including a drive adjustment unit that adjust a first voltage which is a voltage of drains of a PMOS transistor and an NMOS transistor in a constant current circuit and a second voltage which is a voltage of a drain of an NMOS transistor that drives the VCSEL to be the same voltage in a pre-light emission period before the light emission period.