GaN Laser Diode Driver With Gate Current Reuse
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional output stages for driving laser diodes in Lidar applications face inefficiencies due to high power consumption, ground bounce, and ringing on ground nodes caused by large gate current requirements and parasitic impedance, leading to potential errors in pre-driver operation.
Innovation Solution
A driver circuit comprising a first GaN FET in common-source configuration and a second, smaller GaN FET in source-follower configuration, where the gate drive current is provided by a second supply voltage and flows through both FETs, reducing current consumption and parasitic impedance effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a common-source drive FET is used with a pre-driver to overcome large gate capacitance, then the drive FET can be turned on effectively, but power consumption increases and ground bounce occurs
Solution Approach 1:
A source follower FET is introduced as an intermediary between the pre-driver and the common-source drive FET. The source follower's source terminal directly drives the gate of the common-source FET, acting as a buffer that isolates the pre-driver from the large gate capacitance loading effect. This intermediary stage enables effective gate driving while reducing the current burden on the pre-driver and supply voltage VDD.
2Reliability
If large gate current is drawn from supply voltage VDD to drive the gate terminal, then the drive FET turns on, but return current causes ground bounce and ringing
Solution Approach 1:
The source follower FET serves as an intermediary that redirects the gate drive current path. Instead of drawing large current directly from VDD through the pre-driver, the source follower draws current from the higher voltage supply VH through the laser diode load, then delivers it to the gate of the common-source FET. This mediation eliminates the harmful return current through the pre-driver ground path, preventing ground bounce and ringing.
3Ease of operation
If gate current flows through parasitic impedance connections to ground, then the circuit operates, but voltage drops and ringing occur on ground node
Solution Approach 1:
The source follower FET mediates the gate drive current path, preventing current flow through the parasitic impedance of the pre-driver ground connections. By routing current through the load (laser diode) instead of through the pre-driver ground path, the intermediary eliminates voltage drops and ringing on the ground node, ensuring stable ground potential for precise pre-driver operation.
Solution Approach 2:
The invention converts the potentially harmful large gate current into a beneficial configuration where the current flows through the laser diode load instead of through parasitic ground paths. The high current capability is utilized productively to drive the gate while simultaneously contributing to the load operation, transforming what would be waste current into useful current that avoids creating ground bounce problems.
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
A laser-diode driver for Lidar applications with an output stage comprised of two enhancement mode GaN FETs. The output stage includes a driver GaN FET in a traditional common-source configuration, with the drain connected to the cathode of a laser diode and the source connected to ground. The gate of the driver GaN FET is driven by the source of the second, substantially smaller GaN FET in a source-follower configuration, rather than being driven directly by a pre-driver. The source-follower GaN FET has its drain connected to the drain of the common-source driver GaN FET, similar to a Darlington connection used in bipolar devices. The input drive signal from the pre-driver is applied at the gate of the source-follower GaN FET. The current required to turn on the driver GaN FET is thereby drawn from a main power supply through the laser diode, rather than from the power supply for the pre-driver, improving overall current efficiency.