I think a lot of the comments assume too much about the team lead. We've only heard one side of this situation.
I suggest talking to the founders but I would involve the team lead as well. Try to strike a positive chord, you want to improve the work environment by identifying problems, solving tensions and most importantly communicating properly and not circumventing parties. It might turn out the lead also has something to say even though his actions might paint a negative picture.
I also suggest a holiday before doing it without telling anybody about the burnout. Revitalize yourself, reflect on the situation and also look into what you can do better.
Be positive, proactive and suggest solutions. If it turns out that the founders side with the lead at least you tried your best and they can only blame themselves for driving a good engineer away.
Best of luck!
