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A.L.
Until you are ready for your supervisor to know don't send anything to HR. I work in HR and we will tell the supervisor, as they have a business need to know and once you notify us then we certainly can share that information with your immediate supervisor.
There is no set time limit on when you need to notify work, but you should do it before it becomes too obvious (at least your supervisor). There are other restrictions on FMLA than just having 50 employees. You are only entitled to 12 weeks within a 12 month period. so if you recently took leave of all 12 weeks you won't be eligible until 12 months have passed since you came back to work. You also need to have been there for more than 1 year before you are eligible.