FLL is similar to a PLL (Phase locked loop) with just the shifting of an integrator gain in the schematics of the system. To understand how it works, I advice to take a look at my Thesis manuscript with explanation of the working, stability, etc ...

Download the manuscript on:

https://www.researchgate.net/publication/264680762_Modeling_an_Experiments_in_Dynamic_Force_Microscopy_in_Ultra_High_Vacuum

And then, take a look at chapter 4.1.4 (pages 70 to 75, in the text), with the Figure 4.11 that summarizes the analogy between a PLL and a FLL.

Kind Regards.

Thesis Modeling an Experiments in Dynamic Force Microscopy in Ultra...

Thank you.