The schematic in the LM2576S-ADJEVM User Guide shows two components, RFF and CFF, which are mentioned in the BoM but nowhere else. They are connected as a series RC branch in parallel with RFBT, the upper resistor in the potential divider that supplied the adjustable feedback voltage. I can think of two functions for this:
- For larger values of RFBT it provides some filtering for high-frequency signals appearing across this resistor.
- For larger values of CFF it would provide a sort of "soft start" by temporarily shorting out RFBT while CFF charged via RFF. However, since RFF << RFBT it would require quite a large value for CFF to provide any significant effect on the starting transient, and the suggested values in the BoM give an RC time constant around 0.3 microsec.
Can you say more about the function of the RFF / CFF network?