FTP Folder Sync

Hi,
how can I synchronize constantly (every 20 minutes) an android folder with a folder on a server?

I need a software that checks the contents of a local folder with respect to a folder on the server a few minutes, if the folder on the server is changed via FTP will download the contents of the files on the server updating the local folder.

like a dropbox, I tried it but it does not work automatically on android.

thanks to all.