We made a design with Viper16 to convert from 500V to 12V, non isolated. We followed the guidelines indicated in the document ST_ID15232_rev3 and datasheet. This device has a LIM input, to be used for setting the limit current. We left the LIM pin open, as indicated for the technical papers, to set the limit current as default. The device is not working correctly because the LIM pin is very sensitive. Only touching this pin with the finger, the 12V output goes down, making the output unstable. We would like to know if anyone had the same problem and how did you solved.