TPS54239 - Ringing on 5V output

Hello,

I ordered a sample of TPS54239.

My requirements are:

Input 12V; Output 5V; Imax = 1A

For testing purposes I soldered all required components together on a little board. All connections as short as possible (e.g. VFB). All grounds connected without a ground plane. Now I can see a repetitive disturbance on the 5V Output Voltage (see attachment). The periodic time between the ringing is 2us and the signal itself has a time period of 7,5ns.

What is the reason for this disturbance?

Thanks

S. Hilgers