ADP2301 Switching Node Waveform

Question

Does anyone know what causes this waveform at the switching node of a buck? 
 
 This was configured for 12V in 5V out . It uses a 10uH inductor. The load was 100 ohms. 
 I see a typical hysteretic mode when I remove the load: 
 
 When I zoom in in the switching edges, I see something similar to that "oscillation"-like waveform. 
 
 It looks like the transistor turns on partially, which can't be good. 
 If I increase the load to 50 ohms, the switching node looks normal.

Audison · Accepted Answer

Hi cgervasi, 
 What you are seeing is expected when operating at light load. To learn more, you can read page 14 of the data sheet (Power Saving Mode) and Figure 27 for waveform of Light load performance at steady state. The oscillation on SW node is due to parasitics at switches and inductor forming LC tank. 
 We do this because we want to achieve higher efficiency at light load.

Alexxx · Answer

Hi, 
 what you observe is normal at light load. The transistor switches correctly. What you see is the oscillation of the inductor with the output capacitor, after inductor current decayed to zero. This converter is built to have high efficiency at light loads, so it switches in bursts when minimal load is present. So this behaviour is normal and unavoidable as load decreases and keeping output voltage in regulation. You could try to increase inductor to 22&micro;H or 33&micro;H to lower the load current at which discontinous switching occurs.

Electronic Design Services - PartnerZone

ADP2301 Switching Node Waveform

Top Replies