Strange oscillation in buck converter (AP3431)

Question

I am having a lot of trouble getting AP3431 to work reliably. The converter works properly for about 30 seconds, and then oscillations begin, with brief periods of normal function.

Here is how it is hooked up:

I am testing this converter with a custom made constant current sink:

And here's what the physical construction looks like:

For the first 30 seconds, everything is good. Vin = 5V, Vout=2.5V, FB = 0.79V:

Then the oscillations begin, and they are accompanied by a high-pitched buzz either from the converter, or the inductor:

Here's a short video that demonstrates the whole malfunction cycle and the high-pitched buzz: https://www.youtube.com/watch?v=48_JTKHtnE4

A few further observations:

1. Using a different load leads to the same oscillations eventually.
2. Using no load at all, also eventually malfunctions! Here's an example with no load. You can see that the converter operates properly for brief periods of a few seconds, but mostly Vout is at Vcc:
3. After the oscillations begin, restarting the converter does not fix the problem. It seems that unplugging and waiting for about a minute is what returns the converter to a working state for a short while.
4. Nothing gets hot, or even warm.
5. I've been having very similar problems with other converters as well. See these posts:

Help understanding switching voltage regulator (OKR-T/1.5-W12-C)

How to troubleshoot a boost converter (ADP1613)?

I have changed quite a few things since my initial attempts described in the 2 posts linked above:

1. I selected a better inductor, which has it's specs tested at 1MHz (switching frequency of the buck converter).
2. I mounted everything, except the FB voltage divider resistors on the breakout PCB
3. I have used a different breadboard, just to rule out a board defect
4. I've used a different converter
5. I was much more rigorous with calculating the inductor value
6. I used different power sources

Despite all these steps, I am still having the same stability issues.

What could I possibly be missing?

Any advise would be appreciated.

EDIT 1: Is there any significance to the fact that the period of the oscillation is about 600us, and if we just look at the voltage rise part, it's more like 475us. The Soft Start time is listed as 450us typical. It's in the same range, at least. I wonder if the regulator is repeatedly shutting down and entering soft start for some reason?

EDIT 2: In accordance with many suggestions here, I have soldered the divider resistors directly onto the PCB, and I think it became a little more stable. Still started oscillating sooner or later though, depending on load current.

Then I connected everything with direct wire connections. The only stuff still connected to the breadboard was Vin and GND. However, by now something else happened and the chip started heating like crazy, even with no load. I suspect it might be damaged - it is still regulating, but goes to > 60C within the first 30 seconds.

I changed too many things at once, so I think I need to start over. Unfortunately I am out of boards, so I'll need to make another. I may as well redesign it. I'll try the following:

1. Put the resistors on the PCB
2. Widen high current traces
3. Put capacitors closer to the IC
4. Shorten the track to the inductor

Please let me know if I am missing anything in this list!

EDIT 3: Does this look like a reasonable design?

Answer 1

It appears that the problem was with the shape of the traces on my PCB. The following PCB design works correctly:

Unfortunately I still do not understand the real reasons behind this well enough to explain it, but I think it has to do with parasitic capacitance and inductance of the traces at high frequencies. It appears that at 1MHz these things really do matter, which also explains why I had so much trouble with the breadboard earlier.

Answer 2

Add an uninterrupted ground plane.

Decoupling caps connect to the ground plane. Move input side decoupling cap closer to the input of the IC. Use multiple valued output filter capacitors in parallel, e.g. 100nF, 1uF, etc.

Did I mention to add an uninterrupted ground plane!?