AP5100 buck weird behaviour

[Al3X05]

Hello all,

First time post 

I need your expertise..

I have made a buck converter using the AP5100WG-7 exactly as per the datasheet for 3v3 https://www.diodes.com/assets/Datasheets/AP5100.pdf (page 2).
The only changes to the design are the diode B230A-13-F in place of B230A and SM3521-3R3MT for the inductor.

The input voltage is 12V. There is very little load in the order of 10mA. The output always measures 0.8V, EN measures 4.5V >1.2V for ON, BST measures 5.6V and finally FB measures 0.8V.

I am not sure if there is something I am missing, can I use this as a light load buck to power a micro?

[squadchannel]

Why use switching at a light load of 10 mA?
As you can see from the efficiency graph in the datasheet, switching are very inefficient at light loads. And they tend to be unstable.

If it is only 10mA, use an LDO. Is there any reason why you can't use it?

But I think there are other causes for the no output. As far as I can see, there is no problem, Board design, etc. could be considered.

[Al3X05]

I guess I could use an LDO, have in the past, but I think because of the difference in voltage it had to work pretty hard and was getting warm. So wanted to go a more efficient route and make the overall supply smaller as well.

But it should still do the correct output voltage though, regardless of load (besides when current limited)?
On that note, I have also removed all the load from the board and the output voltage is still unaffected.

I have tried to keep the active loop small, there is a ground plane for return.

[Chalcogenide]

If FB measures 0.8 V that means that the IC is working and regulating fine, but you have a shorted R203 or C203, or a very wrong value for R203 (maybe 49.9 ohms instead of 49.9k).

[Al3X05]

Thanks! Will check it out! perhaps put a pot on it to make sure

[Al3X05]

And you were quite right! R203 is indeed the wrong value