I have tried setting up this cheap AOZ1280 buck converter to give output of 3.3V from input of 3.5V to 4.2V without success. Please check my schematics below.
What happens is that I can not achieve an output of 3.3V unless the input voltage is at least 4.2V. If my input voltage is at first 4.2V and then I reduce it to 3.9V, for example, the output will stay at 3.3V. But if I initially connect 3.9V to the input, I will get only around 1.95V at the output. The output voltage will slightly increase with increasing input voltage, but at 4.2V there will be a bump and 3.3V at the output would stabilise.
At that point, while 1.95V can be found at the output, feedback pin gives 0.5V - where should be 0.8V (page 3 of datasheet) and LX pin frequency is around 800kHz. Where 3.3V is at the output, feedback pin is at around 0.8V, and switching frequency is around 1.3MHz, all as it should be.
According to my understanding, this misses datasheet specifications in two points:
- Page 3, Input Under-Voltage Lockout Threshold for VIN raising is 2.9V. In my case it seems to be at the voltage I receive some output voltage, but not what is desired
- Page 3, recommended operating conditions; Output voltage range: 0.8V to Vin
I even tried replacing with higher inductor with bigger one rated 3.3uH 4A, but with no change (as you could guess). All testing done with 1k resistor as a load. I can not understand at which points my circuit is (partly) failing. It should be quite simple and I stuck closely to datasheet recommendations.