I'm currently facing a weird issue. For one of my project I had to design a buck converter circuit in order to step down the voltage of three LiIon batteries to 3.3V so I can power a microcontroller.
I'm using the TPS54240 chip. The datasheet can be found here. A typical application schematic is shown on page 31. Since said circuit outputs 3.3V I was able to more or less copy it. My schematic looks as follows:
After ordering a PCB prototype the buck converter didn't really work, unfortunately. When I applied ~7.4V (2 LiIon batteries) to VCC the output showed ~3.3V as I was expecting. However once I connect a load, in my case the microcontroller, the voltage dropped to ~3.26V. Why is this happening? I thought a buck converter utilizes a feedback loop in order to maintain a constant voltage. The TPS54240 chip itself is rated for up to 2.5A in this case and my microcontroller isn't even pulling 100mA.
Even more confusing was the scenario when I connected ~11.1V (3 LiIon batteries) to VCC. In that case I measured ~2.6V at the output, which again dropped significantly to ~2.4V just when I connected a LED that drawed maybe 20mA. My microcontroller wasn't able to power on on such a low voltage making the circuit more or less useless.
Since I don't have a ton of experience with designing a circuit (this was my first project) I don't know how to troubleshoot this problem. I would appreciate any kind of help. Thanks!