I designed a power converter PCB for up to 24 VAC input and +5 VDC output. I assembled three boards and tested them with a +12 VDC bench power supply, but it always outputs the input voltage minus the diode drop voltage.
The layout should be pretty decent with a solid GND plane, short HF loops etc. according to the datasheet.
Also the schematic looks like the one recommended for +5 VDC in the datasheet and I use exactly the recommended inductor. I really don't know what is going on. I chose capacitors with a high enough voltage rating, so that derating is compensated.
What I measure on the different inputs of the buck converter:
- FB = 1.7 VDC @ 11.3 VDC input voltage
- INTVcc = 3.45 VDC constant
- SW: Gives sometimes a low pulse, but is nearly always high, that is the reason for the same output voltage as the input voltage.
- PG: is low, so indicating a power error
(Note, that I had a pretty bad GND loop with the probe for this measurement, since it was hard to reach it, so ignore the overshoots.)
I really don't know where the mistake is.
Important: R2 and C7 are not placed in my schematic.
Here is the datasheet and the reference schematic from the datasheet:
Here is my schematic:
Here is my layout: