A component in a larger circuit I've designed is a step-down regulator circuit. It provides an output voltage of 18V from an input of up to 29V. For no other reason than the PCB manufacturer had it in stock, I chose the RY8310 buck converter.
I read the datasheet and believe I followed the 'recipe' to obtain 18V output. However, when I ran my initial test using 20V input, there was a brief moment after I closed the Enable switch, where I saw around 300mA flow and then nothing. I'm pretty sure that I've toasted the circuit, but since it's built using SMDs it's going to be pretty difficult for me to isolate which component(s) failed.
Since it's a pretty simple circuit, I'm hoping someone might be able to spot where I might have gone wrong. Here's a link to the datasheet for the RY8310, and I've included an image that shows
- the typical circuit from the datasheet
- the circuit I built
- the calculations I used to determine the voltage control resistor
- the values I used for the caps, inductor and resistor.
I appreciate any help with figuring this out.
As requested, here are the details of the caps and inductor in the circuit.
Based mainly on feedback from @Adam Lawrence, I've made changes to my schematic and posted a copy of this. I've also taken a closer look at the failing PCB and can clearly see that a side of the tantalum cap has blown off, and the Vcc track between the buck converter IN pin and the cap has been overheated. I've attached a picture of the power circuit part of the failed PCB, together with the PCB layout image.
I'm still puzzled by the need for a bypass (1uF?) ceramic cap on the Vin pin of the converter - there's already a 47uF cap between this pin and ground, so is the 1uF still required? Does it serve a different purpose to the larger cap?
Id appreciate feedback on the changes I've made which comprise
- change inductor to be a power inductor
- change caps C5 and C7 to ceramics, by aggregating multiple caps
- added pulldown resistor to EN pin and introduced voltage divider
If the basic components I've changed are deemed ok, I'll then populate the PCB and share this for further comment on component placement before I proceed with re-manufacture. I will also look at downstream protection for the board once I've got consensus that the power circuit is likely to work.
Many thanks for feedback.
The old PCB layout - I've highlighted in bright green the track that failed and the side of the cap that failed