5v Switching Regulator Problems

I have a circuit with two supplies. A 19v supply for stepper motors, and a 5v supply for driving the ATMEGA644, 2 stepper motor driver daughter boards, a bluetooth LE daughter board, and 2 ws2812b rgb leds.

With two supplies everything worked, but I really wanted to get it down to just one supply. So I did another version of the board using just the 19v supply and a VX78-500 switching regulator. I used their recommended input and output caps. (But not the low noise version that include some extra inductors) .

I have a 100uf electrolytic on the 5v rail. Not 100% sure if that's a good idea but it's well below the max capacitive load for the VX78-500 which is 680uf and I thought it might help with noise.

I also have 2 100uf electrolytic on the 19v line next to the stepper driver boards for the same reason.

When I first turned things on I noticed the WS2812b'd flashed slighting but everything was fine. However on maybe the 3 or 4th time I turned it out the bluetooth LE daughter board died. Which is kind of surprising because it's got its own on board 3.3v LDO regulator.

I don't know that much about supply best practices/inrush current, etc. I knew my new setup would be noisier using a switching supply/being and being down stream from the same supply as the steppers. Any ideas what is going on? That flashing makes me think there's a bad spike happening at switch on time. It seems weird that the atmega644 is still ticking but the BTLE daughter board died.

How do I trouble shoot this?

Are those 100uf caps just a bad idea in general? Does my circuit need some other form of protection?

Those daughter boards are $19 so I don't want to blow out too many of them in a search for the solution. I can post pics/eagle files if that would help.

Thank you so much for your time.

by kurt.schaefer
March 03, 2019

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