SparkFun Large Digit Driver Capacitor needed?

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1

I’m using the SparkFun Large Digit Driver, with the TPIC6C596 shift register.

According to the datasheet, the TPIC6C596 needs a bypass capacitor at the Vin and GND pins. But the driver board doesn’t seem to have that. It seems as though it filters out noise that could effect the register.

I am having a lot of problems with getting this to work well, and I recently found out that I think my regulated power supply is having some voltage fluctuations affecting the TPIC6C596. I was thinking about adding a decoupling cap (I have 16v, 1000uF caps handy, although probably overkill for this) across the 5v and GND side anyway to smooth out the voltage.

Also, I have 3 shift registers daisy chained.

So did they not include bypass caps as recommended? Should I add that by each chip or at the start of the chain? And should I add a small .1uF ceramic bypass cap for noise too?

Any help would be appreciated.

2

1000uF is more for bulk storage, not a decoupling cap. I would definitely add a 100nF cap across power and ground right at the chip. Adding a 1000uF cap at the first board would help with bulk capacitance and would avoid the rail dipping as much when many LEDs turn on.

/mike

3

Thank you. The product doesn’t contain the cap, and everything is surface mounted. So adding a cap that close will be a big challenge. I will give it a try.

I am not sure what it’s purpose is. Is it to filter noise? Can I use both caps together, one for power buffer and one for noise filtering? Thanks again.

4

Not sure why the decision was made to not include a decoupling cap on board, but you’re actually the first person to mention you were having issues. It makes me wonder if you’re power supply might be especially noisy?

Anyway, you might try a different power supply just to rule that out but if you need too, popping a 0.1uF cap across the 5V and GND pins on the board would probably help. Ideally that cap should be as close to the VCC and GND pins on the chip but the headers should be OK. It’s also OK to run a 0.1uF and a 1000uF cap together, the 0.1uF will grab short, transients while the 1000uF will grab larger ones.