Hello
I'm doing a project with a CoolRunner-II dev board (http://http://dangerousprototypes.com/docs/CoolRunner-II_CPLD_breakout_board), and I was wondering if I can use my own 3.3V supply? I checked the diagram, and I would connect my 3.3V power to the "3V3" pin, and I would connect the "V+" pin to ground to disable the onboard 3.3V regulator ("V+" is connected to the "EN" pin of the regulator). Does this looks ok? Maybe having 3.3V on the output, and 0V on the input would break the regulator, or having it disabled is ok?
Maybe I would have to connect 3.3V to the "V+" pin too? But then, the regulator would be active, with the same voltage at its input and output...
Or it's not possible, and it would be easier to just use a 5V source to the "V+" pin and use the onboard 3.3V regulator?
Thanks in advance!
That's an excellent and interesting question.
Step 1 is to grab the Vishay Si9183-A data sheet and see what the Absolute Maximum Ratings are. Unfortunately, there's nothing definitive, although the "Output Voltage, Vout ... -0.3 V to Vo(nom) +0.3 V" seems to imply that the output can handle up to 3.6V, I'm just not sure.
Rather than ground the "V+" input, it would probably be much better to use a pull-down resistor of a large value. Anything over 10 kΩ should work.
Assuming that you buy a handful of Si9183-A parts from Mouser or elsewhere, you could try out your experiment. Just replace the part if you let the magic smoke out.
The one thing I would recommend against is connecting 3V3 to both input and output. There's a 1.215 V reference inside the regulator which might fight your 3V3 supply for who's more accurate, possibly attempting to supply infinite current (which would surely burn out the chip), so it's much better to have the regulator Off.
Ideally, you'd cut the trace to pin 3, called !SD in the data sheet, but that might be too drastic, and certainly not easily reversed. If you did cut the !SD trace, you could probably safely get by with connecting 3V3 to input and output, because you could force the part into shutdown (!SD).
P.S. The BYP (BP) pins on all of the regulators should really be connected to 0.1 µF capacitors to improve the supply noise.
Hi kimmitanto,
Just connect your external 3.3volt supply to the 3v3 pin (or the 3v3 pin of the JTAG header will work too). No need to disable the on-board 3v3 regulator, it will be disabled without power.
Your external 3v3 supply will also power the 1.8volt regulator for the CPLD core, and you will still be able to toggle between 1.8v and 3.3volt IO levels with jumpers V1 adn V2.
Please give me a shout if you need any more help.