Ques. About GND and PGND

[_John_]

Hi,

I'd like to build an inexpensive 5v step-down buck converter. I'm not advanced enough to design one myself so have been looking at different circuits that have already been made. I ran across this:
https://www.instructables.com/DIY-High-Efficiency-5V-Output-Buck-Converter/
which seems to fit the bill.

The only question I have about it is the GND and the PGND. I see they are tied together at one point in the circuit. So is it ok to just connect them together? Seems like they wouldn't go through the trouble of differentiating them if so.

I also wouldn't mind suggestions for other buck converter circuits. I'd like to be able to drop down two 18650 batteries or a 12v power adaptor to 5v. Shouldn't need more than one amp.

Thanks in advance for any help!

 

[Diver300]

I think it is just that PGND (power ground) is for the high-current grounds like the output and the bottom of the capacitors, and GND is more for signal connections only and is connected to another pad on the board.

On a buck regulator like that, the output current is either comes from the input VIN on the MAX17502 or from the freewheeling of the inductor, so it comes from PGND on the MAX17502. When the current comes from the VIN, the capacitors C5 and C6 will be providing much of the current, and when the current comes from PGND, the capacitors will be charging back up. That switching is very fast indeed and can cause problems, so it is important to keep the loop made up of PGND, VIN and the capacitors as small as possible.

That design has done a reasonable job of that, but I would have put a capacitor much closer to VIN and PGND to reduce that loop a lot more.

Having PGND and GND as separate connections (even if they are connected) may help in deciding how to make a good layout.

Have look at the data sheet for the regulator IC.
https://www.analog.com/media/en/technical-documentation/data-sheets/max17502.pdf

 

[_John_]

Diver300

Ah yes, I see. That makes a lot of sense. I planned on making the entire circuit much smaller but will concentrate on the VIN, PGND, and cap loop.

Thanks so much for the info! I can move forward with a lot more confidence now.

 

[danadak]

Regards, Dana.

 

[_John_]

Thanks danadak!

That's good info. I sent off for the boards before I saw your post. Hopefully, everything works out. While I'll be charging with 12v, the device will only run with two 18650s. So the voltage difference won't be that great. From my understanding, it may be a little more forgiving under those circumstances.

 

[tepalia02]

"Power Ground. Connect PGND externally to the power ground plane. Connect GND and PGNDpins together at the ground return path of the VCC bypass capacitor."

https://www.analog.com/media/en/technical-documentation/data-sheets/max17502.pdf

 

[_John_]

tepalia02 I'm not sure how I missed that. I should have looked more closely. Thanks for pointing it out.