My first PCB ever (audio amplifier) - Page 1
Forum section: electronics / beginners

author: lutkeveld
author_rank: Regular Contributor
timestamp: August 09, 2014, 06:45:46 pm
content: Hi, new guy here. Long time reader, first time postingI'm working on my first PCB ever, it's going to be an audio amplifier. I'm using Eagle and made my own part.Problem is that the auto placed tStop layer is bigger than the SMD pad. So in the board view there is no soldermask between the pads.Is there a way to modify this or will my board house (seeed) auto fix it?Second problem is that the chip (MAXIM MAX9744) requires 3-3.6v on Vdd? Seems quite odd to me. Never saw an amplifier that converted the main (PVDD) voltage to a lower voltage. What is it used for, can I skip it?Thanks in advance. I will probably post some more questions along the way if I can't find them myself.

author: c4757p
author_rank: Super Contributor
timestamp: August 09, 2014, 06:51:54 pm
content: Quote from: lutkeveld on August 09, 2014, 06:45:46 pm"3-3.6V" means 3.3V, which is "the" standard general-purpose supply voltage for digital circuitry. (5V is for Arduino and the '80s) You need this supply because your amplifier chip is digitally controlled (notice the I2C interface?)As for your soldermask issue - you need to reconfigure the software to generate smaller mask expansions, or else you won't have soldermask. The board house won't fix it. You should check with them and make sure they can handle the size you need - if you specify mask but it's too small for them, they may try to make it anyway, leaving little threads of soldermask around your pads which make soldering difficult. In that case you'd be better off omitting it entirely over the pads (not ideal, but better than having them fuck it up).Unfortunately I don't know how to do that in Eagle.

author: lutkeveld
author_rank: Regular Contributor
timestamp: August 09, 2014, 07:04:54 pm
content: Thanks for your very quick responseAh, but I'm not going to use the I2C interface, I'll be controlling the volume with a potentiometer. Can I leave it unconnected then?I checked some other Atmel 44QFN package which was in Eagle's default library, that one also has too large tStop pads.And a picture to illustrate what I'm saying:

author: c4757p
author_rank: Super Contributor
timestamp: August 09, 2014, 07:07:50 pm
content: No, you must still power up the entire chip, unless the datasheet says otherwise (and I don't see it saying that anywhere).As for the other footprints having their expansion too large as well - I don't know about Eagle, but most other EDA tools allow you to set a soldermask expansion per footprint and per board. It's possible that they are inheriting the mask expansion set for your entire board.It's also possible that they're wrong. Manufacturer-provided stuff like that frequently is.

author: lutkeveld
author_rank: Regular Contributor
timestamp: August 09, 2014, 07:16:03 pm
content: It's going to be battery powered, am I supposed to use a DC-DC converter to get to 3.3v? Doesn't feel like the right thing to do, I've never seen anyone do that.

author: c4757p
author_rank: Super Contributor
timestamp: August 09, 2014, 07:16:42 pm
content: Surely a linear regulator will be fine - it won't draw very much current, at least, compared to the output current of the amplifier itself.Quote from: lutkeveld on August 09, 2014, 07:16:03 pmWell, if you need a voltage, you have to get it somehow. A DC-DC converter is typically the way you do that...

author: lutkeveld
author_rank: Regular Contributor
timestamp: August 09, 2014, 11:06:29 pm
content: After some further investigation it seems that there is indeed a linear regulator on other audio amps. It's just so tiny that I failed to notice before. I'll have a look for one that meets my requirements.

author: ThomasDK
author_rank: Regular Contributor
timestamp: August 10, 2014, 01:16:15 am
content: In your board view, go to ULP -> Masks, and change Min and Max from the default 4 mil.I have gotten away with 1 mil in the past with one of the chinese boards houses (can't remember which, either elecrow or elec-freaks)

author: lutkeveld
author_rank: Regular Contributor
timestamp: August 10, 2014, 07:46:28 am
content: Quote from: ThomasDK on August 10, 2014, 01:16:15 amThomas, if I go to 'File' -> 'Run ULP' there is only a ULP called Maskdata. It makes the pads white. Can't seem to find the option you are referring to.However, I just loaded the DRC of Seedstudio and the Tstop layers are a lot smaller. Soldermask still doesn't expand between the pads tho.

author: ThomasDK
author_rank: Regular Contributor
timestamp: August 10, 2014, 07:16:31 pm
content: Quote from: lutkeveld on August 10, 2014, 07:46:28 amSorry, typo... DRC-->Masks