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 Posted: Jan 18, 2012 - 07:15 PM |
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Joined: May 11, 2011
Posts: 336
Location: Boston, MA
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I'm working on an AT32UC3A3 project that requires getting from 5V to 3.3V. I was using a TI LDO, but it's physically too big for the next revision. I've found this chip, which looks to meet my needs, but I'm curious about the footprint, on Page 21.
http://www.st.com/internet/com/TECHNICA ... 227527.pdf
I have two concerns, about the shape of the pads.
1) The large rectangular pad (the thermal pad) has rounded corners. I don't see any problem with just making the pad a rectangle, but I want to get a second opinion.
2) The six pins are kind of a funny shape, one that I cannot easily replicate in my PCB program (KiCAD). I might be able to put a circular pad on top of a rectangle and approximate the shape, or I could also just make it a rectangle. Is there anything very wrong with just making it a rectangle?
Also, if anyone has suggestions on other LDOs I'm happy to listen. I found this one by searching on Mouser, mostly trying to find something that was relatively small.
Thanks! |
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Posted: Jan 18, 2012 - 07:19 PM |
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Joined: Jul 27, 2001
Posts: 7429
Location: St. Leonards-on-Sea (UK)
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| Rounded corners are better for lead-free reflow soldering. They are available with the PCB package I use. |
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Leon Heller
G1HSM
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Posted: Jan 18, 2012 - 07:30 PM |
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Joined: May 11, 2011
Posts: 336
Location: Boston, MA
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Leon, what package do you use?
I'm looking through some of the KiCAD mailing lists and I've found references to DFN footprints, but I haven't found any yet. I guess I'll try to approximate it as best I can for now and see what happens.
Thanks! |
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Posted: Jan 18, 2012 - 07:46 PM |
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Joined: Nov 22, 2002
Posts: 12053
Location: Tangent, OR, USA
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For the pin pads, I would lay a filled rectangle on top of a filled circle. For the thermal pad, I would put small circles at each corner and use a filled polygon for the remainder, making a diagonal through each corner circle.
I would hope that KiCAD has filled rects and filled circles and filled polygons. EAGLE does.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jan 18, 2012 - 08:11 PM |
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Joined: May 11, 2011
Posts: 336
Location: Boston, MA
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Jim,
KiCAD does have filled circles and polygons, but I'm not sure how to get them onto a footprint. For instance, they work great when laying out ground/power planes, but I've never used them in a footprint itself before. I might be able to fudge it by laying out a number of surface mount "Pads" which are filled circles or rectangles, it just becomes somewhat of a pain when trying to do the math to make sure everything is placed right.
I've got a placeholder footprint for now, which will at least let me start working on the layout, and I can go back and modify the footprint later. |
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Posted: Jan 18, 2012 - 08:14 PM |
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Joined: Jul 27, 2001
Posts: 7429
Location: St. Leonards-on-Sea (UK)
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I use Pulsonix:
http://www.pulsonix.com
I created the footprint in about 10 minutes. |
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Leon Heller
G1HSM
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Posted: Jan 18, 2012 - 10:46 PM |
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Joined: May 01, 2003
Posts: 577
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| How about microchip mcp1702.... std sot package . I hate using DFNs unless the board is going to be made/loaded by an SMT subcontract house, then no problem. |
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Posted: Jan 19, 2012 - 03:53 PM |
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Joined: May 11, 2011
Posts: 336
Location: Boston, MA
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chartman,
This isn't going to be hand soldered, I'm using a 144 pin BGA chip (AT32UC3A3) which makes hand soldering...difficult  . While ideally, I'd like to be able to do my own rework, size is the most important factor for me at the moment.
Taking a look at the mcp1702, certainly an interesting possibility, looks to be about the same size as the DFN part I found. |
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Posted: Jan 19, 2012 - 06:22 PM |
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Joined: May 11, 2011
Posts: 336
Location: Boston, MA
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Actually, did some looking around, and the MCP1700 is a better pick. Max input voltage is only 6V (vs 13.2V) but as I'm running off of USB, that'll be sufficient.
I'll try using this instead of the DFN part. That'll make it easier for me to rework everything but the BGA.
Thanks guys! |
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Posted: Jan 20, 2012 - 04:05 PM |
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Joined: May 01, 2003
Posts: 577
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| That family of regulators is brilliant, up to 250mA... Iq down to 1uA.... I have to use the 1702 because I run off a 12V battery but my 6V version uses the 1702.Makes my radio system battery life out last the competition |
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