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 Posted: May 29, 2012 - 03:58 PM |
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Joined: Nov 10, 2009
Posts: 71
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Hi,
I am designing a pcb and due to size constraints, planning to put a 5V regulator (78M05) right below an AVR microcontroller, well on the other side of the board. (All SMD's)
Can somebody suggest if this is a good idea? The regulator ofcourse heats up a little depending on the battery used, but does it affect the microcontroller on a long run?
Thanks,
Praveen |
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Posted: May 29, 2012 - 04:05 PM |
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Joined: Dec 11, 2007
Posts: 6849
Location: Cleveland, OH
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Moderator,
Perhaps this would do best in the General Electronics Section?
JC |
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Posted: May 29, 2012 - 05:11 PM |
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Joined: Nov 22, 2002
Posts: 12046
Location: Tangent, OR, USA
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Should not be an issue.
Jim |
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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: May 29, 2012 - 06:15 PM |
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Joined: Nov 10, 2009
Posts: 71
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| Thanks Jim. I am planning for a commercial board and just confirming so that it does not hit back huge. |
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Posted: May 29, 2012 - 06:21 PM |
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Joined: Nov 28, 2004
Posts: 3552
Location: San Diego, Ca
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| What voltage is the battery ? Why not just use a MCU that can run on a 3V battery and forget about regulators and waste thru heat ? |
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Posted: May 29, 2012 - 06:59 PM |
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Joined: Aug 21, 2002
Posts: 895
Location: Austria
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| Don't know which battery you speak about because the quiescent current of 78M05 is 4-6mA. So maybe you will never have a long run. |
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/Martin.
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Posted: May 29, 2012 - 07:50 PM |
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Joined: Nov 22, 2002
Posts: 12046
Location: Tangent, OR, USA
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Of course, double-sided SMT boards are possible, but they are to be avoided when possible. Manufacturing is more costly. Trouble-shooting is harder. Repair hardly exists.
Jim |
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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: May 29, 2012 - 08:07 PM |
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Joined: Oct 30, 2002
Posts: 5720
Location: The Netherlands
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I don't see how repair and trouble shooting are really that much harder on a doubled side board.
What gets repaired on a component level these days? (Sub)boards are just swapped out and thrown away. |
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Posted: May 29, 2012 - 08:34 PM |
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Joined: Nov 22, 2002
Posts: 12046
Location: Tangent, OR, USA
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With parts on both sides, vias often get covered. That makes it harder to follow traces. Of course, thats also true for multi-layer.
Of course, its correct that boards are often not repaired.
Jim |
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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: May 30, 2012 - 10:08 AM |
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Joined: Nov 10, 2009
Posts: 71
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Quote:
What voltage is the battery ? Why not just use a MCU that can run on a 3V battery and forget about regulators and waste thru heat ?
This may not be possible as the board need to support other sensors (running at 5V) powered by the same board.
Quote:
Of course, double-sided SMT boards are possible, but they are to be avoided when possible. Manufacturing is more costly. Trouble-shooting is harder. Repair hardly exists.
Manufacturing is fine for the effort. Troubleshooting? No. I do everything and when user gets it, he shouldn't bother more about it.
Quote:
Don't know which battery you speak about because the quiescent current of 78M05 is 4-6mA. So maybe you will never have a long run.
Ok. When I say long run, I meant throughout the life of the board. Yes that 4-6mA is huge, but did not find a better one.
Can somebody suggest a regulator which can meet a few of the following conditions?
1. Lower Quiescent current
2. Minimum of 500mA of output current (more the better)
3. Easily available and cheaper
4. Smaller footprints
For the records, the battery (7V-12V) current will be anywhere between 1A to 1.5A.
Thanks |
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Posted: May 30, 2012 - 11:05 AM |
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Joined: Nov 10, 2009
Posts: 71
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| Just searched for LDO's and found this "NCP1117". Does it serve the purpose? |
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Posted: May 30, 2012 - 12:14 PM |
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Joined: Oct 30, 2002
Posts: 5720
Location: The Netherlands
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| It is not better in terms of quiescent current. How much current does it need to deliver? And what's the exact application? This quiescent current might not be important at all. It is if you have some board that has to run months or years on a battery, but not for a robot that only has to run for 30 minutes on a big battery and where motor current are dominant. |
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Posted: May 30, 2012 - 12:45 PM |
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Joined: Jul 02, 2005
Posts: 5942
Location: Melbourne, Australia
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Praveen,
Your description has left me confused. You say the battery (7 to 12 volts) will be supplying between 1 and 1.5 amps. What does this have to do with the load on a 5 volt linear regulator?
You say the 5 volt regulator must supply 500 mA or more. Why so uncertain? Surely you know what your load is going to be? |
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Ross McKenzie
ValuSoft
Melbourne Australia
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Posted: May 30, 2012 - 01:27 PM |
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Joined: Nov 10, 2009
Posts: 71
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Quote:
This quiescent current might not be important at all. It is if you have some board that has to run months or years on a battery, but not for a robot that only has to run for 30 minutes on a big battery and where motor current are dominant.
This explains it all. The board is designed such that it can support sensors and motors. It sure does not run for more than an hour, but just asking if saving a bit of current matters.
Thanks for your inputs.
Quote:
Your description has left me confused. You say the battery (7 to 12 volts) will be supplying between 1 and 1.5 amps. What does this have to do with the load on a 5 volt linear regulator?
Sorry to confuse you, but reading all the comments I am more confused.  I wrote this because someone asked for battery voltage details above.
Quote:
You say the 5 volt regulator must supply 500 mA or more. Why so uncertain? Surely you know what your load is going to be?
I am uncertain because I need something which is better and cheaper. Most regulators supply 500mA and few of them supply 1A.
If an end user decides to use a couple of servos, then it sure sucks most of the current (just in case). Since the board has an option of choosing current directly from the battery or from the regulated input, I am bit over cautious.
Reading all the inputs, I just guess that I go with my initial plan of using 78M05, without worrying much. |
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Posted: May 30, 2012 - 01:39 PM |
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Joined: Oct 30, 2002
Posts: 5720
Location: The Netherlands
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Remember that the regulator has to dissipate (12-5)*0.5=3.5W with a 12V input and a 0.5A load.
If you use the DPAK package, the junction will heat up to over 300C if you have no heatsink. Of course, the thermal protection will kick in at ~140C, so after a couple of seconds power is gone 
So you have to do some thermal analysis beforehand. |
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Posted: May 30, 2012 - 01:58 PM |
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Joined: Nov 10, 2009
Posts: 71
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@jayjay1974 ,
Thanks a lot for your valuable suggestions.
This is the reason I raised my initial question about heating up AVR. The bottom part of my board has copper spread across which can act as heatsink, but now wondering if that is enough.
As mentioned, will have to do a prototype board for thermal analysis and then go ahead. |
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Posted: May 30, 2012 - 02:00 PM |
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Joined: Nov 10, 2009
Posts: 71
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| Just saw the Arduino Uno which looks like it uses 1117. I do not see any heat sink in there and input voltage is rated to be 7-12V. Will check the schematic once. |
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Posted: May 30, 2012 - 02:16 PM |
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Joined: Oct 30, 2002
Posts: 5720
Location: The Netherlands
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| They use the PCB as a heatsink. That provides some cooling, enough for the application but not enough to use the maximum capabilities of the regulator. |
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Posted: May 30, 2012 - 03:16 PM |
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Joined: Nov 10, 2009
Posts: 71
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| Thanks a lot for your time and appreciate your help. With a bit of testing, let me see where I will end up. |
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Posted: Jun 01, 2012 - 01:40 AM |
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Joined: Nov 28, 2004
Posts: 3552
Location: San Diego, Ca
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| Jim, above, had a design that used the PCB as a heatsink and over time the heat lifted traces all over the board ( took about 1 yr. before it became a problem, IIRC ) . Your testing may not be good enough to catch this possibility. 3.5W or more has a good shot at doing that too. |
_________________
1) Studio 4.18 build 716 (SP3)
2) WinAvr 20100110
3) PN, all on Doze XP... For Now
A) Avr Dragon ver. 1
B) Avr MKII ISP, 2009 model
C) MKII JTAGICE ver. 1
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