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Forum Index : Microcontroller and PC projects : STM32F407 discovery problem

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boss

Senior Member

Joined: 19/08/2011
Location: Canada
Posts: 268
Posted: 10:15am 02 Apr 2015
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My ARMite board suddenly develop strange behavior and unpredictable response. The situation went worse as I tested board thoroughly. The larger code the more unpredictable results. After couple of days a new board arrived the problem was solved. See picture.


Older Discovery versions MB997A and B tantalum capacitors on positions C33 & C36 while latest board MB997C uses ceramic capacitors. As the B version of board was sitting in a drawer since 2011 the tantalum capacitors ESR went higher (3.9Ohm) when a fresh capacitor has ~1.1Ohm. I fixed board temporarily with replacement C33 and C36 for tantalum capacitors with low ESR and I purchased X7 2.2uF 1206 ceramic capacitors on Ebay.

Reagards
Bo
 
Zonker

Guru

Joined: 18/08/2012
Location: United States
Posts: 772
Posted: 10:38am 02 Apr 2015
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Nice Catch Boss..!

Good thing you got the second one..! Will check mine out... It might actually be easier to just order a new board rather than fuss with ordering the new parts and doing the iron work...
 
redrok

Senior Member

Joined: 15/09/2014
Location: United States
Posts: 209
Posted: 11:04am 02 Apr 2015
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Hi boss'
  boss said  tantalum capacitors on positions C33 & C36
Bo
What are these capacitors used for in the circuit?
redrok
 
matherp
Guru

Joined: 11/12/2012
Location: United Kingdom
Posts: 10400
Posted: 11:26am 02 Apr 2015
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  Quote  What are these capacitors used for in the circuit


These are the VCAP capacitors required for the STM32 internal regulator - same use as the 10uF-47uF VCAP for the PIC32MX.

Great find Boss!!!!!!
 
JohnS
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Joined: 18/11/2011
Location: United Kingdom
Posts: 4071
Posted: 11:50am 02 Apr 2015
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+1 well found!

John
 
boss

Senior Member

Joined: 19/08/2011
Location: Canada
Posts: 268
Posted: 12:01pm 02 Apr 2015
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@redrok

I opened Discovery BOM file and they recommended to use X7 ceramic capacitor size 1206 2.2uF/16V. I didn't have X7 capacitors at all so I have temporarily replaced these with the fresh tantalum low ESR 4.7uF/16V 1206 and I purchased X7 4.7uF/16V 1206 on Ebay (100pcs $4 + free shipping).

Regards
Bo
 
paceman
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Joined: 07/10/2011
Location: Australia
Posts: 1329
Posted: 01:09am 03 Apr 2015
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Bo, strange that they recommend X7 (I presume that means X7R as the ceramic's electrolyte?) Low ESR ceramics usually require the X5R electrolyte like we use with the MicroMites.

Greg
 
WhiteWizzard
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Joined: 05/04/2013
Location: United Kingdom
Posts: 2948
Posted: 02:33am 03 Apr 2015
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A general question: why would you use Tants when ceramics are much 'better' in terms of specification?
I am asking this in respect to using them for vRegs or vCaps (on a MicroController).

I have always recommended/preferred ceramics for this very reason. Can someone say why a Tant 'could' be better? (I have 'limited' analogue electronics knowledge' which I guess may be 'restricting' my view of the answer.

WW
 
matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10400
Posted: 03:41am 03 Apr 2015
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  Quote  why would you use Tants when ceramics are much 'better'


Only reason I can think of is that high capacity ceramics are only available in SMT packages.
 
boss

Senior Member

Joined: 19/08/2011
Location: Canada
Posts: 268
Posted: 05:24am 03 Apr 2015
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@paceman
Totally agree with you, but see the BOM. Because the capacitors are easily replaceable I decided to test it.

@WhiteWizzard
I have no ceramic capacitor in 1206 case right now, so only way how to verify this theory was remove capacitors and check the ESR and compare with fresh one. I have couple of fresh tantalum 4.7/16V 1206 so I decided to temporarily use them while waiting for ebay package.

Bo
 
WhiteWizzard
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Joined: 05/04/2013
Location: United Kingdom
Posts: 2948
Posted: 06:24am 03 Apr 2015
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@boss

Let me apologise with the way my question was worded. I meant 'why would anyone design a circuit and then use Tants instead of ceramics?'

I did not mean 'why did you . . . '!!

It amazes me to see Tants being used in circuit designs (especially in digital circuits) when ceramics are pretty much 'indestructible' - hence asking the question! Why do 'big' companies do this??

Having replaced so many tants/electrolytics in circuits (typically power supply circuits) I've always used ceramics as the replacements - never had to perform a rework after doing that.

Another bonus is that ceramics are considerably cheaper than tants (especially SMDs).

WW



 
boss

Senior Member

Joined: 19/08/2011
Location: Canada
Posts: 268
Posted: 06:39am 03 Apr 2015
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@WhiteWizzard

Good question, but to wrong person. Actually my first job after graduation was a researcher and measurement equipment designer. And the company bureaucrats always interfered with our lives with some stupid questions like "Why you need to order extra ceramic capacitor when we have in stock tantalum one the same value". I just guessing, but I could imagine it be similar reason.

Bo
 
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