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'[SX] SX28 Bypass Capacitor selection question'
2005\11\22@135211 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Greetings,

Not being an engineer, I would like to ask for the opinion of those who better understand the subtleties involved in the selection of bypass capacitors. After reading about twenty articles printed from the web (including one I found on the stamp mailing list and one on  sxlist.com entitled PCB Layout and Prototyping Considerations ) I have come to a near comfortable decision on my selection. But since this is for a proposed product for manufacturing, I need to better understand a few things.

Background:


  1. I am using eight SX-28 chips in a non-critical timing situation using the internal clock at 4 MHz. Most of the SX's are used as smart drivers for LEDs. Only one is used as a master controller. The eight chips are physically on four separate but small boards - necessary for geographic distribution in a small case for optical coupling to fiber optic lines.
  2. I am considering a multilayer ceramic by AVX with .2" lead spacing to be placed next to pins 2 & 4 of the DIP package.
  3. Capacitor specs. = .01uf, 50v, XR7 temp. coefficient.


Assuming that I may be on track so far, I would like some advice with the selection of the Temperature Coefficient. My understanding at present, without going into all the math I've seen on this subject, is that the capacitance value is not too critical in this bypass application and therefore a COG (NP0) temp coefficient may not be needed, thus saving some money.

But, saving money is not my primary goal. I want the best reliability I can put into my product, but even more so, I just want to understand this issue! Some of what I've read has left me somewhat hesitant on the XR7 choice.

I've also searched the UBICOM data sheet for the SX28 and none of the spec info helps me understand the switching rate that I am facing. Maybe I'm making too much of this. Maybe there is a general cook book rule that most everyone follows.

I would greatly appreciate the help and wisdom of someone further down the road than I am at present. Thanks for reading.

Tom Taylor
taylort@lanecc.edu

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2005\11\22@154252 by Coriolisn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Coriolis wrote:

For bypass applications, I wouldn't be that concerned about the temperature coefficient. The application isn't value critical, also temperature coefficient is only important when exposed to varing temperatures. Neither should be a concern to you. I posted a link to a guide on choosing bypass capacitors: http://forums.parallax.com/forums/default.aspx?f=15&m=95311

Your  0.01 F value seems a little low for higher operating frequencies, but it might be sufficient for 4MHz.

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2005\11\22@160843 by Forrestn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Forrest wrote:

FYI Parallax's SX Tech board uses a 0.1uF monolythic capacitor.

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2005\11\23@075215 by Chris Savagen/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Chris Savage wrote:

.1uF has always been a good typical bypass/filter capacitor value for digital logic, controllers, etc.

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2005\11\23@083052 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Thanks guys for helping me settle this issue.

Paul, I did go to the article you posted. (Thanks) The question I have is that I could not find that posting when I did an initial search for "bypass capacitors" prior to posting my initial question. I even copy/pasted the phrase from your post and ran all the possible combinations of choices from the search window and never came up with your post. So this morning I tried again to see if I was doing something wrong - I am new to the forum. Again, I did not see your post (unless my eyes are just skipping over it!),... but I did see mine! ???

As I now see the content of the sandbox, that would have been a more appropriate place for my post. And now my problem(?) with the search window is drifting far from the current subject... but I just wanted you to know I had tried the search feature prior to posting.

Can anyone tel me why the phrase "bypass capacitors" would NOT show up when selecting "All of the keywords and phrases", but will show up when selecting the other option, "Any of the keywords and phrases"?

Thanks again.

Tom Taylor
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2005\11\23@092326 by prcoyn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, prcoy wrote:

Try the new search at 'search.parallax.com' it uses the Google engine.

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2005\11\23@092522 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Thanks, Paul
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2005\11\23@093023 by Jon Williamsn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Jon Williams wrote:

And on the Advanced Search page you can enter "forums.parallax.com" into the domain field to limit the search to forums posts.

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2005\11\23@104544 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Thanks, Jon.

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2005\11\23@120635 by g_daubachn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Thomas,
please have a look at this forum thread for more info about filtering, EMI, and all such crystal ball reading techniques...

http://forums.parallax.com/forums/default.aspx?f=7&m=75182&g=75648#m75648
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2005\11\29@150604 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Guenther,
Thanks for the tip about the EMI thread. It is very informative - containing lots of "in the trenches stuff".

I do have a question if you have a moment. I noted with interest your dropping down to 20 MHz and the results you found. Since I am using the internal 4 MHz. clock (for an application where timing is non-critical) I am still facing the same transition slope of any switching element in the SX. In my mind I see two elements of the EMI problem (1) switching frequency (driven by clock speed) and (2) transition time (driven by the chip technology). Am I correct in thinking of it this way? I sensed this is what you were saying in your last post of the EMI thread. I looked in the SX spec sheet I had and could not find any info on transition timing. Do you have any info on this?

Related to this subject:
I just found this site this morning which may be of value on EMI. They have back-issues of newsletter articles and even offer seminar classes called
EMI/EMC/SI Made Simple  (Grounding, Shielding, EMC/SI in PCBs, and EMC troubleshooting)  http://www.emiguru.com/index.html
Thanks to all for your help and support. It's great to be able to "reach out and touch" someone's experience and advice.

Tom
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2005\11\30@015818 by williamn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, william wrote:

Thomas,
How is the Master SX going to communicate with all the Driver SXes?
If you are going to use some serial UART, I think you'd better put a 4Mhz resonator to all the driver SXes instead of using the internal oscillator.  The internal oscillator freq can run by as much as 10% even with trimming.

William
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2005\11\30@030311 by g_daubachn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Thomas,
unfortunately, I don't have any information about transition timing either. I agree with you that sources for EMI problems can be the switching frequency and the transition time. In theory, an "ideal" transition, i.e. with no rise or fall time, is composed of an indefinite number of harmonics. As the SXes are designed to run at clock speeds of 50 MHz and even higher, I'm pretty sure that the transition times are shorter than 5 ns independently from the clock speed.

Therefore, good filtering of the supply voltage, and other methods for reducing EMI are always important, no matter which clock frequency and clock source you are using. During my experiments in the EMI lab, I found out that reducing the supply voltage helps a lot. So if you have the chance, you should consider running the SXes at 3.3V, for example.

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2005\11\30@090126 by Thomas Taylorn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Thomas Taylor wrote:

Guenther,
Thanks for the confirmation on the switching and frequency issue. On other papers concerning EMI issues there was some mention of transition times around 2 ns, and this was for a higher clock frequency, so your estimate puts it right in the ballpark.

On another response of yours (thread on voltage regulators, 78XX) you gave some info on the LM2574 stating it was a favorite of yours. That gave me an idea that will help solve a heat issue I was facing so I printed out National's data sheets and will start experimenting with it when I get hold of one on my next parts order. Thanks for the info. BTW, that sure is a compact board you have! That photo is what really sparked my interest. No more large heatsink!

Thomas
---
William,
I hope I have not made my project sound more glorified than it is. The Slave SX28s are simply smart LED drivers. Each has a unique program routine that they perform but only under the direction of the Master SX. The timing is really slow and non-critical. The Slaves (and a sound chip) basically wait looking for their coded address to appear in the A register, then off they go, performing a specific task that when accomplished they return and continue to wait looking for the next code in the A register.

I do appreciate your question and concern about the oscillator tolerance. Thanks
Thomas
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2005\11\30@103847 by Peter Van der Zeen/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Peter Van der Zee wrote:

Hi Thomas and Guenther;
The internal switching times and the propagation delays inside the silicon are constant, and independent of the oscillator clock used. The transition time us mortals can measure are the ones on the output pins, and I understand that those have been purposely rate-limited; obviously to reduce noise. The transition time of an output is also heavily influenced by the amount of capacitance presented by the lines/load connected to it; it is amazing to see how much effect 5 pF can have.

Using a special 1 pF oscilloscope probe and no connections to an output pin (just the pin itself) shows a bit less than a 10 nano second transition. Adding the few pF of a "normal" scope probe will readily slow that to about 15 nSec. Poorer (lower frequency) probes will fare worse.

Cheers form Maui, Hawaii,
Peter (pjv)
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2005\11\30@125351 by g_daubachn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Peter,
I absolutaly agree (maybe, I did not make this clear enough in my last post) - internal switching times and propagation delays inside the silicon are more-or-less constant, and independent of the clock oscillator.

Thomas,
when I mentioned the 5 ns transition time, this was just an estimate. Assuming that the SX silicon was designed for a maximum of 100 MHz clock, i.e. for a 10 ns clock period, I made the assumption that the internal transition time must be at least half the clock period, i.e. 5 ns but I guess that the 2 ns you had mentioned are much more realistic.

Concerning the DC/DC converter: For one of my commercial projects, I needed a 24/5 step-down DC/DC converter and I was lucky enough to acquire 300 Rohm BP5220 converters at a very reasonable price on eBay, assuming that this supply of converters would be more than I ever needed. (Un)fortunately, it soon turned out that I had under-estimated the number of units that could be sold. Instead of buying  BP5220 at a remarkably higher price, I designed this little PCB around the LM2574, pin-compatible to the BP5220 and my costs for this replacement were lower than the regular price for the BP5220. Another big advantage of the LM2574-based design is that it has short-circuit protection where the BP5220 does not. Shorting the BP5220 usually immediately shorts the internal series transistor. This means that the full input voltage will be fed into the output pin after removing the short, causing a "nice" chain reaction - did you ever see how an SX looks like after 24V have been fed into the Vdd/Vss pins?

In the meantime, I have re-designed most of my SX-based boards to have an LM2574 on-board instead of using the small add-on board.

In the end, I can only report very positive results with the LM2574 - you are right - no large heatsinks required, although there are some "hidden" ones - the ground planes on both sides of the PCB.

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