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I am currently finishing a circuit with TIVA C TM4C123G where the user manual indicate the following:

"decoupling capacitor typically 0.1uF in value and should be accompanied by a bulk capacitor. The combined VDD and VDDA bulk capacitance of the microcontroller is typically between 2uF and 22uF" "For optimal performance, locate one decoupling capacitor adjacent to each VDD power and ground pin par. At a minimum there should be one decoupling capactiro on each side of the microcontroller package connected betxeen VDD and Ground"

1./ Looking at the launchpad board TMC123G and other board EVM, the number of decoupling capacitor and bulk capacitor are not respecting those indication. So, how to know how many capacitor and of which value I should put (particularly for the bulk) ?

2./ What can happen if I put too many decoupling capacitor on one pin

3./What can happen if I put too many bulk capacitor on one pin ?

I am a bit lost here , sorry if my question look stupid

Launchpad guide reference: http://www.ti.com/lit/ug/spmu296/spmu296.pdf DK guide: http://www.ti.com/lit/ds/spms375e/spms375e.pdf Chip guide: http://www.ti.com/lit/an/spma059/spma059.pdf

I don't understand because in the chip they say some decoupling capacitor should be placed on each side of the mcu, but on the LQFP144 package there are several VDD +GNd pair... so i am supposed to put decoupling capacitor and bulk capacitor on all ?

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  • \$\begingroup\$ Show the schematic of the LaunchPad, and tell us which specific part you think is wrong. \$\endgroup\$
    – CL.
    Jun 14, 2016 at 9:08
  • \$\begingroup\$ You can't put too many of them. At some point (pretty quickly, actually), it becomes useless, but it will never have bad consequences. \$\endgroup\$
    – dim
    Jun 14, 2016 at 9:29
  • \$\begingroup\$ @CL I have added the link to the guide which incldue the scehmatic inside \$\endgroup\$
    – chris
    Jun 14, 2016 at 9:40
  • \$\begingroup\$ @dim, this is the point, I don't know if I should place on each pair or not... \$\endgroup\$
    – chris
    Jun 14, 2016 at 9:40

2 Answers 2

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On what I've seen from the Launchpad schematic, they seem to put one decoupling cap near each pin, and a single bulk for both VDD and VDDA. The decoupling caps are a mix of 10nF and 100nF, and the bulk is 1µF.

Indeed, this is different from the values in the recommendations, but the principle is the same. 10nF is quite low for a decoupling cap, but still makes the job. These low values are usually recommended for high-speed FPGA designs, but they also recommend using 0402-sized caps to be even closer to the chip, etc... In your case, just put 100nF everywhere (everywhere means: one for each VDD/VDDA pin, very close to it, as per the recommendations - the sides don't really matter, what matters is each pin).

Now, for the bulk, do as you prefer: a single 1µF minimum (for the whole chip, including VDD and VDDA supplies), close to the chip (and with short traces running to each pin). But if you use 2.2µF, 10µF, it will be all right. And I'm even sure if you don't put it, it will still work (unless the regulator is very far away).

Whatever you choose to do, don't worry: you can't (well, almost - see below) put too many decoupling/bluk caps. At some point (pretty quickly, actually), it becomes useless, but it is unlikely to have bad consequences. It could make some older LDO regulators to oscillate (the one that are unstable with ceramic output caps), if you put really too many of them and they are all close to the regulator.

Som interesting insights are also provided in this (more general - which is why I don't consider it's a duplicate) question: What is a decoupling capacitor and how do I know if I need one?

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    \$\begingroup\$ I'd like to add that you can have to many decoupling caps. Each one lowers the overall ESR of the capacitance seen by the voltage regulator driving the microcontroller. Now some voltage regulators are fine with this while others (older) may become unstable and break into oscillation. \$\endgroup\$
    – Nils Pipenbrinck
    Jun 14, 2016 at 10:46
  • \$\begingroup\$ @NilsPipenbrinck That is right. You need all those three conditions, though: 1) The regulator is an old LDO regulator. 2) You put a lot of decoupling caps. 3) The regulator is close to all these decoupling caps. But I'll mention it. Thanks. \$\endgroup\$
    – dim
    Jun 14, 2016 at 10:53
  • \$\begingroup\$ The bulk cap is a "bucket of electrons" that is responsible for supplying energy for the low frequency spikes. Basically bulk cap is needed to counteract line inductance which may very well be neglible if you've got an actual closely coupled power and ground planes plus adequate regulator cap. For high frequencies you should be looking at impedance curves, generally smaller packages have smaller inductance hence smaller impedance. The whole business of decoupling can get quite involved with higher frequencies with vias having their own inductance and providing return path via close by.. \$\endgroup\$
    – Barleyman
    Jun 14, 2016 at 12:21
  • \$\begingroup\$ Not only old LDOs have this behavior; synchronous buck regulators (maybe more common than marginally stable LDOs) also do, because they need to see the inductor ripple current to work properly. Anyway, it is a minor point. \$\endgroup\$
    – Oleksandr R.
    Jun 14, 2016 at 13:00
  • \$\begingroup\$ @dim , thank you very much for your insight. It was very usefull \$\endgroup\$
    – chris
    Jun 14, 2016 at 13:42
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I am not sure you understand the purpose of decoupling and bulk capacitors so this answer should adddress that. Hopefully, all your other answers would be answered with this new understanding.

Digital circuits may require alot of current. Specifically, they require a lot of FAST current during. What fast current means is that it requires current for a very short amount of time. In digital signals, this fast current is during the edge transistions (going from high to low, or from low to high). If you have alot of digital transistions, you could very easily be drawing 1-2A easily.

So why can't we just get the current from the power supply ? A power supply supplies as much current as needed right ?

Yes, but they supply as much (quantity) current as needed but not how fast they are needed. Power supplies are slow, and cannot react to the current demand.

What type of passive device exists which resists voltage change but allows instantanous current change ? A capacitor !!

Capacitor's can react very quickly and provide the necessary current needed.

This is why you need capacitors as close as possible to the power pins of your device, so that when you device needs the current, it doesn't have to go far to find it. So this capacitor, decouples, the current from power supply. Decoupling capacitors.

Capacitors cannot store infinite amounts of energy. With heavy current draw, the charge on the capacitor dies out. The capacitor needs to be charged up again. So where can it get it from ? The power supply ? Sure, that's an option, and in cases, that's ok! But what if you want your decoupling capacitors to be charged faster, so that the the next round of current bursts from your digital circuits have the necessary current ? Yup, another capacitor. A bulk capacitor (also called a reservoir cap or a tank cap).

A bulk capacitor just acts as a bridge between the power supply and the decoupling capacitor. It's only purpose is to shorten the recharge time of the decoupling capacitors so that the capacitors. They tend to be much bigger, and generally recharge an area of capacitors. You might have 1 bulk cap for 2-3 ICs in the area. The bulk capacitors replenish its charge directly from the power supply.

Hopefully the information should be clear for you to answer your own questions now!

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  • \$\begingroup\$ I know already what is a decoupling capacitor is (to amortize ripple from switching etc...) and the bulk as a tank I know also. Your explanation is nice by the way, this is very easy to understand. The question however was why the hardware guide say to put a decoupling and a bulk capacitor on each pin pair while the demo board and the EVM don't follow this, other question was about how to define the value of those decoupling and bulk capacitor \$\endgroup\$
    – chris
    Jun 14, 2016 at 13:40
  • \$\begingroup\$ @chris Well think about what the purpose of decoupling is and what bulk is for. It's to provide fast currents. Its recommended but not always required and the EVM didnt use it so for THEIR design, it wasn't necessary. They probably figured out what their peak current draw is and determined it wasnt necessary. If you can do that, then you can selectively choose which caps you need. This is not an easy task, so its recommend/rule of thumb to put decoupling caps on all pins. \$\endgroup\$
    – efox29
    Jun 14, 2016 at 13:43
  • \$\begingroup\$ what about the value of those cap ? How do you calculate it ? \$\endgroup\$
    – chris
    Jun 14, 2016 at 13:48
  • \$\begingroup\$ @chris thats a difficult question. To get the exact values all depends on how much current you need and how fast you need it. To determine that is complex and difficult subject. That is why rules of thumbs exists (because its "good enough"). 100nF for decoupling and 1-10uF for bulk. These should cover majority of cases. If you are interested in learning more about this, have a look at PDN (Power distribution network). Not only is the value of the capacitor relevent, but so is the physical size. \$\endgroup\$
    – efox29
    Jun 14, 2016 at 13:54
  • \$\begingroup\$ @chris Also, decoupling caps are not necessarily to reduce the ripple (in your first comment). While they may act divert some high frequency noise from the rails - that is not their primary purpose. So your understanding of the decoupling capacitor is not complete. \$\endgroup\$
    – efox29
    Jun 14, 2016 at 14:01

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