Decoupling and bulk capacitor number and value

Question

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 ?

Answer 1

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?

Answer 2

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!