Decoupling caps on STM32 - Page 1
Forum section: electronics / general

author: init
author_rank: Contributor
timestamp: April 23, 2016, 04:16:37 am
content: Hey guys, so I'm working on the power supply for an STM32f446RET6 and looking at decoupling caps, specifically the one that connects Vcap_1 to ground. Since I'm using the 64 pin package, it has one Vcap_1 pin and the data sheet recommends 4.7uF with <1ohm ESR. From a quick search, I see that there isn't anything in terms of eletrolytic radial cans that have an ESR meeting this condition. Is the implication here that I should use tantalum instead? I've heard tantalum to be unreliable, is there any truth to this?Cheers

author: SeanB
author_rank: Super Contributor
timestamp: April 23, 2016, 04:49:13 am
content: 22uF SMD ceramic will do there, and even with the capacitance drop with voltage it should work, though you can simply place pads for 2 next to each other in case you get a poor batch which magically becomes a 2uF part at 5V. You could use a tantalum part, just use a 16V part on 5V and it will be fine, but the ceramic would likely be a lot cheaper.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2016, 07:03:25 am
content: Ceramic or tantalum is fine.Ceramic: choose X5R or better, 10V or higher, 4.7 or 10uF. Will probably be ~0805 size. No big deal.Is this a 2 or 4 layer PCB? The VCC pins should all be bypassed; you can skimp on these if you use internal VCC/GND planes, but you should include one for each on a two-layer build.Tim

author: init
author_rank: Contributor
timestamp: April 23, 2016, 08:14:41 am
content: Quote from: T3sl4co1l on April 23, 2016, 07:03:25 amJust a thought with the ceramics, I seem to recall larger ceramics that have multiple interleaved plates can exhibit piezoelectric effect at certain resonant frequencies. If I have a switching converter on my board, is there any chance that could cause issues?Unfortunately I am limited to 2 layers with the level of software I'm using (freeware diptrace), so yeah I've put a separate 100nF ceramic per VDD/VSS pair as advised in the data sheet. On a separate note, the datasheet also asks for a 4.7uF across VDD/VSS and a 1uF across VDDA/VDDA, but doesnt specify ESR for these, what sort of cap would you use?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2016, 09:49:24 am
content: The 4.7uF on VCC will simply be whatever bulk cap you have on the supply, e.g. from the converter, input bypass, whatever. Tantalum or electrolytic would be typical. (You can make thingsworseby using ceramic or aluminum polymer here: the ESR is so low, the bulk cap acts as a short circuit; the remaining caps resonate against their trace inductances. Ideal ESR is around R = sqrt(Lstray / Cbyp).)The VCCA bypass can be whatever. They probably suggest ferrite beads too; that's fine. I don't think much current is drawn from that supply to need much consideration, but if you like, you can use a small-ceramic + large-lossy strategy there, too.Tim

author: Kjelt
author_rank: Super Contributor
timestamp: April 23, 2016, 11:24:48 am
content: You're right Tim,I was just going to post I never ever saw tantalums used with the STM32 uC's but even the F4discovery has tantalums as their Vcap caps. Good call.

author: Monkeh
author_rank: Super Contributor
timestamp: April 23, 2016, 11:36:22 am
content: Quote from: Kjelt on April 23, 2016, 11:24:48 amerh?F4 Discovery has two 2u2 ceramics for Vcap. Not easy to meet the ESR requirement with tantalum..The tantalum caps are for the analog supply and reference (and elsewhere, audio and USB).

author: Kjelt
author_rank: Super Contributor
timestamp: April 23, 2016, 11:48:32 am
content: Not on my board.C33 and C36 the Vcaps are polarized caps. Will take a picture.

author: Kjelt
author_rank: Super Contributor
timestamp: April 23, 2016, 11:57:37 am
content: wow! hold on my 3rd F4 board does have ceramics on them , but note that the silkscreen still has the polarized markers on it. Now I am totally confused, probably just a BOM reduction or indeed a change to increase stability?

author: bktemp
author_rank: Super Contributor
timestamp: April 23, 2016, 11:58:22 am
content: Quote from: T3sl4co1l on April 23, 2016, 09:49:24 amVCCA decoupling seems to be important, because it is also the PLL supply. Recently I had some trouble with the PLL: The CPU worked fine, but ocassionally there was a glitch on the output signals from the LCD controller (and also on any other signals derived from the PLL clock). I traced it back to an instable PLL ocassionally going out of lock whenever there was an increased ripple on the 3.3V rail due to the SDRAM drawing more current when transferring data.On STM32F469 discovery board there are 1uF tantalum + 100nF ceramic for Vcc and VccA but 2.2uF ceramic for VCAP.