Renesas Datasheet doesn't account for DC-Bias?! - Page 1
Forum section: electronics / projects

author: Return Current
author_rank: Newbie
timestamp: October 21, 2022, 10:01:15 pm
content: Hello,I am currently working on designing a not so low current (6-10A) step down converter to supply WS2812B LEDs (24V to 5V). For that I selected the ISL8117A Step Down Controller IC from Renesas (Datasheet). This IC requires an external Feedback Loop Compensation network/filter. While reading this section of the datasheet in order to design an appropriate filter for my design, I noticed something odd.The datasheet provides a design example with actual values, which happens to be the same values as one version of their evaluation boards for this IC (Datasheet EvalBoard). In this version/example they use 200uF of output capacitance. In the datasheet of the evaluation board it is specified which capacitors they use to attain the 200uF output capacitance: They use two 100uF ceramic capacitors from Murata:DC-Bias Characteristics.When accounting for their DC-Bias characteristics those two capacitors result in 2*100uF* (1 - 0.363) = 127.4uF (-36,3% change at 3.3V DC Bias).However in the example (in the datasheet of the IC) for designing the compensation network for the evaluation board circuit, they do not seem to consider the DC Bias. On page 19 they calculate a angular velocity/frequency of a pole in the control loop with equation 19 (EQ19). The datasheet states that the angular velocity yielded is 14.7*10^3*s^-1. However you will only get this result when you calculate with 200uF. Using 127.4uF would yield a significantly different result. My calculations:So they seem to have ignored the DC-Bias?! I am a bit irritated because of this and do not know what to think of this. When it comes to designing a network for my own application I intend to consider the DC-Bias because not doing this just feels wrong. Do you think I should consider the DC-Bias or should I not?While I personally do not like ignoring the DC-Bias I do understand that a somewhat-off frequency for this pole / zero (in the compensation network) doesn't result in immediate malfunction of the control loop. However, I would assume it worsens the quality of the regulation of the output voltage.I look forward to hearing your thoughts on the matter.

author: moffy
author_rank: Super Contributor
timestamp: October 21, 2022, 11:36:10 pm
content: You appear to be more correct in your taking into account DC bias (assuming all else that is stated is accurate, I checked the DC bias) than the application, but I must say this is the first I have seen of the dramatic effect of DC bias on some caps. I don't think I would use such caps, -55% @ 5V! It makes a nonsense of their 100uf+/- 20% spec. Thanks for the info.

author: Return Current
author_rank: Newbie
timestamp: October 22, 2022, 12:31:05 am
content: Thank you for your reply. Yeah I think I am probably going to use the "real" capacitance.When you get to these levels of capacitance it is quite normal to have pretty strong capacitance losses with MLCCs and an applied DC Bias voltage. Even if this cap has -55% at 5V, it still provides 45uF with only about a milliohm of ESR. Put three of them in parallel and you got 135uF of output capacitance (which is not that small) with very low ESR and therefore output ripple.

author: moffy
author_rank: Super Contributor
timestamp: October 22, 2022, 12:47:38 am
content: I have been watching a number of electronic repair videos, NorthbridgeFix etc. and the large ceramic caps are notorious for going short circuit. If there is a short on the board look for a blown MOSFET or ceramic cap. Do you know if the MLCC caps are piezoelectric at all? i.e. they change dimensions with applied voltage.If I had to use one, I think I would go for higher voltage even if more were required.

author: Return Current
author_rank: Newbie
timestamp: October 23, 2022, 09:29:52 pm
content: Thank you for these tips. I actually do not know to what extent these caps are piezoelectric. I think I will go with theseCaps. 6.3V Caps would be fine but 10V Caps for a 5V rail should conservative enough.

author: moffy
author_rank: Super Contributor
timestamp: October 23, 2022, 10:31:07 pm
content: I looked up the piezo electric effect of these caps and it seems to be quite strong. I wonder if that is the reason for their failures i.e. mechanical fatigue.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 23, 2022, 11:05:30 pm
content: This is very common from vendor datasheets. Sometimes they admit that MLCC derating exists, sometimes they don't. Even when they admit it, they rarely tell you whether they mean "nameplate" or "effective" capacitance in any particular place, and they flop back and forth between them as it pleases them. It's frustrating.Fortunately it also doesn't usually matter that much. There's enough slop in DC-DC converters that you can get away with a lot, unless you are really trying to cost-optimize or space-optimize, or you're on the ragged edge of performance some other way.The voltage rating of a particular MLCCis not relatedto its DC bias characteristics. It just isn't. Itusedto be, years ago, but as the manufacturers have pushed the C-V product harder, it has lost whatever relationship it once had. There is a kinda-sorta-general trend, yes, but you simply cannot trust a 50V X5R part to be any better at 5V than a 6.3V X5R unless you have pulled the specs for the exact part in question. So, also given that MLCCs are remarkably tolerant of overvoltage in general (but never rely on this), a 10V part is really no more "conservative" here than a 6.3V part.They are all piezoelectric or ferroelectric, so, again, don't think you are escaping that somehow. If you care, you need to use the soft/flexible termination parts. If you really care, you can't go larger than 1210 or so (0805 if you're especially paranoid), because the bigger ones are much worse.

author: moffy
author_rank: Super Contributor
timestamp: October 24, 2022, 12:10:28 am
content: Quote from: exmadscientist on October 23, 2022, 11:05:30 pmIt is a more complex subject than I realized. I would have thought that higher voltage = thicker dielectric for the same material, but I guess not. I wonder what the limiting factor is?P.S. I am talking about the difference between a 6.3v Murata part GRM32EC70J107ME15 vs 10v Murata, GRM31CR61A107MEA8K

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 24, 2022, 01:57:13 am
content: The #1 factor is and will always be physical size. 1210 outperforms 1206 outperforms 0805 outperforms 0603 outperforms 0402 outperforms are-you-crazy-even-0402-barely-works. And remember that thickness counts, too; a thin 1206 might be worse than a normal 0805. Everything else is secondary.The #2 factor is the physical material they're made of, including (perhaps) its processing during manufacturing. Murata codes this or some part of this as the two digits after the "E", so your two options pit "15" against "A8". Those codes mean little to nothing to me, but you can see if you play around that they are directly related to capacitance derating, and generally consistent across (at least) part size codes.Those two factors, at least for Murata, explain the majority of MLCC capacitance loss with DC bias.

author: moffy
author_rank: Super Contributor
timestamp: October 24, 2022, 09:05:21 am
content: If you haven't seen it, look at the graph posted by "wraper":https://www.eevblog.com/forum/projects/is-ceramic-capacitor-package-relevant/very informative.