What does 2x22µF mean? - Page 1
Forum section: electronics / beginners

author: cmumford
author_rank: Contributor
timestamp: September 02, 2022, 12:33:46 pm
content: In theAP63205 datasheet, the sample schematic has what I think is an output decoupling capacitor, but it's labelled "2x22µF". What does "2x" mean? Should I have two 22µF capacitors in parallel, series, or something else?

author: gamalot
author_rank: Super Contributor
timestamp: September 02, 2022, 12:48:38 pm
content: Means two 22uF capacitors in parallel.

author: rooppoorali
author_rank: Regular Contributor
timestamp: September 02, 2022, 01:49:00 pm
content: It means two 22µF capacitors. I am wondering, why the author didn't draw two capacitors in parallel instead.

author: cmumford
author_rank: Contributor
timestamp: September 02, 2022, 02:29:13 pm
content: Yeah, I figured it's just an uncommon shortcut notation used by this company. I just wanted to check before making any boards.I'm also curious why the datasheet doesn't specify a single 44µF capacitor. Is it because 22µF is a standard cap value, but 44µF is not, or is there an electrical reason why two 22's in parallel is better than a single 44?

author: JohanH
author_rank: Frequent Contributor
timestamp: September 02, 2022, 02:46:01 pm
content: In most cases, the difference between 47 µF (which is the standard value) and 44 µF doesn't matter. What matters more in this case is having two capacitors in parallel, which effectively halves the ESR.The data sheet also says:"An output capacitor with large capacitance and low ESR is the best option. For most applications, a 22μF to 68μF ceramic capacitor is sufficient."You will probably fine with a single low ESR capacitor, although it could be more expensive than using two "cheap" capacitors.Then it also depends on your specific application. Use the formulas provided in the data sheet to find out.If you are creating a one-off product, it doesn't matter much, throw a lot of low ESR capacitance on it (but do not exaggerate). If you are planning for production, consider which components are available and what will be a cheap option, of course so it still fulfills your demands (i.e. allowed ripple level etc.).

author: ledtester
author_rank: Super Contributor
timestamp: September 02, 2022, 02:48:53 pm
content: Quote from: cmumford on September 02, 2022, 02:29:13 pmThe datasheet probably specifies a minimum capacitance so anything above a certain value is recommended.Also, parallel caps generally have a lower ESR than a single cap.Finally, the selection of 22uF caps might be due to BOM optimization -- i.e. reduce the number of different cap values for the entire board or due to cost or supplier.

author: fourfathom
author_rank: Super Contributor
timestamp: September 02, 2022, 04:09:41 pm
content: Also, the 2X may be because most MLCC chip capacitors will have an actual lower capacitance when operated near their voltage rating. For example, the capacitance of a Murata 0.1uF 50V X7R cap is down 15% at 25V, and down 50% at 50V. Since the regulator spec suggests a 22uF minimum, a single 22uF cap might not be enough. Using two of them instead of just a single 47uF cap might be for all the other reasons mentioned above.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 02, 2022, 05:18:48 pm
content: I agree, the most interesting question is why it is specified that way. Some other possible reasons that occur to me:1. Packaging. The two capacitors may fit the available space better or meet part size guidelines in his application.2. Part standardization. This value may be used in a lot of products at the company, allowing bulk buys and reduced cost.3. A variant on 2. The author may have only had 22 microfarad caps in his lab stock.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2022, 07:15:24 pm
content: Quote from: cmumford on September 02, 2022, 02:29:13 pmTwo 22 microfarad capacitors in parallel have lower ESR and ESL and higher ripple current rating than a single 44 or 47 microfarad part, and it is the ESR, ESL and ripple current rating which matter in this application.

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2022, 08:17:55 pm
content: Quote from: ledtester on September 02, 2022, 02:48:53 pmThat's true, but in some cases there may be a maximum capacitance a power supply can handle without becoming unstable or shutting down due to overload. A minimum ESR specification is also common, especially for low drop-out regulators. One must read the data sheet very carefully.QuoteTrue.In this case ceramic capacitors are recommended. A much larger capacitance and voltage rating might be required to ensure the capacitance is high enough and the output voltage and the ESR is low enough. It might be more practical to use a larger aluminium electrolytic and tantalum capacitor in parallel with a smaller ceramic capacitor.https://www.diodes.com/assets/Datasheets/AP63200-AP63201-AP63203-AP63205.pdfNote the layout is very important. Throwing it together any old how will result in failure or poor performance. Follow the recommended layout in figure 25, as closely as possible.

author: cmumford
author_rank: Contributor
timestamp: September 02, 2022, 08:54:20 pm
content: Quote from: Zero999 on September 02, 2022, 08:17:55 pmThanks Zero999. I completely missed that section of the datasheet. I'll follow that layout.

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2022, 10:51:10 pm
content: From the datasheet you linked:"The output capacitor keeps the output voltage ripple small, ensures feedback loop stability, and reduces the overshoot/undershoot of the outputvoltage during load transients. During the first few milliseconds of a load transient, the output capacitor supplies the current to the load. Theconverter recognizes the load transient and sets the duty cycle to maximum but the current slope is limited by the inductor value.The output capacitor, COUT, requirements can be calculated from equations Eq. 9 and Eq. 10.The ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from Eq. 9:𝑽𝑶𝑼𝑻𝑹𝒊𝒑𝒑𝒍𝒆 = ∆𝑰𝑳 ∙ 𝑬𝑺𝑹 Eq. 9An output capacitor with large capacitance and low ESR is the best option. For most applications, a 22μF to 68μF ceramic capacitor is sufficient.To meet the load transient requirement, COUT should be greater than the following calculated from Eq. 10:"Equation 10 didn't copy and paste properly: see the end of the data sheet.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 03, 2022, 07:15:34 am
content: In a buck converter, much more critical is the input capacitance.Why? Draw the buck converter schematic (the two switches, input cap, inductor output cap) and add the series inductance of the capacitors (ESL) explicitly, i.e., the small unwanted inductors in series with the capacitors.Now look at it and ask what happens:1) when the input current suddenly tries to change from 0 to I_L and vice versa, through the ESL of the input cap.2) with the ESL of the output cap just acting in series with the explicit inductance which is massive anyway.And note, ESL is not just a feature of the component itself, more inductance is added by the layout.Simply put, it's just that buck converter needs input current to change QUICKLY, so the energy reserve needs to be close, otherwise stray inductance generates larger and larger voltage peaks trying to supply those current changes.So instead of a single 10uF massive input cap somewhere, you might want to parallel say 2-3 4.7uF parts, in smaller package (say 0805), which you can place closer to the switches, and use shortest possible traces or enough vias so that there are multiple parallel paths for the current, as short as possible.For the output capacitor, the ESL matters for the load and sensing, but not for the switching itself. The output current waveform is just triangle, a lot less nasty than the square-ish wave seen by the input.