At the bottom of this data sheet: https://datasheet.lcsc.com/lcsc/1811151645_CoreChips-SL2-1A_C192893.pdf the reference circuit diagram shows huge 100uf decoupling capacitors connected to the USB plugs. My circuit board is tiny and I am hoping to use a 0402 or even 0603 part, can I get away with 10uf (0402), or say 47uf (0603) caps instead? The hub is for a very specific use, I will be connecting two wireless keyboard dongles to it (semi-permanently), it is not going to be used as a general purpose hub.
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\$\begingroup\$ IIRC, the maximum capacitive load USB allows is 10 µF; anything more, and you need inrush current limiting. I've not seen a 0402-sized 10µF cap aside from the ones with 4V rating, and you will want > 5V + some safety. Also, your USB plug / wire connection will be multiple times larger than an 0805, I don't really think you're saving space at the right end here. \$\endgroup\$– Marcus MüllerApr 14, 2022 at 23:44
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\$\begingroup\$ They're probably there to deal with noncompliant devices that have a large capacitive load. A big, relatively high esr electrolytic will source enough charge to hopefully keep the hub from browning out or blowing a fuse on insertion. I've seen hubs with and without them so they are not strictly necessary. \$\endgroup\$– user1850479Apr 15, 2022 at 0:28
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2\$\begingroup\$ At 5v bias a relatively good 0402 will give you at most 1uF effective capacitance regardless of nominal capacitance, so no point in 10uF. I would instead put them on the hub as decoupling and let the power supply (or host) deal with bulk capacitance. \$\endgroup\$– user1850479Apr 15, 2022 at 0:31
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\$\begingroup\$ @MarcusMüller The 10uF load is a limit placed on the devices. The hubs can have more source capacitance, although there is also a limit. Maybe 100uF off the top of my head. \$\endgroup\$– Kuba hasn't forgotten MonicaApr 15, 2022 at 2:55
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2 Answers
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According to Table 7.7 of the USB 2.0 Specification any compliant USB hub must include at least 120uF of bypass capacitance to prevent droop when a device is plugged into one of the downstream ports. Note that this is "per hub" rather than per port.
All of the electrical requirements are found in section 7 of the USB 2.0 specification.
But in your case, you're not designing a compliant hub, so you don't need to worry about droop during attachment. In that case its certainly possible to use much less capacitance.
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Yes, you can use much smaller capacitors - 10uF would be enough, or even 4u7, as long as it’s an actual capacitance that you measured at 5V.
Do measure the capacitance under DC bias. The physically small capacitors use dielectrics that cause monumental loss of capacitance as the voltage on the capacitor goes up from 0.
The “47uF” part you think you’re using at 0603 will probably be an 8uF capacitor at best at 5V. But measure it yourself, and ensure you are set up to do capacitance measurements under DC bias. Unbiased measurements of surface mount capacitors are mostly fiction unless you actually used them with no DC bias. You should instead measure them under an operating voltage present in the actual circuit.
Many cheap capacitance meters can’t deal with DC bias, so you’ll need a better meter I bet. In fact, meters not designed for DC bias can get destroyed if you attempt to bias the capacitor.
answered Apr 15, 2022 at 2:59
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
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