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I’m making a USB powered device and that will need relatively large bypass capacitors and I’m worried about the inrush current to the caps.

Yes I know about the 10 uF inrush limit of the USB 2.0 spec, but the these larger capacitors will be charged after the initial inrush (power sequencing I think it’s called) so I guess it can handle higher current spikes by then.

But how much capacitance can I have without creating too big current spikes during charging of the capacitors?

(I already know about the TPS2141 USB power management IC but I hope I don’t need it)

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  • \$\begingroup\$ “I’m worried”, ”so I guess it can handle”, ”but I hope”. This is not how you engineer something. Define your specifications, read guidelines, look at application notes and do simulations. \$\endgroup\$ – winny Nov 13 '19 at 17:03
  • \$\begingroup\$ @winny, be reasonable, you're defining idealised electronics design. OP's question is on problem solving in practical engineering. Let's help them instead. \$\endgroup\$ – TonyM Nov 13 '19 at 17:08
  • \$\begingroup\$ @TonyM That would be by reading the specification, look at application notes for commercial circuits for the very purpose and start simulating. \$\endgroup\$ – winny Nov 13 '19 at 17:11
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    \$\begingroup\$ I think it's a reasonable question. Engineers have intuitions too. Sometimes this is easily settled by datasheets but sometimes the answer is not spelled out as in this case and then someone on SE might have a quick answer. \$\endgroup\$ – Anders Petersson Nov 13 '19 at 17:24
  • \$\begingroup\$ @winny, it's a considerably larger world out there with far more obstacles than what you're describing, so such closed advice is often useless. Anyway, back to the OP's problem... \$\endgroup\$ – TonyM Nov 13 '19 at 17:44
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But how much capacitance can i have without creating to big current spikes during charging of the capacitors?

You can have as much capacitance as you want. You just have to limit how fast you ramp up the voltage on those capacitors.

You can google "inrush current limit circuit" to find circuits that solve this problem. Or, if you're using a switching supply, you can look for one with a "slow start" circuit.

If you have some part that has a maximum voltage ramp-up time (some digital circuits are like this) powered by the same supply as the big capacitors, that might limit how slowly you can ramp the voltage, and thus limit how big your capacitors can be without violating the current limits for USB.

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Inrush current limiting is regulated by USB Specifications (2.0) in section 7.2.4.1. Essentially is says that you can't have more than 10 uF connected DIRECTLY to the port, and, if you need to engage "high-power mode" (>100mA), you need to implement your internal power supplies/converters such that they limit start-up currents:

enter image description here

(see p.177). In general, power of a USB device with "high-power function" (aka >100 mA) has to be implemented in two stages: first stage should not take more than 100 mA and should report its descriptor to USB host, and engage the "high-power function" only after the host sets "SET_CONFIGURATION" command. So you will need the TPS2141, or use any other means of control of your secondary on-board supplies after SET_CONFIGURATION is received.

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