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For a system I am designing I need go over 500mA limit for a very short time (30mSec). I have designed my system such that during the over 500mA period I am able to sustain my circuits using internal caps. (Internal caps can maintain voltage for my operation). However on the bench using a bench supply I see that I drag the voltage down (500mA constant current mode) to 4.5 during this 30mSec.

Is this allowed? What can go wrong ? (Would USB disconnect ?)

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  • \$\begingroup\$ double the capacitance, try to get the drop to more like 4.75V. 4.5 is very close to/out of USB standard. \$\endgroup\$ – KyranF Oct 29 '14 at 7:20
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    \$\begingroup\$ Consider using a current limiting switch. This will ensure that you never draw more than 500 mA from the host. When the demand exceeds 500 mA, the switch will limit, and the Voltage on your board will start to droop. You can use whatever size capacitors are needed to keep the droop acceptable, but at least you won't have to worry about the host dropping you. Note: the current limiting switch will also help out during USB attach. If you have a lot of capacitance, that can drag down VBUS when you connect, but the switch will limit inrush to 500 mA. \$\endgroup\$ – mkeith Oct 29 '14 at 7:26
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    \$\begingroup\$ onsemi.com/pub/Collateral/NCP380-D.PDF \$\endgroup\$ – mkeith Oct 29 '14 at 7:28
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If you exceed 500 mA of current draw, then the result is going to be highly dependent on the exact implementation of the source current limiting.

It is possible that there will be no current limiting and you will have no issues at all. It is possible that there will be a PTC or similar electronic fuse. This will likely allow you to draw over the limit for brief periods. It is also possible to have a hard current limit that will either drop the voltage to clamp the current, or it will trip and disconnect the input voltage entirely.

The proper solution is to add enough capacitance so the transient over 500 mA will be supplied by the capacitors instead of the source. It might be a good idea to add some sort of current limiting to the power rail to make absolutely sure your device does not exceed 500 mA. A small-value resistor may be sufficient. Or you may need to turn to some sort of active current limit circuit. Something like a PTC will not be fast enough to prevent an external current limit from tripping.

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