kb1lqc
commented
8 years ago OK I've spent a rediculous amount of time thinking about this tonight. I don't have a good way to test USB inrush current and while on my previous USB testing in #5 I was OK with this since I knew I had a soft start circuit, now I am talking about removing the soft start circuitry which therefore makes me nervous.
Should I Be Nervous?
Probably not. Most people seem to be OK with putting well over 10uF on the USB bus. However, I would really prefer that Faraday doesn't have issues like causing a USB hub to reset during insertion. This would make me feel much better to know I simply don't need to worry about it.
Why Do I Need > 10uF on VBUS?
The TPS562201DDCR needs a minimum capacitance and in order to gauruntee the correct operation of the switcher over the 17V input range of Faraday I need to account for voltage bias loss with the capacitors. This puts over 20uF on to the bus, closer to 30uF.
Concluding Thoughts
In the end I'm trying to reduce BOM line items and part count. However, the MIC94072YC6 is about $0.30 each and doesn't even need input/output bypass capacitors. For the cost of one line item and one assembly placement I get assurance that the 1uF minimum load is sufficient upon initial USB power application but not too much. I still get to remove a ton of power supply parts and I'd like to remove one of the 10uF capacitors, C54, leaving C17 and R8 as bulk capacitance. This seems like a happy medium for all. However, if I don't use the 2 Ohm resistor elsewhere maybe I should just forget about it.
Final Thought on Testing
Why not just test with and without the MIC94072YC6 and look at the voltage drop at the USB hub/port when I insert Faraday onto the bus. Per USB compliance documents the real test is whether USB voltage drops more than 330mV.
We decided that while the power supply of Rev C is awesome! This is a board that will be produced in large quantities and we don't need this much complexity.