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USB 2.0

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Full Member level 3
Mar 20, 2021
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I am using USB 2.0 full speed for a design. Power to the electronics is 5V from the USB, into a FTDI device ( which has an internal 3V3) regulator to power my electronics.

I have a couple of questions, FTDi recommend a LC filter, they have also added an extra 10nF at the input and 4.7uF to the output of the LC filter for VUSB.

My first question is the value of the ferrite is 600R @ 100MHz. This is the DC resistance at 100MHz, will this act as a voltage divider with my cable? Do I have to ensure that the attenuation is not below the acceptable levels for VBUS in terms of voltage? USB 2.0 is 500mA current, the ferrite is from the BLM18SND series, I presume the current handling capability needs to be greater than 500mA? Also would the ferrite prevent inrush current as it has a small DC resistance.

The other question I have is differential pair tracking. I have read that if the wavelength is 1/10th of the signal, then impedance of the tracking is not an issue. Can someone explain this 1/10th rule of thumb at full speed USB 2.0 with calculations as I do not understand it fully.

Lastly I have chosen a TVS array based on protecting 2 I/O lines D+/D- and VBus, the capacitance is less than 2pF, the TVS array also is well above the rating for +/- 4kV / +/-8kV air and contact discharge. Does that seem correct?

My FTDi device FT231x is designed for full speed Comms, it has also extra flexibility of configurable GPIO pins IE. CBuS lines. Is there anything else I need to consider?

Any help would be appreciated.

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