App note: USB port protection


USB port protection from EMI and ESD, a good app note from Würth Elektronik introduce you to their common mode chokes and TVS diodes solution. Whole article here in PDF.

The USB-Interface might be the most distributed PC interface in the world. The usage in industry applications is more and more common. Let’s have a closer look to the special environmental conditions of industry applications. That there are real concerns regarding the robustness against EMI and ESD is written in Intel’s “High Speed USB Platform Design Guidelines”. Intel recommends the usage of a common mode choke for EMI suppressions and another component for protection against ESD pulses.

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  1. I know it is common practice to provide overvoltage protection to a pin by tying it to a diode to the positive rail, Vbus in this case. I was wondering how much overvoltage protection this actually provides in low power circuits driven by 7805 or a LDO variant. Such linear regulators source current from their output pins but looking at the 7805 schematic, the output pin is tied to ground by ~10K of resistance. This will not be able to sink much current in an overvoltage state.

    This was not really an issue when dealing with circuits that consume many mA or even amps with lots of bypass caps. In low power circuits that consume 1 mA or less as baseline, it wouldn’t take much to overwhelm the minimal sink capacity of the regulator and circuit combined.

    I see that the IC in this case has a zener to prevent this problem. Should we always be adding a zener when tying a pin to the positive rail using diode?

    1. typical regulators cannot sink current so you are correct if you inject more current than the circuit uses the voltage will increase and potential kill components

      this is especially problematic in very low power like mcu in sleep mode

      The zener will help but you need some head room, can’t use a 3.3V zener on 3.3V it’ll be semi on all the time, if you use a 5V zener is might not be on until it is too late

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