USB Port Design with ESD and Overcurrent Protection

esdserialusbusb device

I am working on a small project in which I use a CH340 to communicate with an MCU, but during prototyping I probably should have a way to protect the controlling computer's USB port from shorts. I designed this circuit to handle the task at hand:

The USB Circuit

I have two questions regarding this design:

  1. I am faced with the decision to use the NUF2042XV6T1G for ESD protection, which is surprisingly the only ESD protection IC my supplier has. Checking the datasheet, I noticed there were some capacitors and resistors shown in the circuit description – which I think are not going to cause any issues since the datasheet clearly states that the IC can be used in USB hubs. I do not want to use discrete TVS diodes to keep the circuit a lot more compact (and also because even 2 of them costs around the same as the IC here). Would there a better way to handle the ESD protection, or would this be sufficient?

  2. I am planning on using a PPTC fuse (FSMD025-1206-R) which has a tripping current of 500mA to prevent damage to the USB port of the controlling PC/laptop in case of a short somewhere on the board. I know most computers have internal means of protecting the ports, but I want this project to be something anyone can use easily without the risk of damaging their devices. My question here is I am unsure of what hold/trip currents I should pick, and if a PPTC in this configuration is enough to actually protect the USB port from overcurrent.

I am also open to any criticism, since I want to learn more about proper circuit design.

Thanks in advance!

Best Answer

I'd probably just use an 0805 or 1206 size 10uF (25V) ceramic cap on VBUS (but I can't remember what the USB spec is). 0805 or 1206 size will allow a wide variety of capacitance values if you need to change it.

Polymer fuses work great, but with limitations. They don't trip very fast and the tripping current is rated at 20C or 25C. So, the trip current will be less as the temperature goes up (look at the datasheet for the I vs. Temp curves). I think you probably don't need a fuse on your device. The upstream port should have have overcurrent protection built in.

If you want overcurrent protection, set the limit for how much current your device will use, (plus some margin), not how much power the upstream port can supply. If you device only uses 50mA, a 100mA or 200mA fuse is fine - a 500mA fuse is overkill.

I'd go with one ground for the whole design - like you have. Everyone always tries to overcomplicate grounding. I've always had very good results with 1 ground for everything.

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