ESD Protection Help + General Advice for a USB to UART Bridge PCB  - Page 1
Forum section: electronics / beginners

author: Quiggster
author_rank: Newbie
timestamp: November 25, 2023, 06:22:01 am
content: Howdy folks, just sharing my entry-level PCB design – a USB to UART bridge converter. It's a small project aimed at diving into PCB design as a beginner. Planning to use it for hardware hacking or maybe tweak it later for a custom microcontroller programmer.I placed an attached PNG of my schematic for reference of what I have so far.I've based it on theCP2102Nchip, sticking closely to its data sheet and other breakout boards using it. My primary worry is ensuring I've nailed the ESD protection for the USB bus since it's my first venture into the USB realm. I wanted to verify that my choice of theTPD4S012was a good fit for the job.I also looked at this pretty helpful video by Phil’s Lab on TVS diodes:I think everything seems to align with his recommendations except for the clamping voltage which I am unsure how to define for my project. I’m still not sure that I actually understand this parameter entirely to be completely honest but I’ve seen it both described as the voltage where the device would output the most current and both the voltage where transient spikes will be normalized to by the TVS diode.I'm not exactly sure what to make of this, I know for the TPD4S012 it has a clamping voltage of 8V according to the datasheet. In the Phil’s lab video, He seems to suggest to just minimize the clamping voltage as much as possible when choosing a diode which seems somewhat obvious but I don’t want to go off of this metric without fully understanding what is going on. Would love any advice you seasoned folks can throw my way on approaching this topic.Thanks everyone

author: Terry Bites
author_rank: Super Contributor
timestamp: November 25, 2023, 05:45:52 pm
content: There's a very comprehensive app note from TI.https://www.ti.com/lit/pdf/slvaf82?keyMatch=SLVAF82AYou select a diode with a stand off voltage a little higher than the supply.The clamping voltage only needs be a few volts higher. A 5V bus will still be in spec at 5.25V, a diode with a stand off voltage of 5.5V is needed.Any lower and it will shunt signals to ground and degrade the bus speed.You also need to minimise the capacitance of the protection diode for the same reason.The IO pin has its own internal clamp but it can't deal with high energy transients.The protection diode is there to divert the energy in an ESD or transient overload away from the IO pin.Note that the protection diode has a very high switching speed, higher than the IO pin's internal clamp.You need to add the same protection to the positive supply rail.

author: Quiggster
author_rank: Newbie
timestamp: November 26, 2023, 10:44:17 pm
content: Thank you for your response, I would also like to ask about the breakdown voltage parameter. This may be trivial but wouldn’t I also have to consider this by looking for a voltage thats well below the max ratings for the IC that I’m trying to protect if the diode conducts at that value?

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: November 27, 2023, 01:56:59 am
content: In the case of this specific design, you've got the right part since it's designed for USB. What matters more here is the capacitance introduced by the TVS since USB is a high-speed protocol.In the more general case, Vclamp is the voltage that is presented to the USB transceiver in the event of a fault (the part of the TVS V/I curve when a fault current is flowing). The breakdown voltage is the same, but generally at a much lower current (specified in the data sheet) than what would flow in a fault condition.