Shottky diodes for USB ESD

[Blitzschnitzel]

Hi,
I am designing a small USB numpad and I am adapting someone else’s design. I want to keep everything except the MCU through hole. The design has two ESD protection diodes in it. Which makes sense but I don’t know what to make of the rating of the Schottky diodes.
I’ve looked at the shottky diodes from my local supplier and forward voltage ends at 2.5v and reverse voltage starts at 8v. So, I really don’t know how to read the marked 3.6v. Does the guy perhaps actually mean Zener diodes? And if it were Zener diodes, wouldn't you pick something close to USB's 5v? What kind of diode would you pick for this?

[Twoflower]

Usually this are TSV diodes. But I don't think that you'll find some through hole types as USB is a bit demanding. There are single diodes and full networks available (protecting also the supply lines). But the only ones I found was SMD, sorry.

Maybe here's some information about the different types of USB ESD protection: https://www.we-online.de/web/de/electronic_components/news_pbs/blog_pbcm/blog_detail-worldofelectronics_108785.php.

Edit: Fixed the link (the . at the end does not belong to the link as the Forum-SW thinks)

[HwAoRrDk]

And if it were Zener diodes, wouldn't you pick something close to USB's 5v?

The data lines for LS and FS USB use 3.3V (or is it 3V? I forget).

[Blitzschnitzel]

Usually this are TSV diodes. But I don't think that you'll find some through hole types as USB is a bit demanding. There are single diodes and full networks available (protecting also the supply lines). But the only ones I found was SMD, sorry.

Maybe here's some information about the different types of USB ESD protection: https://www.we-online.de/web/de/electronic_components/news_pbs/blog_pbcm/blog_detail-worldofelectronics_108785.php.

Ah, It's called TVS. I am wiser already. Yes through hole TVS diodes seem to start at minimum 6v.

The data lines for LS and FS USB use 3.3V (or is it 3V? I forget).

Thanks I didn't know that.

So, in this case I would either try normal Zener diodes or leave out the ESD protection. Would a normal Zener still somewhat help or would it actually be detrimental to the design?

[Twoflower]

Not sure why you're insist to use through-hole. Especially since you say you have a SMD processor.

And if you use a SOT8 to DIL8 adapter? OK, that's not fully through hole anymore, but that brings your on the safe side to protect the PC and your own circuit. For example the ST USB61. That device provides a core or less complete protection. And SOT8 is normally easy to solder (pin to pin 1.25mm). It seems that there was a B2 version in a DIL8 package. But I haven't seen anything else but the datasheet.

[TheHolyHorse]

Thanks I didn't know that.

So, in this case I would either try normal Zener diodes or leave out the ESD protection. Would a normal Zener still somewhat help or would it actually be detrimental to the design?

Why not use TVS diodes.

[wraper]

You don't need 3.6V TVS. ESD protection devices made specially for USB are usually rated for 6V minimum breakdown voltage. Also you shouldn't just slap any TVS or varistor onto USB data lines, they need to have low capacitance.

[TheHolyHorse]

There're plenty of TVS diodes intended for use on high speed data lines. Something like this https://www.mouser.se/ProductDetail/Texas-Instruments/TPD1E04U04DPLR?qs=sGAEpiMZZMsItbzKdwyIe3d9rBYGXZaY0kRQqbQ6Xu9OvH3BvdPQ6Q%3D%3D.

[Blitzschnitzel]

Not sure why you're insist to use through-hole. Especially since you say you have a SMD processor.

I have used a SMD one in my last design but this is meant to become a soldering kit. I would just presolder the USB-C port and the MCU. It is meant to be a bit of component porn. Somewhat like this: https://5z6p.com/products/plaid-through-hole/
So, from the comments I guess there is no trough hole solution? The design from my first post is from the linked keyboard. They seem to work but maybe rather despite the through hole zener diodes.

[pigrew]

I'd normally suggest diodes sold as TVS diodes (as they will have a somewhat different die layout), but the ones listed on DigiKey all have way too high of a capacitance (1 to 10 nF) for an IO signal. For low-speed and full-speed USB, application notes suggest 5V stand-off, even though the logic shouldn't go above 3.3V. Probably a "standard" zener would be the way to go (like BZX79-C4V7,143 which has ~300 pF capacitance). However, the datasheet of the NXP zeners do not mention ESD ratings.

Another note is that FTDI suggests adding a small capacitor (~47 pF) to the data lines which also helps with ESD.

Double-diode arrangements would work, but require twice as many diodes.

If this were my design, I'd go with a SMD component, as appropriate THT diodes don't seem to exist. Something like ST USB6B1 in an SOIC package should be manageable.

[EEEnthusiast]

Stay clear of adding through hole components on the USB lines. They act like stubs, can pick up noise and cause multiple problems due to the vias added. Even if you plan to make a kit, you could use 0603 components which can be easily hand soldered.

[Blitzschnitzel]

I'd normally suggest diodes sold as TVS diodes (as they will have a somewhat different die layout), but the ones listed on DigiKey all have way too high of a capacitance (1 to 10 nF) for an IO signal. For low-speed and full-speed USB, application notes suggest 5V stand-off, even though the logic shouldn't go above 3.3V. Probably a "standard" zener would be the way to go (like BZX79-C4V7,143 which has ~300 pF capacitance). However, the datasheet of the NXP zeners do not mention ESD ratings.

Another note is that FTDI suggests adding a small capacitor (~47 pF) to the data lines which also helps with ESD.

Double-diode arrangements would work, but require twice as many diodes.

If this were my design, I'd go with a SMD component, as appropriate THT diodes don't seem to exist. Something like ST USB6B1 in an SOIC package should be manageable.

Thanks I'll try the BZX79-C4V7 And if that causes interference I'll change the design to a USBLC6-2SC6. I had to buy a 50 strip for the last project anyways.

Stay clear of adding through hole components on the USB lines. They act like stubs, can pick up noise and cause multiple problems due to the vias added. Even if you plan to make a kit, you could use 0603 components which can be easily hand soldered.

hmmm... I've looked at many open harware keyboards which use none of the best practices of USB 2.0 design. So far my impression was that HID-device speeds are so low that you can send the signal through a rusty spoon.

[jhpadjustable]

If this were my design, I'd go with a SMD component, as appropriate THT diodes don't seem to exist. Something like ST USB6B1 in an SOIC package should be manageable.

Lead inductance defeats the entire purpose of providing a path to ground that has very low inductance, relative to the trace inductance to the protected devices. ON Semi application note AND8232 is an excellent explanation of how to use TVS for ESD protection.

hmmm... I've looked at many open harware keyboards which use none of the best practices of USB 2.0 design. So far my impression was that HID-device speeds are so low that you can send the signal through a rusty spoon.

1.5Mbps USB could be sent through a rusty fork, but it's still not a good engineering practice, and will still contribute to the smog in the HF and MF radio bands. (Still waiting for Dave to take some EMC probes to that 4-layer Gigatron he was soldering a while back...)

[Bassman59]

hmmm... I've looked at many open harware keyboards which use none of the best practices of USB 2.0 design.

Don't confuse the spec revision with the wire speed. USB 2.0 added the 480 Mbps High Speed transfers, but devices did not have to run that fast. USB 2.0 supports the 1.5 Mbps Low Speed transfers commonly used by keyboards.

[langwadt]

You don't need 3.6V TVS. ESD protection devices made specially for USB are usually rated for 6V minimum breakdown voltage. Also you shouldn't just slap any TVS or varistor onto USB data lines, they need to have low capacitance.

It looks like the hack used for bitbanging low speed USB with a 5V mcu (V-USB)

I've seen some use blue leds