USBLC6-2SC6 - Is it possible to connect pins 1 + 6 (and 3 + 4) on the PCB? - Page 1
Forum section: electronics / general

author: Tarik
author_rank: Newbie
timestamp: October 18, 2022, 06:15:36 pm
content: Hello,I'm designing a board for hand soldering and would like to use the USBLC6-2SC6 as ESD protection.Its speed is not really needed on an Arduino board, but it might help make SDA/SCL more reliable.Footprint is SOT23-6.Although I got soldered it manually, I'm afraid it's not everyone's cup of tea or will scare others off.For this reason I would like to make the use of USBLC6 optional.The footprint will be on the board of course, but the signal lines are broken and it won't workI see 2 ways to make it optional but functional:1) Connect pin 1 and pin 6 on the board (3 and 4 also)This would give the option to start without ESD protection, but can be installed at any time / subsequently without further changes.It should not be a big deal, since these pins are connected internally anyway.I just haven't seen this in any example yetor2) Add a solder bridge if one does not want the adventure of USBLC6-2SC6What do you think?Tarik(USBLC6 Symbol attached)

author: ataradov
author_rank: Super Contributor
timestamp: October 18, 2022, 06:56:29 pm
content: What solder bridge do you want to add? All ESD protection devices can be removed without affecting functionality of the board.Show the schematic you are planning on using.

author: Tarik
author_rank: Newbie
timestamp: October 18, 2022, 07:19:37 pm
content: Yes, of course.Please have a look at the attached picture.As soon as I don't apply the protection circuit, the connection for the SCL line (as well as for SDA) will be interrupted, unless you take further precautions (In the picture: the solder bridges).

author: ataradov
author_rank: Super Contributor
timestamp: October 18, 2022, 07:21:58 pm
content: This is not how this works. 1+6 and 3+4 are internally shorted and should be externally shorted permanently. There is no protected and unprotected side. This is not a pass-though device, it just clamps excess voltage on the data lines to the power rails.The reason to have this arrangement of pins is to simplify the layout and because extra pins are available on the package.

author: Tarik
author_rank: Newbie
timestamp: October 18, 2022, 09:20:20 pm
content: I fully agree - this is also my "Variant 1" solution.The only thing that made me doubt it is that I did not see that it is not drawn that way in the datasheet.I have changed the scheme (see below) according to the advice.Thank you for your quick help and advice!Tarik

author: tom66
author_rank: Super Contributor
timestamp: October 18, 2022, 10:04:43 pm
content: Be aware a "side effect" of devices like these with an internal 3V3 clamp and zener is if there is an external low impedance short circuit, it can bring your 3V3 rail up to a higher voltage easily.For instance, damaged connector causing short on D+ to 5V USB. A solution to this is to put a series resistor of about 100 ohms in line with the zener supply. You can add a 10nF or so cap to ground here, which can also provide a low impedance buffer for any fault current to go into for momentary transients, though it is not necessary if the zener conducts quite close to 3V3 + diode drop.By the way, why are you using USB connector with I2C signals? That seems quite nonstandard, but perhaps you have a good reason.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 18, 2022, 10:17:47 pm
content: First, the "shorted out" layout is suboptimal. The design idea behind the USBLC6-2SC6's pinout is forcing the signal to flow into the chip at one end, and flow out of the chip at another end. There is no connection from input to output on the circuit board, it's bridged by the chip itself. If you externally short the pins, the high-speed ESD energy will take the path of least inductance and partially bypass the chip, reducing its effectiveness. To maximize its ESD protection, I would use two additional jumpers resistors to create a bridged connection across USBLC6-2SC6, instead of shorting it.On the other hand, a circuit board "flow through" routing is found in the pinouts of many other ESD protection chips with acceptable performance, so it's probably not a really big deal and it's not strictly necessary. So do whatever you like with my information.BTW, as a fan of old-school schematics, I dislike the default KiCad symbol of this chip, the drawing makes it almost impossible to show the natural signal flow so I happened to have another symbol at hand. Feel free to use it.

author: Tarik
author_rank: Newbie
timestamp: October 19, 2022, 02:49:15 pm
content: I use a USB connector for IIC because it is an connection between the Arduino board and the external sensor board.USB connections are reliable, shielded, connectors are cheap and easy to solderAnyway, I am on open for different ideasI was not aware of the problem that a damaged connector can cause, but yes, that can be a problem. I took your advice.See below.The changed symbol looks goodThanks a lot to all of you!

author: wraper
author_rank: Unknown
timestamp: October 19, 2022, 03:10:22 pm
content: IMHO use a different device. This is for USB, therefore trades off protection for low capacitance. Also it has higher clamping voltage than you need. Device which has separate TVS for each channel better suits your needs.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2022, 07:25:55 pm
content: Quote from: Tarik on October 19, 2022, 02:49:15 pmThere’s a very simple reason why you should never use USB connectors for anything other than USB: people WILL connect USB to it. USB power is 5V. What happens to your 3.3V circuit when 5V is applied?No, really. Choose something else.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 20, 2022, 03:51:31 am
content: Quote from: tooki on October 19, 2022, 07:25:55 pm+1. Don't abuse customer electronics connectors. If you must abuse the USB connector, at least use some tricks to ensure the device cannot be destroyed. For example, change the power input specification of your board from 3.3 V to 5 V, and use a 3.3 V regulator at VBUS. A 100% duty cycle buck converter sounds like an even better choice. Then to protect the signals... USB uses either 3.3 V (LS/FS) or LVDS (HS) signaling for D+/D-, so this voltage is safe for I2C. You don't need to do anything else. But if you want another signal, change the USB connector to USB 3.0 instead of USB 2.0, and use the two SSRX+/- pins. SSRX is driven by the host to the device, and it's AC-coupled with no DC flow. So it should be safe to plug into a computer.

author: Tarik
author_rank: Newbie
timestamp: October 20, 2022, 04:34:45 pm
content: USB 2.0 doesn't seem to be in your favor because it's too tempting to plug the other end into a computer? Hmm. I have never evaluated this kind of impact of USBOriginally, I had also considered using LAN connectors, e.g. a 1 m patch cable.But that seemed somehow like overkill, also because the short patch cables are not shielded.If I understood you correctly, the suggestion is to use USB 3.0 and the (DC-free) SSRX pins.Sounds reasonable

author: tooki
author_rank: Super Contributor
timestamp: October 20, 2022, 04:38:47 pm
content: LAN patch cables exist in shielded types. It’s up to you what you buy. (Not that I2C is likely to need shielding.)While using a USB-C connector makes it easier to avoid damage, I still wouldn’t use it, because users will think it’s USB when it’s not.An RJ-11 or RJ-45 jack works, or something like DIN or mini-DIN.