what happen while I use 3.3v instead of 5v in common mode ESD filter - Page 1
Forum section: electronics / projects

author: Boss
author_rank: Newbie
timestamp: February 07, 2020, 12:57:02 pm
content: The common mode ESD filter is used in the USB2.0 for filtering of noise, They have specified that voltage rating as 5v, but my USB2.0 is going to work in 3.3v. If we use this filter what happen in the transmission of data and is there any other option to solve this problem. I have attached the datasheet of the filter and circuit design to implement in this.Datasheet link:https://www.murata.com/en-eu/products/productdata/8796781969438/CMESD-E.pdf] https://www.murata.com/en-eu/products/productdata/8796781969438/CMESD-E.pdfCircuit design:

author: brabus
author_rank: Frequent Contributor
timestamp: February 07, 2020, 01:13:18 pm
content: IMvHO it will work just fine.Yes, every junction presents a variable capacitance, dependant on the applied voltage, but in this case I would expect no problem whatsoever, given the negiglible capacitance of the TVS diodes.Please note: if you supply 3,3V to your connector, it will not be USB anymore. Call it how you want it, use the same connector, but do not call it USB.

author: Ian.M
author_rank: Super Contributor
timestamp: February 07, 2020, 01:17:03 pm
content: IIRC, The USB specification (at least up to USB 2) calls for the data lines to withstand an indefinite short to Vbus, or ground without damage. Therefore it is undesirable to clamp them below 5.5V on a compliant device, even though the signals use 3.3V levels.

author: Boss
author_rank: Newbie
timestamp: February 07, 2020, 02:06:28 pm
content: pls, Explain in detail, quite I'm unable to understand

author: Ian.M
author_rank: Super Contributor
timestamp: February 07, 2020, 02:56:09 pm
content: See sections 'Electrical' and 'Power (VBUS)' here:https://www.beyondlogic.org/usbnutshell/usb2.shtmlUSB 2.0 VBUScan legitimately be up to 5.25V so a minimum clamping voltage of 5.5V is advisable so a data to VBUSshort doesn't pass continuous current and fry the clamping device.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2020, 09:17:53 pm
content: Heh, nice that they have plots of everything -- ESD waveforms, attenuation, eye diagrams!Does show that it's not all that great, though. Peaks of 100-200V, and that will have a low Thevenin source resistance -- still dangerous to the transceiver. What ESD level that counts as (it may be closer to a CDM level condition!), and whether the PHY is rated to handle it, who knows.If there are series resistors between CMESD and PHY, that'll help a lot; I'd be comfortable with it. With no resistors, I'd like something better.Note that the CM filtering is very meager indeed, offering significant attenuation only from fractional GHz up. I never found much point in such things. But, maybe it's a good idea if you have a lot of radio modules nearby (cell, BT, WiFi, etc.)?And going in the other direction, note that larger CMCs are straight out -- USB itself depends upon unbalanced symbols (J and K states) to frame packets. These get corrupted by CM filtering with lower cutoff frequencies. Anyway, there's very little impedance at the PHY for a CMC to work against (just the ~10pF pin capacitance), so the filtering obtained even from very large value CMCs is mediocre (even if it didn't screw up the framing).In short, this is why USB is designed for, and must always be carried by, shielded media. The shield needs to be solid, contiguous between transceivers, at the very least well RF-grounded (i.e., bypassed with multiple capacitors, if not galvanically connected). (Galvanic grounding doesn't matter, as far as I've seen: the GND wire is tied to the shield on every port I've seen.)Tim