USB connector design - Page 1
Forum section: electronics / beginners

author: hsn93
author_rank: Regular Contributor
timestamp: April 26, 2018, 02:04:11 pm
content: hello,this is from XPlained-Pro evaluation boardand i have two questions:1. why the shield is connected to ground through capacitor .. and how its calculated exactly2. the TVS is connected to GND -> is this right ? i can see other design guide from FTDI:http://www.ftdichip.com/Support/Documents/AppNotes/AN_146_USB_Hardware_Design_Guidelines_for_FTDI_ICs.pdfthey put connect TVS to the Shield not the GND direct

author: ataradov
author_rank: Super Contributor
timestamp: April 26, 2018, 02:54:28 pm
content: 1 .That's how USB standard recommends doing it. There are no exact calculations, the exact value is not all that important and you can drop the whole thing entirely for most designs.2. Most of the designs I've seen use the ground. But realistically, I doubt it makes a difference.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 26, 2018, 02:55:12 pm
content: The C and R are a RC filter-noise reduction stuff.Practically I would connect the TVS to shield, but to gnd is also almost the same.Sent from my GT-I8260 using Tapatalk

author: wraper
author_rank: Unknown
timestamp: April 26, 2018, 03:33:03 pm
content: Quote from: Chriss on April 26, 2018, 02:55:12 pmBy doing so you cripple ESD protection of IC connected to USB data lines and also increase risk damaging capacitor between shield and GND during ESD event (yes capacitors can be damaged by ESD too). As of placing something between connector shield and GND, IMO it should be done only if connector shield is connected to GND by some other means like conductive gasket between connector and grounded metal enclosure. FWIW, the vast majority of micro USB cables have shield and GND wire sorted together at connector. And the strangest is that placing ferrite bead instead of capacitor seem to be just as popular, despite that it has completely opposite effect.Quote from: ataradov on April 26, 2018, 02:54:28 pmAnd where goes return path for current from ESD discharge? If your intention is protecting usb connector from discharging between it's pins and shield, then sure, you can connect protection device to connector shield. But you are not protecting IC. It will mean that voltage will rise on both data pins and shield in reference to GND to which IC is connected, and IC will still receive hit on it's USB pins. However if metal enclosure which is connected to connector shield is used, then sure you could connect ESD protection to connector as it should act as current return pant.

author: wraper
author_rank: Unknown
timestamp: April 26, 2018, 03:43:09 pm
content: Just look at part of Arduino schematic. They nullified the effect of ESD varistors by connecting them through ferrite bead

author: wraper
author_rank: Unknown
timestamp: April 26, 2018, 03:49:58 pm
content: I'd say ESD protection device manufacturer knows better.http://www.st.com/resource/en/datasheet/usblc6-2.pdf

author: Neilm
author_rank: Super Contributor
timestamp: April 27, 2018, 09:26:41 am
content: There is no generic correct way to do this - I have designed with several different ways depending on the requirements. If there is an earth path, I would directly connect to that (assuming the 0V was star pointed or isolated from Earth). Otherwise connecting to 0V via the RC is usually sufficient. The cap shunts the high frequency and the resistor slowly discharges it.

author: wraper
author_rank: Unknown
timestamp: April 27, 2018, 10:48:01 am
content: And here is a document from TI which suggests to place ferrite bead to GND pin isolate it at high frequency and allow low resistance path for DC. The catch is that shell shall be connected directly to metal enclosure. Or if there is no metal enclosure, suggests connecting shell directly to the ground plane.https://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdfQuote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2018, 11:20:01 am
content: Quote from: wraper on April 27, 2018, 10:48:01 am[Ed: emphasis]Basically no appnote gets this.So you end up with everyone using literally the flimsiest possible ground connection. It's embarassing.For 99% of what you're doing with USB:GROUND THE SHELL!Ground it hard, in multiple points, to the ground plane. And obviously, use a ground plane, too!Tim

author: ataradov
author_rank: Super Contributor
timestamp: April 27, 2018, 04:36:38 pm
content: Quote from: T3sl4co1l on April 27, 2018, 11:20:01 amOn the host side only. If you ground it on the device side, then the current will flow through the shield, eliminating any positive EMI effects it may have.Also, it appears that there is really no consensus on this at all. And whatever opinion is expressed here by anyone can't be 100% true without a really good justification.On the other hand, even with all the different implementations, things still works fine.

author: Karel
author_rank: Super Contributor
timestamp: April 27, 2018, 04:46:52 pm
content: Quote from: T3sl4co1l on April 27, 2018, 11:20:01 amI can confirm this. I burned myself once. Now I always connect the shell to the groundplane. No resistors, no capacitors, no ferrites.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2018, 08:51:25 pm
content: Quote from: ataradov on April 27, 2018, 04:36:38 pmDude, that's precisely what it's there to do!QuoteI haven't read a single book, since vacuum tubes were in vogue, that ever claimed open shields was a good thing.Do you connect your cable TV without the shield? Audio/video cables? HDMI cables?Do you connect your Ethernet with one wire in each pair floating?Tim