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USB connector design
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hsn93:
hello,

this is from XPlained-Pro evaluation board



and i have two questions:

1. why the shield is connected to ground through capacitor .. and how its calculated exactly


2. 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.pdf
they put connect TVS to the Shield not the GND direct
ataradov:
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.
Chriss:
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

wraper:

--- Quote from: Chriss on April 26, 2018, 02:55:12 pm ---Practically I would connect the TVS to shield, but to gnd is also almost the same.

--- End quote ---
By 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 pm ---2. Most of the designs I've seen use the ground. But realistically, I doubt it makes a difference.

--- End quote ---
And 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.
wraper:
Just look at part of Arduino schematic. They nullified the effect of ESD varistors by connecting them through ferrite bead  :palm:

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