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I am thinking to use TVS diode TPD1E10B09 for ESD surge protection. The general convention is outer edge of the microUSB is connected to the ground, so any ESD strike happens it will be grounded. But in one reference design, the outer casing of the USB is not grounded. (pic attached below). enter image description here 1) Will it not make more sense to attach outer casing to ground, what is the significance of attaching it to 33 ohm resistor and then giving it to TVS diode. If VBus has to be given to the TVS diode, then what is the need of connecting 33 ohm resistor to the ground?

2) I am studying another reference diagram, in this USB interface and power supply section, 33K ohm resistor is connected from microUSB outer casing to ground. (pic attached below) enter image description here 3) Why 180 ohm resistor is being used in the path of VBus and the ESD IC (TPD4E004XXXX). It could be connected directly, what is the significance if attaching 180 resistor in the path of the VBus.

I saw one more development board in which outer casing is being connected to the ground. I am wondering what should be my design strategy, should I connect microUSB outer casing to the ground or to any ESD protection diode.

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    \$\begingroup\$ It's a ferrite bead, not a resistor. \$\endgroup\$ – Long Pham May 21 '18 at 15:48
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Your question is strongly related to the question of how to connect a cable shield/screen to GND and has only little to do with the TVS diode.

In a „high speed“ transmission standard like USB, the shield/screen of the cable is mainly there as a defence against capacitive coupling of the E-field. This coupling induces currents into the shield, which must drain off somewhere.

Depending on the expected noise frequency and overall device grounding scheme, there can be different strategies to handle this noise current. That‘s why you see different resistors from shield to GND.

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