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TIDEP0013: USB shield grounding

Abhi Choksi
Intellectual 500 points
Part Number: TIDEP0013

As per my knowledge, we should use a 1.5nF /2KV SMD1808 capacitor between Shield and Ground of USB Connector. This capacitor will make AC coupling between the USB Shield and Board Ground.

Below are my queries.

(1)When to Connect USB Shield Pins directly to Board main Ground without Capacitor?

(2)When to have an AC coupling capacitor between USB Shield and Board Main Ground?

(3)How to select AC coupling capacitor value,its voltage rating and package.?

(4)When to reference ESD diodes to shield of USB Connector?

  • 0
    TI__Guru 55305 points

    I’m not familiar with origin of this reference, but connecting the USB connector shield to the PCB ground only through an inductor may be problematic. With this topology, any high frequency return current or noise will not have a low impedance path back to the PCB ground which is the reference for the USB signals. If the return currents aren’t allowed to return to the signal reference, it may induce an unexpected voltage potential on the shield which could radiate.

     

    I have seen this topology used before where someone feels noise from the PCB is coupling out onto the USB cable shield and radiating. So they use the inductor to block the noise. This may help when the actual noise source is something else on the PCB, but this noise can still conduct out through the actual USB signal conductors which are referenced to the PCB ground. So now you have noisy USB signals without a good shield. The best solution is eliminate the actual noise source.

     

    I have seen may different connection topologies and not sure if there is a single good answer. You need to understand your product requirements and implement a topology that meets all of these requirements. You may need to consult with a product compliance specialist that can review your product requirements and make appropriate recommendations.

     

    You absolutely need external ESD protection on the USB signals. Internal ESD protection was only designed to protect the semiconductor device to industry standard levels expected during PCB assembly and handling. Any system level ESD protection must be implemented on the PCB. You should place the ESD protection devices as close as possible to the connector and they need to be referenced to the PCB ground since you are trying to minimize the potential to the semiconductor which is referenced to the PCB ground.

     

    Regards,

    Paul