SPRS989F December 2016 – December 2018 DRA74P , DRA75P
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
Short the cable connector sockets directly to a small chassis ground plane (GND strap) that exists immediately underneath the connector sockets. This shorts EMI (and ESD) directly to the chassis ground before it gets onto the USB cable. This etch plane should be as large as possible, but all the conductors coming off connector pins 1 through 6 must have the board signal GND plane run under. If needed, scoop out the chassis GND strap etch to allow for the signal ground to extend under the connector pins. Note that the etches coming from pins 1 and 4 (VBUS power and GND) should be wide and via-ed to their respective planes as soon as possible, respecting the filtering that may be in place between the connector pin and the plane. See Figure 7-32 for a schematic example.
Place a ferrite in series with the cable shield pins near the USB connector socket to keep EMI from getting onto the cable shield. The ferrite bead between the cable shield and ground may be valued between 10 Ω and 50 Ω at 100 MHz; it should be resistive to approximately 1 GHz. To keep EMI from getting onto the cable bus power wire (a very large antenna) a ferrite may be placed in series with cable bus power, VBUS, near the USB connector pin 1. The ferrite bead between connector pin 1 and bus power may be valued between 47 Ω and approximately 1000 Ω at 100 MHz. It should continue being resistive out to approximately 1 GHz, as shown in Figure 7-32.