Technical information

USBUF

4/13

DocID7041 Rev 8

2.1

Application example

Figure 3. Implementation of ST solutions for USB ports

2.2

EMI filtering

Current FCC regulations requires that class B computing devices meet specified maximum

levels for both radiated and conducted EMI.

•

Radiated EMI covers the frequency range from 30 MHz to 1 GHz.

•

Conducted EMI covers the 450 kHz to 30 MHz range.

For the types of devices utilizing the USB, the most difficult test to pass is usually the

radiated EMI test. For this reason the USBUF device is aiming to minimize radiated EMI.

The differential signal (D+ and D-) of the USB does not contribute significantly to radiated or

conducted EMI because the magnetic field of both conductors cancels each other.

The inside of the PC environment is very noisy and designers must minimize noise coupling

from the different sources. D+ and D-must not be routed near high speed lines (clocks

spikes).

Induced common mode noise can be minimized by running pairs of USB signals parallel to

each other and running grounded guard trace on each side of the signal pair from the USB

controller to the USBUF device. If possible, locate the USBUF device physically near the

D+

D-

CABLE

D-

D+

D+

D-

Upstream port

Downstream port

USBDF01W5

D+

D-

Rt

D+ in

Gnd

D- in

D+ out

D- out

Rt

Rd

Rd

Ct

Ct

Gnd

3.3 V

Rp

Ct

Rt

Ct

Rt

D1

Gnd

D2

D4

D3

3.3V

USBUF01W6

D+

D-

CABLE

D-

D+

D+

D-

Upstream port

Downstream port

USBDF01W5

D+

D-

Rt

D+ in

Gnd

D- in

D+ out

D- out

Rt

Rd

Rd

Ct

Ct

Gnd

3.3 V

Rp

Ct

Rt

Ct

Rt

D1

Gnd

D2

D4

D3

3.3V

USBUF01W6

FULL SPEED CONNECTION

LOW SPEED CONNECTION