USBUF Datasheet, PDF (4/13 Pages) STMicroelectronics – EMI filter and line termination for USB upstream ports

Technical information
USBUF
2.1
Application example
Downstream port
D+
Gnd
D-
Figure 3. Implementation of ST solutions for USB ports
USBDF01W5
D+ in
Rt
Ct _Rd
D+ out
CABLE
D+
D+
Gnd
Ct ^Rd
D-
D-
D- in
Rt
D- out
USBUF01W6
D2
Gnd D1
Ct
Rt
D3
Ct
Rt
3.3 V
Rp
3.3V ^D4
Upstream port
D+
D-
FULL SPEED CONNECTION
Downstream port
USBDF01W5
D+
Gnd
D-
D+ in
Gnd
D- in
Rt
Ct _Rd
Ct ^Rd
Rt
D+ out
CABLE
D+
D+
D-
D-
D- out
USBUF01W6
D2
Gnd D1
Ct
Rt
D3
Ct
Rt
3.3 V
Rp
3.3V ^D4
LOW SPEED CONNECTION
Upstream port
D+
D-
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
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DocID7041 Rev 8