Datasheet

USBDFxxW5 Application information
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Figure 2. Application example
2.1 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 USBDF device aims 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 the two conductors exactly cancels each
other.
The inside of the PC environment is very noisy and designers must minimise noise coupling
from the different sources. D+ and D- must not be routed near high speed lines (clocks...).
Induced common mode noise can be minimised 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 USBDF device.
If possible, locate the USBDF device physically near the USB connectors. Distance between
the USB controller and the USB connector must be minimized.
The 47 pF (C
t
) capacitors are used to divert high frequency energy to ground and for edge
control, and must be placed between the USB Controller and the series termination
resistors (R
t
). Both C
t
and R
t
should be placed as close to the mSB Controller as
practicable.
The USBDFxxW5 ensure a filtering protection against electroMagnetic and radio-frequency
Interference thanks to its low-pass filter structure. This filter is characterized by the following
parameters :
● cut-off frequency
● Insertion loss
● high frequency rejection
Figure 4. shows the attenuation curve for frequencies up to 3 GHz.
D+
Peripheral
transceiver
D-
CABLE
Host/Hub USB
port transceiver
D- (2)
D+ (1)
D+ (2)
D- (1)
(1) for a low-speed port
(2) for a full-speed port
+Vbus
Upstream portDownstream port
USBDF
xxW5
USBUF
xxW6