USBUF01P6_10 Datasheet, PDF (4/9 Pages) STMicroelectronics – IPAD™ EMI filter and line termination for USB upstream ports

Technical information
USBUF01P6
2.1.1
EMI filtering
Current FCC regulations require 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 device compliant with the USB standard, the most difficult test to pass is
usually the radiated EMI test. For this reason the USBUF01P6 device aims to minimize
radiated EMI.
The differential signal (D+ and D-) of USB devices does not contribute significantly to
radiated or conducted EMI because the magnetic field of each conductor cancels out the
other.
The inside of a PC product is very noisy and designers must minimize noise coupling from
the different sources. D+ and D- must not be routed near high speed lines (clock 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 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 are placed between the driver chip and the series termination resistors (R_t).
Both C_t and R_t should be placed as close to the driver chip as is practicable.
The USBUF01P6 ensures a filtering protection against electro-magnetic 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. USBUF01P6 typical attenuation
curve
0.00
dB
-2.50
-5.00
-7.50
-10.00
-12.50
-15.00
-17.50
-20.00
-22.50
-25.00
1.0M
3.0M
10.0M
30.0M 100.0M 300.0M
f/Hz
1.0G
3.0G
Figure 5. Measurement configuration
50Ω
Vg
TEST BOARD
50Ω
4/9
Doc ID 9883 Rev 7