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USBUF01P6
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 30MHz to 1GHz.
- Conducted EMI covers the 450kHz to 30MHz 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 USBUF01P6 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 USB connectors. Distance between the USB con-
troller and the USB connector must be minimized.
The 47pF (Ct) capacitors are used to bypass high frequency energy to ground and for edge control, and
are placed between the driver chip and the series termination resistors (Rt). Both Ct and Rt should be
placed as close to the driver chip as is practicable.
The USBUF01P6 ensures a filtering protection against ElectroMagnetic and RadioFrequency Interferences
thanks to its low-pass filter structure. This filter is characterized by the following parameters :
- cut-off frequency
- Insertion loss
- high frequency rejection.
EMI FILTERING
f/Hz
1.0M 3.0M 10.0M 30.0M 100.0M 300.0M 1.0G 3.0G
-25.00
-22.50
-20.00
-17.50
-15.00
-12.50
-10.00
-7.50
-5.00
-2.50
0.00
dB
-
-
-
-
-
-
-
-
-
-
Fig. A3: USBUF01P6 typical attenuation curve.
TEST BOARD
50
Ω
Vg
50
Ω
UUx
Fig. A4: Measurement configuration
In addition to the requirements of termination and EMC compatibility, computing devices are required to be
tested for ESD susceptibility. This test is described in the IEC 61000-4-2 and is already in place in Europe.
This test requires that a device tolerates ESD events and remains operational without user intervention.
The USBUF01P6 is particularly optimized to perform ESD protection. ESD protection is based on the use
of device which clamps at:
VV RI
cl BR d PP
=+.
This protection function is splitted in 2 stages. As shown in figure A5, the ESD strikes are clamped by the
first stage S1 and then its remaining overvoltage is applied to the second stage through the resistor Rt.
Such a configuration makes the output voltage very low at the output.
ESD PROTECTION
