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USBUF01W6

EMI FILTER AND LINE TERMINATION FOR USB UPSTREAM PORTS

STMicroelectronics 

USBUFxxW6

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 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 USBUFxxW6 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 USBUFxxW6 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.

Fig. A3: USBUFxxW6 typical attenuation curve.

S21 (dB)

0

Fig. A4: Measurement configuration

-10

50Ω

TEST BOARD

Vg

50Ω

-20

-30

1

10

100

Frequency (MHz)

1,000

ESD PROTECTION

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 USBUFxxW6 is particularly optimized to perform ESD protection. ESD protection is based on the use

of device which clamps at :

Vcl = VBR + Rd.IPP

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.

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