USBDFXXW5(2000) Просмотр технического описания (PDF) - STMicroelectronics

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USBDFXXW5
(Rev.:2000)

EMI FILTER AND LINE TERMINATION FOR USB DOWNSTREAM PORTS

STMicroelectronics 

USBDFxxW5

EMI FILTERING

Current FCC regulations requires that class B computing devices meet specified maximum levels for both

ratiated 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 USBDF 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 the two conductors exactly cancels each other.

The inside of the PC environment is very noisy and designers must minimise noise coupling from the differ-

ent 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 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

must be placed between the USB Controller and the series termination resistors (Rt). Both Ct and Rt

should be placed as close to the USB Controller as practicable.

The USBDFxxW5 ensure 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 shows the attenuation curve for frequencies up to 3GHz.

Fig. A2: Measurement configuration

Fig. A3: USBDFxxW5 attenuation curve.

Insertion loss (dB)

0

50 Ω TG OUT

Vg

TEST BOARD

RF IN

50 Ω

-10

-20

-30

1

10

100

F (MHz)

1000 3000

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 remain operational without user intervention.

The USBDFxxW5 is particularly optimized to perform ESD protection. ESD protection is based on the use

of device which clamps at :

Vinput = VBR + Rd.IPP

This protection function is splitted in 2 stages. As shown in figure A4, the ESD strikes are clamped by the

first stage S1 and then its remaining overvoltage is applied to the second stage through the resistor R.

Such a configuration makes the output voltage very low at the Vout level.

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