DocID7041 Rev 8 5/13
USBUF Technical information
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USB connectors. Distance between the USB controller and the USB connector must be
minimized.
The 47 pF (C
t
) 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 USBUF ensures a filtering protection against Electromagnetic and Radio-frequency
Interferences thanks to its low-pass filter structure. This filter is characterized by the
following parameters:
• cut-off frequency
• Insertion loss
• high frequency rejection.
2.3 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 USBUF is particularly optimized to perform ESD protection. ESD protection is based on
the use of device which clamps at:
V
cl
= V
BR
+ R
d
.I
PP
This protection function is spitted in 2 stages. As shown in Figure 6, 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.
Figure 4. USBUF typical attenuation Figure 5. Measurement configuration
1 10 100 1,000
-30
-20
-10
0
Frequency (MHz)
S21 (dB)
TEST BOARD
50
Ω
Vg
50
Ω
UUx
