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USBUFxxW6
EMI FILTER AND LINE TERMINATION
FOR USB UPSTREAM PORTS
March 2002 - Ed: 3A
A.S.D.
TM
SOT323-6L
FUNCTIONAL DIAGRAM
TM: ASD and TRANSIL are a trademarks of STMicroelectronics.
EMI Filter and line termination for USB upstream
ports on:
- USB Hubs
- PC peripherals
APPLICATIONS
s
Monolithic device with recommended line termi-
nation for USB upstream ports
s
Integrated Rt series termination and Ct bypass-
ing capacitors.
s
Integrated ESD protection
s
Small package size
FEATURES
The USB specification requires upstream ports
to be terminated with pull-up resistors from the
D+
and
D-
lines
to
Vbus.
On
the
implementation of USB systems, the radiated
and conducted EMI should be kept within the
required
levels
as
stated
by
the
FCC
regulations. In addition to the requirements of
termination
and
EMC
compatibility,
the
computing devices are required to be tested for
ESD susceptibility.
The USBUFxxW6 provides the recommended line
termination while implementing a low pass filter to
limit EMI levels and providing ESD protection
which exceeds IEC61000-4-2 level 4 standard.
The device is packaged in a SOT323-6L which is
the smallest available lead frame package (50%
smaller than the standard SOT23).
DESCRIPTION
s
EMI / RFI noise suppression
s
Required line termination for USB upstream
ports
s
ESD protection exceeding IEC61000-4-2 level 4
s
High flexibility in the design of high density
boards
s
Tailored to meet USB 1.1 standard
BENEFITS
3.3 V
Rp
Ct
Rt
Ct
Rt
D1
Grd
D2
D4
D3
3.3 V
Rt
Rp
Ct
CODE 01
33
1.5k
47pF
CODE 02
22
1.5k
47pF
Tolerance
10%
10%
20%
USBUFxxW6
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IEC61000-4-2, level 4
15 kV (air discharge)
8 kV (contact discharge)
MIL STD 883E, Method 3015-7
Class 3 C = 100 pF R = 1500
3 positive strikes and 3 negative strikes (F = 1 Hz)
COMPLIES WITH THE FOLLOWING ESD
STANDARDS:
Symbol
Parameter
Value
Unit
V
PP
ESD discharge IEC 61000-4-2, air discharge
ESD discharge IEC 61000-4-2, contact discharge
ESD discharge - MIL STD 883E - Method 3015-7
16
9
25
kV
kV
kV
T
j
Maximum junction temperature
150
C
T
stg
Storage temperature range
- 55 to +150
C
T
L
Lead solder temperature (10 second duration)
260
C
T
op
Operating temperature range
0 to 70
C
P
r
Power rating per resistor
100
mW
ABSOLUTE RATINGS (T
amb
= 25C)
TECHNICAL INFORMATION
Host or
Hub port
Twisted pair shielded
Zo = 90ohms
5m max
Hub 0 or
Full-speed function
Untwisted unshielded
3m max
FULL SPEED CONNECTION
LOW SPEED CONNECTION
3.3V
3.3V
D+
D-
D+
D-
D+
D-
D+
D-
1.5k
1.5k
Hub 0 or
Low-speed function
Low-speed USB
Transceiver
Full-speed or
Low-speed USB
Transceiver
15k
Rt
Rt
Ct
Ct
Host or
Hub port
Full-speed or
Low-speed USB
Transceiver
15k
Rt
Rt
Ct
Ct
Rt
Rt
Ct
Ct
Full-speed USB
Transceiver
Rt
Rt
Ct
Ct
15k
15k
Fig. A1: USB Standard requirements
USBUFxxW6
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Fig. A2: Implementation of ST' solutions for USB ports
APPLICATION EXAMPLE
D+
D-
CABLE
Host/Hub USB por transceiver
t
D-
D+
D+
D-
Upstream port
Downstream port
USBDF01W5
D+
D-
Rt
D+ in
Gnd
D- in
D+ out
D- out
Rt
Rd
Rd
Ct
Ct
Gnd
3.3 V
Rp
Ct
Rt
Ct
Rt
D1
Gnd
D2
D4
D3
3.3V
P
eripheral transceiver
USBUF01W6
FULL SPEED CONNECTION
D+
D-
CABLE
Host/Hub USB por transceiver
t
D-
D+
D+
D-
Upstream port
Downstream port
USBDF01W5
D+
D-
Rt
D+ in
Gnd
D- in
D+ out
D- out
Rt
Rd
Rd
Ct
Ct
Gnd
3.3 V
Rp
Ct
Rt
Ct
Rt
D1
Gnd
D2
D4
D3
3.3V
P
eripheral transceiver
USBUF01W6
LOW SPEED CONNECTION
USBUFxxW6
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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.
EMI FILTERING
1
10
100
1,000
-30
-20
-10
0
Frequency (MHz)
S21 (dB)
Fig. A3: USBUFxxW6 typical attenuation curve.
TEST BOARD
50
Vg
50
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 USBUFxxW6 is particularly optimized to perform ESD protection. ESD protection is based on the use
of device which clamps at :
V
V
R I
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
USBUFxxW6
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ESD Surge
Vinput
Voutput
Rload
Rg
Rt
S1
Rd
V
BR
V
BR
V
PP
Device
to be
protected
USBUF01W6
Rd
S2
Fig. A5: USBUFxxW6 ESD clamping behavior
TEST BOARD
ESD
SURGE
16kV
Air
Discharge
Vin
Vout
UUx
Fig. A6: Measurement board
To have a good approximation of the remaining voltages at both Vin and Vout stages, we give the typical
dynamical resistance value Rd. By taking into account these following hypothesis : Rt>Rd, Rg>Rd and
Rload>Rd, it gives these formulas:
Vinput
R V
R V
R
g
BR
d
g
g
=
+
.
.
Voutput
R V
R Vinput
R
t
BR
d
t
=
+
.
.
The results of the calculation done for Vg=8kV, Rg=330
(IEC61000-4-2 standard), V
BR
=7V (typ.)
and Rd = 1
(typ.) give:
Vinput = 31.2 V
Voutput = 7.95 V
This confirms the very low remaining voltage across the device to be protected. It is also important to note
that in this approximation the parasitic inductance effect was not taken into account. This could be few
tenths of volts during few ns at the Vinput side. This parasitic effect is not present at the Voutput side due
the low current involved after the resistance Rt.
The measurements done hereafter show very clearly (Fig. A7) the high efficiency of the ESD protection :
- no influence of the parasitic inductances on Voutput stage
- Voutput clamping voltage very close to V
BR
(breakdown voltage) in the positive way
and -V
F
(forward voltage) in the negative way
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