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USBDF01W5-USBDF02W5
EMI FILTER AND LINE TERMINATION FOR USB DOWNSTREAM PORTS
1/9
USBDFxxW5EMI FILTER AND LINE TERMINATION
FOR USB DOWNSTREAM PORTS
May 2000Ed:1C
A.S.D.
FUNCTIONAL DIAGRAMTM: ASDand TRANSIL areatrademarksof STMicroelectronics.
EMI Filter and line termination for USB down-
stream ports on: Desktop computer Notebooks Workstations USB Hubs
APPLICATIONS Monolithic device with recommended line termi-
nationfor USB downstream ports IntegratedRt series termination andCt bypass-
ing capacitors. Integrated ESD protection Small package size
FEATURESThe USB specification requires USB
downstream ports to be terminated with
pull-down resistors from the D+ andD- linesto
ground. 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 additionto
the requirements of termination and EMC
compatibility, the computing devices are
requiredtobe tested for ESD susceptibility.
The USBDFxxW5 provides the recommended line
termination while implementinga low pass filterto
limit EMI levels and providing ESD protection
which exceeds IEC 61000-4-2 level4 standard.
The deviceis packagedina SOT323-5L whichis
the smallest available lead frame package (50%
smaller than the standard SOT23).
DESCRIPTION EMI/ RFI noise suppression Required line termination for USB downstream
ports ESD protection exceedingIEC61000-4-2 level4 High flexibilityin the designof high density
boards Tailoredto meet USB 1.1 standard
BENEFITS
USBDFxxW5IEC-61000-4-2, level4
±15 kV (air discharge)kV (contact discharge)
MIL STD 883C, Method 3015-6
Class3 C= 100pF R= 1500Ω positive strikes and3 negative strikes (F=1Hz)
COMPLIES WITH THE FOLLOWING ESD
STANDARDS:
ABSOLUTE MAXIMUM RATINGS (Tamb = 25°C)
ELECTRICAL CHARACTERISTICS (Tamb = 25°C)
USBDFxxW5
APPLICATION EXAMPLE
Fig. A1: USB Standard requirements
APPLICATION INFORMATION
USBDFxxW5
Fig. A2: Measurement configuration
Fig. A3: USBDFxxW5 attenuation curve. additionto the requirementsof termination and EMC compatibility, computing devices are requiredtobe
testedfor ESD susceptibility. This testis describedin the IEC 61000-4-2 andis alreadyin placein Europe.
This test requires thata device tolerates ESD events and remain operational without user intervention.
The USBDFxxW5is particularly optimizedto perform ESD protection. ESD protectionis basedon the use device which clampsat:
Vinput V RIBR dPP=+ .
This protection functionis splittedin2 stages. As shownin figure A4, the ESD strikes are clampedby the
first stage S1 and thenits remaining overvoltageis appliedto the second stage through the resistorR.
Sucha configuration makes the output voltage very lowat the Vout level.
ESD PROTECTIONCurrent FCC regulations requires that classB computing devices meet specified maximum levelsfor both
ratiated and conducted EMI. Radiated EMI covers the frequency range from 30MHzto 1GHz. Conducted EMI covers the 450kHzto 30MHz range.
For the typesof devices utilizing the USB the most difficult testto passis usually the radiated EMI test. For
this reason the USBDF deviceis aimingto minimize radiated EMI.
The differential signal (D+ and D-)of the USB does not contribute significantlyto radiatedor conducted
EMI because the magnetic fieldof the two conductors exactly cancels each other.
The insideof the PC environmentis very noisy and designers must minimise noise coupling from the differ-
ent sources. D+ andD- must notbe routed near high speed lines (clocks...).
Induced common mode noise canbe minimisedby running pairsof USB signals parallelto each other and
running grounded guard traceon each sideof the signal pair from the USB controllerto the USBDF device. possible, locate the USBDF device physically near the USB connectors. Distance between the USB con-
troller and the USB connector mustbe minimized.
The 47pF (Ct) capacitors are usedto bypass high frequency energyto ground andfor edge control, and
must be placed between the USB Controller and the series termination resistors (Rt). BothCt andRt
shouldbe placedas closeto the USB Controlleras practicable.
TheUSBDFxxW5 ensurea filtering protection against ElectroMagnetic and RadioFrequency Interferences
thankstoits low-pass filter structure. This filteris characterizedby the following parameters: cut-off frequency Insertion loss high frequency rejection
Fig. A3 shows the attenuation curvefor frequenciesupto 3GHz.
EMI FILTERING
USBDFxxW5 havea good approximationof the remaining voltagesat 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 RV RV
gBR dg= + ..
Voutput RV R Vinput
tBR d= + ..
The resultsof the calculation donefor VPP=8kV, Rg=330Ω (IEC61000-4-2 standard), VBR=7V (typ.)
and Rd= 1Ω (typ.) give:
Vinput= 31.2V
Voutput= 7.95V
This confirms the very low remaining voltage across the devicetobe protected.Itis also importantto note
thatin this approximation the parasitic inductance effect was not taken into account. This couldbe few
tenthsof volts during fewnsat the Vin side. This parasitic effectis not presentat the Vout side due the low
current involved after the resistanceR.
The measurements done here after show very clearly (Fig. A6) the high efficiencyof the ESD protection:no influenceof the parasitic inductanceson Vout stage output clamping voltage very closeto VBR (positive strike) and -VF (negative strike)
Fig. A5: Measurement board
Fig. A4: USBDFxxW5 ESD clamping behavior
USBDFxxW5
Fig. A6: Remaining voltage at both stages S1 (Vinput) and S2 (Voutput) during ESD surge.
Please note that the USBDFxxW5is not only actingfor positive ESD surges but alsofor negative ones. For
these kindsof disturbancesit clamps closeto ground voltageas shownin Fig. A6b.
The early ageing and destructionof IC’sis often dueto latch-up phenomena whichis mainly inducedby
dV/dt. Thanksto its structure, the USBDFxxW5 providesa high immunityto latch-up phenomena by
smoothing very fast edges.
LATCH-UP PHENOMENA
CROSSTALK BEHAVIOR
Fig. A7: Crosstalk phenomena
The crosstalk phenomenais dueto the coupling between2 lines. The coupling factor( β12or β21) increases
when the gap across lines decreases, thisis the reason why we provide crosstalk measurements for
monolithic deviceto guarantee negligeable crosstalk between the lines.In the example above the ex-
pected signalon load RL2is α2VG2,in fact the real voltageat this point has gotan extra value β21VG1. This
partof the VG1 signal representsthe effectof the crosstalk phenomenonof the line1on the line2. This phe-
nomenon hastobe taken into account when the drivers impose fast digital dataor high frequency analog
signalsin the disturbing line. The perturbed line willbe more affectedifit works with low voltage signalor
high load impedance (few kΩ).
