The Protection of USB 2.0 Applications

Version 10

    Protection of USB 2.0 ApplicationsWE-Logo.jpg

     

    USB Port Protection

     

    ANP002C BY JOCHEN BAIER

     

    1. ESD-confirm USB Interfaces

    The USB-Interface might be the most distributed PC interface in the world. The usage in industry applications is more and more common. Let’s have a closer look to the special environmental conditions of industry applications.

     

    That there are real concerns regarding the robustness against EMI and ESD is written in Intel’s “High Speed USB Platform Design Guidelines”. Intel recommends the usage of a common mode choke for EMI suppressions and another component for protection against ESD pulses.

    Würth Elektronik offers all these types of products.

     

     

    - The WE-CNSW common mode choke was developed for EMI suppression on high speed signal lines

    - For ESD suppression the TVS diode arrays WE-TVS with very low capacitances (<2pF) or the ESD Suppressors WE-VE (capacitances down to 0.05pF) are your preferred choice.

     

    USB_Controller.JPG

     

     

    But for outstanding EMI behaviour it is absolutely necessary to protect the power supply (Vbus) as well. A lot of design engineers forget this important point and wonder themselves when their product don’t pass all the tests in an EMI lab.

     

    Two typical schematics for optimized protection of one or two USB ports are shown below:

     

    With one TVS diode array WE-TVS you can fully protect two USB lines. All four signal lines as well as the common power supply are well protected. For further optimization, i.e. to filter incoming common mode noise as well as differential mode noise from USB line, a LC filter is built with the common mode choke WE-CNSW and the capacitors. For power supply lines very good noise suppressions can be eached with a chip bead ferrite, like the Würth Elektronik WE-CBF series.

     

     

    USB_Controller II.JPG

     

     

    Single line protection components like ESD suppressors series WE-VE also have to be connected from signal line to GND. For power line protection it is not necessary to use a low cap ESD suppressor, you might prefer a standard SMD varistor which can withstand higher surges and higher transient energies.

     



     

     

     

    2. About TVS Diodes

     

    Würth Elektronik launched the TVS diodes array series called WE-TVS. These TVS diodes arrays include three main functions:

     

    - protection against ESD pulses according to EN 61000-4-2

    - protection against surge pulses according to EN 61000-4-5

    - protection against EFT pulses according to EN 61000-4-4

     

    WE-TVS series are high performance designed TVS diode arrays which include surge rated diodes. They are the best choice to protect high speed data lines like USB 2.0, DVI or LAN from over-voltages. The WE-TVS series exceeds the requirements outlined in EN 61000-4-2. Due to their ultra low capacitance (<2.0 pF) they are nearly invisible on the signal lines.

     

    3. About ESD Suppressors

     

    The ESD suppressors are special raised varistors with a low and defined intrinsic capacitance. Würth Elektronik has three different series:

     

    - WE-VE standard series: Capacitances of 1pF to 120pF and voltages from 5V to 24V

    - WE-VE “ULC” series: Capacitance of 0.2pF, suitable for signal lines up to 12V

    - WE-VE femtoF series: Capacitance of 0.05pF and rated voltages of 6V, 14V and 26V

     

    For the first two series are also four-fold array packages available.

     

    3.1. Why ESD Protection?

     

    Since the first electronic developments, all the parts are subjected to electrostatic discharge. ESD events have peak voltages up to 30kV and therewith they are very dangerous for all kind of integrated circuits.

    However, some state-of-the-art ICs are protected against ESD pulses but real world conditions show daily that an additional protection device is indispensable. On the one hand to have the whole board ESD free and on the other hand to develop more reliable products to satisfy the needs of your customers.

     

    3.2. Why EMI Suppression?

     

    With the increase in electronic/electrical products and the need to have wireless connectivity in today’s global markets, there is an increased requirement to ensure that your product is immune to the impacts caused by both the intentional and un-intentional RF environment. Only if you consider now which EMI phenomena impacts on the product, will you be able to design in the appropriate protection and reduce time to market.

    In addition, the product must not generate excessive levels of EMI and this is typically assessed by an EMC Test Laboratory, and should the product fail then costs to implement remedial work will increase aggressively.

     

     

    4. Different Protection Devices for Different Requirements

     

    There are four different TVS diode arrays and five different ESD Suppressors available:

     

    Order Code

    Package

    Pin

      Conf.

    I/O / VDD

    Lines

    VRWM

    CIN

    IPP

    VESD

    VCl ESD

    VDD can

    float

    824011

    SOT23-5L

    1

    2/1

    5V

    2.0 pF

    12 A

    20 kV

    13 V

    NO

    824015

    SOT23-6L

    2

    4/1

    5V

    2.0 pF

    12 A

    20 kV

    13 V

    NO

    824001

    SOT23-6L

    2

    4/1

    5V

    1.0pF

    5A

    15kV

    14V

    NO

    824014

    SOT23-6L

    3

    4/1

    5V

    0.5 pF

    4A

    16kV

    12V

    YES

    82307050029

    0402

    4

    1

    5V

    0.2 pF

    -

    8kV

    17V

    -

    82306050029

    0603

    4

    1

    5V

    0.2 pF

    -

    8kV

    30V

    -

     

    Pin_Configuration I.JPGPin_Configuration II.JPGPin_Configuration III.JPG

    Pin Configuration 4

    Pin_Configuration IV.JPG

    Pin Configuration 1Pin Configuration 2Pin Configuration 3Pin Configuration 5

     

     

    4.1. How to find the Suitable Part?

     

    - You have supply voltage to connect for rail-to-rail clamping

    --> take TVS diode

     

    - You don’t have supply voltage or do prefer ceramic components

    --> take TVS diode where VDD pin can float or take WE-VE ESD suppressor

     

    - Which maximum capacitance can you add to your data lines to pass the eye pattern test?

    --> for USB 2.0 line capacitances up to 5pF shouldn’t cause any trouble

     

    - What’s the maximum expected ESD voltage that can occur?

     

    - Do you want to protect one or two USB lines? However, connecting one USB data line with two I/O

    ports of TVS diode lets you get a better performance --> select array type

     

    Answer all these questions and you will find the most suitable part for you. We recommend for one USB 2.0 port 824011824011 and for two USB 2.0 ports 824015824015. Designers preferring single line components may choose p/n 8230705002982307050029 or 8230605002982306050029.

     

     

    5. Eye Pattern Test

     

    After making a choice of protection element you have to ensure that your whole interface design meets the requirements of USB specification. Therefore you can use our protection devices with a small capacitance which is nearly invisible on the signal lines.

     

            Port 1: D+Port 1: D-Port 2: D+Port 2: DEye
    Eye_Pattern_Test I.JPG

    Pattern test without any protection component placed on two USB 2.0 differential signal lines

     

           Port 1: D+Port 1: D-Port 2: D+Port 2: DEye
    Eye_Pattern_Test II.JPG

    Eye pattern test with TVS diode WE-TVS p/n 824 015

     

    The eye pattern test shows that the WE-TVS do not disturb the USB 2.0 signal. You get the same results with our other listed TVS diodes and with the ESD suppressors as well. The following table shows the cutting edge ultra low capacitance technology of Würth Elektronik. This technology enables a channel to channel capacitance which is almost invisible at normal operating conditions.

     

    Order Code

    Order Code Package

    I/O

    VRWM

    CIN

    CX

    824015

    SOT23-6L

    4

    5

    2pF

    0,1pF

    Competitor S

    SOT23-6L

    4

    5

    3pF

    1,5pF

    824001

    SOT23-6L

    4

    5

    1pF

    0,1pF

    Competitor P

    SOT23-6L

    4

    5

    3pF

    1,5pF

    I/O

    Number of I/O lines the TVS-Diode can protect

    VRWM

    Max. reverse working voltage

    CIN

    Input capacitance at I/O pin via. GND (VIN = 2,5V, VDD=5V)

    CX

    Input capacitance between I/O pins (VIN = 2,5V, VDD=5V)

     

    6. Testing the Protection Level of ESD Protection Devices

     

    The easiest way to measure the protection level is to apply an ESD pulse to your electronic circuit with the ESD protection device and measure the peak voltage of this ESD pulse before and after the protection device. But this way carries out different problems.

     

    Due to high frequency and wide range of ESD pulse (few MHz to some GHZ) high frequency refractions will occur during the measurement. The absolute peak voltage and the spike voltages during the refractions are an indicator for better and worse protection, but you cannot indicate the definitive clamping voltage. And by the way, this is not a reliable and repeatable measurement!

     

    Engineers being familiar with semiconductor products do know the TLP measuring method. The TLP (transmission line pulsing) method is an accurate measurement system which allows - due to all measurements are done in a 50Ω-System - repeatable and very accurate measurements.

     

    A defined current impulse (left) is charged into the protection component and the resulting voltage across it is measured (middle). This procedure is repeated with increasing TLP currents. As a result you get the TLP curve (right).

     

    TLP_Curve.JPG

    TLP measurement system with corresponding TLP curve

     

    This measurement can be done for VDD and I/O pins. The lower the voltage you measure, the better is the protection device and therewith the reliability of your electronic circuit.

     

    Thanks to the built in snap-back technology, Würth Elektronik TVS diodes have the lowest ESD clamping voltage on the market. Compared to competitors’ products, the WE-TVS do outperform them clearly! This you can see with a short view at the TLP curves.

     

     

    Order Code

    Package

    I/O

    VRWM

    VCl,IO

    VCl,VDD

    VESD

    CIN

    IPP

    824015

    SOT23-6L

    4

    5

    12

    7,5

    12kV

    2pF

    12A

    Competitor S

    SOT23-6L

    4

    5

    22

    15,5

    15kV

    3pF

    12A

    824001

    SOT23-6L

    4

    5

    14

    9

    8kV

    1pF

    5A

    Competitor P

    SOT23-6L

    4

    5

    28

    19

    8kV

    3pF

    6A

    I/O

    Number of I/O lines the TVS-Diode can protect

    VRWM

    Max. reverse working voltage

    VCl,IO

    ESD clamping voltage at I/O pin (IEC 61000-4-2, contact mode at 6kV)

    VCl,VDD

    ESD clamping voltage at VDD pin (IEC 61000-4-2, contact mode at 6kV)

    VESD

    Max. withstanding ESD pulse (IEC 61000-4-2, contact mode)

    CIN

    Input capacitance at I/O pin via. GND (VIN = 2,5V, VDD=5V)

    IPP

    Max. Withstanding surge current (8/20μs)

     

     

    TLP_Curve_VDD.JPGTLP_Curve_IO.JPG
    TLP Curve of different parts for VDDTLP Curve of different parts for I/O lines

     

    7. Recommended Layout for Single USB Port

     

    The two differential signal lines (D+ and D-) are routed from connector to TVS-Diode (p/n 824 011824 011) and via a common mode choke (p/n 744 232 090744 232 090) to the USB controller as shown on the left picture below. This results in an awesome ESD protection and EMI suppression of both data lines.

     

    VBUS is routed like the signal line, but instead of the common mode choke a chip bead ferrite (p/n 742 792 641742 792 641) is used. After the chip bead you may add a capacitor and a second chip bead as well to get the highest possible EMI suppression effect.

    For very sensible IC’s and / or high reliable applications you can get an optimized ESD suppression effect if you double contact a four-fold TVS array (p/n 824 015824 015) like shown on right picture.

     

    USB_Protection I.JPGUSB_Protection II.JPG
    Protection of single USB portDouble protection of single USB port
    USB_Protection III.JPG

     

     

     

     

    Designers preferring single channel components can also use the ESD Suppressors WE-VE. The connection has to be carried out from D+ / D- to GND.

     

    The other parts are connected in same way as above.

    Protection of single USB port


    7.1. Recommended Layout for Double USB Port

     

    According to single USB port protection the routing is very similar. Use exactly the same parts as for single port protection and the protection level will also be the same.

     

    USB_Protection IV.JPG

    Protection of double USB port

     

     

    Bill of Material:

     

    In this application note the following parts are used:

     

    TVS-Diodes WE-TVS:

    824011: Protection of 2 I&O lines and VDD line: Size SOT23-5L | 2pF CI/O | 5V VRWM | 5A IPP

    824015: Protection of 4 I&O lines and VDD line: Size SOT23-6L | 2pF CI/O | 5V VRWM | 5A IPP

     

    ESD Suppressors WE-VE:

    82307050029: Size 0402 | 0.2pF Ctyp | 5V VRWM | 17V VCL

    82306050029: Size 0603 | 0.2pF Ctyp | 5V VRWM | 30V VCL

     

    Common Mode Chokes WE-CNSW:

    744232090: Size 1206 | 370mA IDC | 300 mRDC | 90W Impedance

     

    Chip Bead Ferrites WE-CBF:

    742792641: Size 0603 | 2 000mA IDC | 150 mRDC | 300W Impedance

     

    USB-Connectors WR-COM:

    61400826021: USB Typ A Connector | THT | Dual stacked

    614004135023: USB Typ A Connector | THT | Vertical

     

    8. Interface Design Kit

     

    For an easy design of interfaces Würth Elektronik has launched a special Interface Design Kit. This design kit includes a design guide for USB 1.0 to USB 2.0, CAN, Ethernet, VGA, DVI, RS232 and RS485 interfaces and all the components used. These are ESD Suppressors, SMD Common Mode Chokes, Chip Bead Ferrites, LAN Transformers and the corresponding Connectors.

     

    In total are 35 different passive components with 235 parts and 4 evaluation boards in. The colour scheme makes it easy to locate the suitable parts for your application. Just follow the specific application colours and arrange your needed parts.

     

    For each application you find a simple block schematic where you see how to place the different components to get the best result...try it!

     

     

     

     

    -Würth Elektronik USB 3.0-