Securing USB 2.0 Applications - Semiconductor Business -Macnica

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ESD Check USB Interface

The USB interface is probably the most widespread PC interface in the world. Its use in industrial applications is becoming more and more common. Let's take a closer look at the special environmental conditions of industrial applications. There are concerns regarding robustness against EMI and ESD, which are mentioned in Intel's High Speed USB Platform Design Guidelines. Intel recommends the use of common mode chokes for EMI suppression and another component for protection against ESD pulses.

Würth Elektronik offers all these types of products.

-WE-CNSW common mode chokes are developed for EMI suppression on high speed signal lines.
For ESD suppression, very low capacitance (<2pF) TVS diode arrays WE-TVS or ESD suppressors WE-VE (capacitance down to 0.05pF) are recommended.

But for good EMI behavior, the power supply (V_BUS) must also be protected. Many design engineers forget this important point and start wondering when their products do not pass all the tests in the EMI lab. Below are two typical schematics for proper protection of one or two USB ports. With one TVS diode array WE-TVS, two USB lines can be fully protected. All four signal lines and the common power supply are properly protected.

Figure 1: Two-port USB interface with ESD protection

For further optimization, i.e. filtering common and differential mode noise coming from the USB lines, an LC filter is built with a common mode choke WE-CNSW and a capacitor. On the power lines, very good noise suppression can be achieved using chip bead ferrites like the WE-CBF series from Würth Elektronik.

Figure 2: Configuration without low-capacitance ESD suppressors on power supplies as opposed to shielding data lines

Single line protection components like ESD suppressor series WE-VE should also be connected from the signal line to GND. For power line protection, it is not necessary to use low capacitance ESD suppressors. Standard SMD varistors are recommended as they can withstand higher surges and higher transient energy.

About TVS Diodes

Würth Elektronik has a series of TVS diode arrays called WE-TVS. These TVS diode arrays have three main functions:

- Protection against ESD pulses according to EN61000-4-2
- Protection against surge pulses in accordance with EN61000-4-5
- Protection against EFT pulses according to EN61000-4-4

The WE-TVS series is a TVS diode array designed for high performance surge withstand capability. It is the perfect choice for protecting high speed data lines such as USB2.0, DVI, and LAN from overvoltages. The WE-TVS series meets the requirements outlined in EN61000-4-2. The ultra-low capacitance (<2.0 pF) makes it virtually invisible on the signal line.

About ESD Suppressors

ESD suppressors are special varistors with a defined low specific capacitance. Würth Elektronik offers three different series:

・WE-VE Standard Series: Capacitance 1pF to 120pF, Voltage 5V to 24V
・WE-VE "ULC" series: Capacitance 0.2pF, for signal lines up to 12V
・WE-VE femtoF series: Capacitance 0.05pF, rated voltage 6V, 14V, 26V

The first two series are also available in four element array packages.

Why ESD Protection?

Ever since electronic devices were developed, all components are exposed to electrostatic discharges. The peak voltage of an ESD event can reach up to 30kV, which is extremely dangerous for any integrated circuit. Although some state-of-the-art ICs are protected against ESD pulses, it becomes clear every day that additional protection devices are essential in real-life environments. On the one hand, we make the whole board ESD-free, and on the other hand, we develop more reliable products to meet our customers' needs.

Why EMI Suppression is Needed

In today's global marketplace, the proliferation of electronic and electrical products and the need for wireless connectivity has increased the need to ensure products are protected from intentional and unintentional RF environmental effects. Understanding what EMI phenomena can affect your product will allow you to design in the right protection, helping to reduce time to market.

Additionally, products must not generate excessive levels of EMI, which is typically assessed by an EMC test lab - if the product fails, the costs of remediation will increase significantly.

Different protection devices for different requirements

Four types of TVS diode arrays and five types of ESD suppressors are available.

Figure 3: Multiple pin configurations of the WE-TVS series

How to find the right part

■ If there is a power supply voltage to connect for rail-to-rail clamping
　 Use a TVS diode.

■ When there is no power supply voltage or when ceramic components are used
Use a TVS diode that allows the VDD pin to float or use a WE-VE ESD suppressor.

■ What is the maximum capacitance that can be added to a data line to pass the eye pattern test?
For USB 2.0, a line capacitance of up to 5pF is acceptable.

■ What is the maximum ESD voltage that can occur?

■ Do you want to protect one or two USB data lines?
　 By connecting one USB data line to two I/O ports with TVS diodes, performance is improved.
Select the array type.

Answering all these questions will help you find the best part. For one USB 2.0 port we recommend the WE-TVS 824 011, for two USB 2.0 ports we recommend the 824 015. Designers who prefer single line components can choose the WE-VE ULCC 823 07 050 029 or 823 06 050 029.

Eye Pattern Test

Once the protection elements have been selected, it must be ensured that the overall interface design meets the requirements of the USB specification - for this purpose protection devices from Würth Elektronik can be used, which have a tiny capacitance that is almost invisible on the signal lines.

Figure 5: Eye pattern test using TVS diode WE-TVS 824 015

Eye pattern testing shows that the WE-TVS does not interfere with the USB2.0 signal. The same results are obtained with the other TVS diodes and ESD suppressors listed here. The following table shows Würth Elektronik's cutting edge ultra-low capacitance technology. This new technology makes the capacitance between channels nearly invisible under normal operating conditions.

Table 1: Comparison of Capacitive Components

Testing the protection level of ESP protection devices

The simplest way to measure the protection level is to apply an ESD pulse to an electronic circuit using an ESD protection device and measure the peak voltage of this ESD pulse before and after the protection device. But this method has another problem: ESD pulses are at high frequencies ranging from several MHz to several GHz, so high-frequency refraction occurs during the measurement. The absolute peak and spike voltages during refraction are an indication of the quality of the protection, but they do not provide a definitive indication of the clamping voltage. By the way, this is an unreliable and unrepeatable measurement.

Engineers familiar with semiconductor products know how to perform TLP measurements. The TLP (Transmission Line Pulse) method is a highly repeatable and extremely accurate measurement system since all measurements are made in a 50 Ohm system. A defined current impulse (left) charges the protection component and the resulting voltage is measured (center). This procedure is repeated with increasing TLP currents. The result is the TLP curve (right).

Figure 6: TLP measurement system and corresponding TLP curves

This measurement can be performed on VDD and I/O pins. The lower the voltage you measure, the better the protection device and the more reliable your electronic circuit is. Würth Elektronik TVS diodes offer the lowest ESD clamping voltages on the market thanks to their built-in snapback technology. Compared to competitor products, the WE-TVS is clearly superior. A quick look at the TLP curves shows this.

Table 2: Comparison of VDD components

Figure 7: TLP curves of each component vs. VDD

Figure 8: TLP curves of each component for I/O lines

Recommended Layout for Single USB Port

The two differential signal lines (D+ and D-) run from the connector through TVS diodes (WE-TVS 824 011) and through a common mode choke (WE-CNSW 744 232 090) to the USB controller as shown in Figure 9. This provides excellent ESD protection and EMI suppression on both data lines. VBUS is routed similarly to the signal lines, but with a chip bead ferrite (WE-CBF 742 792 641) instead of a common mode choke. A capacitor and a second chip bead can be added after the chip bead to provide the best possible EMI suppression.

For very sensitive ICs and/or high reliability applications, optimized ESD suppression can be achieved by doubling the four-element TVS array (WE-TVS 824 015) as shown in Figure 10.

Designers who prefer single channel components can also use the ESD suppressor WE-VE. It requires a connection from D+/D- to GND (Figure 11). The other components are connected in the same way as above.

Figure 10: Dual protection for a single USB port

Recommended layout for double USB ports

Protecting a single USB port is very similar and the routing is very similar - we use the exact same parts as protecting a single port and the level of protection is the same.