How to Protect Communication Ports from Unwanted Interruptions – Part 2: Protecting High-Speed Interface

October 27, 2021

Blog

Accurate and reliable data transmission is the goal for all communication circuit designs. Electronics designers can choose from several communication protocols to address their application’s needs. To ensure minimal signal interference, maximize signal/noise, and optimize error correction algorithms, designers need to protect the input and output ports from damage.

Current overloads, voltage transients from lightning, electrical fast transients, and electrostatic discharge (ESD) can all result in a loss of communications between devices. The communication port circuits require protection from these external threats; however, the transmission protocol cannot be compromised. To ensure reliable performance, designers must implement circuit protection schemes that allow communication circuitry to provide both transmission of uncorrupted data and accurate detection for complete recovery of the original data.

 This article assists design engineers with recommendations for protecting the ports of high-speed interfaces without degrading the interface’s transmission and reception performance or impacting constraints for the design goals on the size of the product. The protection methods for the following four high-speed interfaces are defined: 

• Universal Serial Bus (USB) which continues to evolve with higher speed formats 
• High-Definition Multimedia Interface(HDMI)
• DisplayPort interface 
• External Serial Advanced Technology Attachment (eSATA). 

See Table 1 for the functions and maximum bandwidths of these standards. 

Note: Part 1 in this series focuses on protecting power-over-Ethernet (PoE) communication ports.

Table 1: High-speed communication protocols, function, and maximum data rate

USB interfaces

USB ports are commonly used on personal computers, peripherals, communications interfaces, electronic test and measurement instruments, and many other products. The USB interface enables fast, easy connections between computers, smart devices, and other peripheral devices. First standardized in 1996, USB has been evolving with higher speeds and allowing more power capacity for fast charging battery-operated devices. 

The USB-Implementers Forum (USB-IF) has upgraded the standard through four major revisions.¹ The wired USB standard started with version 1.0 and has progressed through version 2.0, 3.x versions, and is currently up to revision 4, USB4. USB 2.0 through USB4 and the maximum throughput of each version are shown in Table 2. The different data rates allow USB ports to interface with devices ranging from slow keyboards to high-speed video devices.

Designers can take advantage of a generalized interface in which the signal lines are not dedicated to a specific function for one type of device. Also, designers can set up USB interfaces to have low latency for time-critical functions or to enable large transfers of data operating in the background. In addition, the standard defines power delivery (PD) performance for USB versions 1 through 3. The PD revisions allow devices to be charged and powered through the USB interface. The power capacity has increased from 2.5 W (5 V @0.5A) to 100 W (20 V @ 5A).

 
Table 2: USB interfaces and their maximum data transfer rates

The USB connectors have also evolved to enable higher data rates and greater power availability. Figure 1 shows the pin configurations and styles for the various connectors used for each USB version. The maximum data rate that each connector can achieve is shown in Table 3.

 
Figure 1: USB connectors designed for the various USB standards (individual illustrations are not to scale)

 
Table 3: USB connector types maximum data rates

Protection for a USB 2.0 Interface 

The USB 2.0 interface consists of a VBUS power line and two data lines as shown in Figure 2 (left diagram).  The VBUS line which can receive its power from the AC power line, is subject to current overloads and voltage transients propagated on the AC power line. A resettable fuse should be installed on the VBUS line to protect against overloads so that when the overload is resolved, the fuse will reset and the circuit can continue to function. Polymer positive temperature coefficient (PPTC) fuses are resettable fuses. Their resistance increases significantly as a result of heat generated by an overload current. The PPTC fuse’s internal structure changes during an overload, thus resulting in an increase in resistance. When the device cools, the PPTC’s low resistance structure is restored. These fuses are designed for low voltage circuits where the maximum voltage rating is commonly 24 V. Other PPTC fuse features include:

• Ultra-low resistance, ranging from mΩ’s to about 2 Ω, when current below the fuse’s trip rating is flowing
• Wide range of current trip ratings from 100 mA to 9 A
• Fast time to trip, typically under 5 s
• Space-saving, surface mount packaging in 0402 up to 2920 sizes
• UL component recognition and TUV approval.

For protecting the circuit fed by the VBUS line from power line induced transients and electrostatic discharge (ESD) strikes, consider a unidirectional transient voltage suppressor (TVS) diode array. This type of diode array can include the following features:

• Safely absorbs up to 40 A from an electrically fast transient and 5 A from a lightning strike
• Ability to withstand a ±30 kV ESD strike either propagated over the air or via direct contact
• Maximum low leakage current of 0.5 µA in 5 V circuits
• Space-efficient 0201 surface mount package

Be sure to protect the data lines from voltage transients that can corrupt data transmission. Consider using a 4-channel TVS diode array for data line protection. Diode arrays, such as the one shown in Figure 3, have the following capabilities:

• Minimal impact on the data lines with a capacitance of 0.3 pF per pin to ground 
• Safe absorption of a +22 kV ESD through-the-air or a direct contact strike and a – 10 kV ESD strike via air or direct contact
• Low leakage current of 10 nA for minimum loading on the circuit. 

With only three components, a USB 2.0 port is fully protected from both current overload and voltage transients.

Protection for a USB 3.2 Interface 
As shown in Figure 2 (right diagram), the USB 3.2 interface comprises a VBUS line and six data and control lines. Use the same components recommended to protect the VBUS line from overcurrent and overvoltage events as discussed for the USB 2.0 interface. To protect the six data lines from voltage transients, consider a discrete TVS diode array on each port. TVS diode arrays can have the following capabilities:

• ESD protection to ±18 kV over the air and ±12 kV from direct contact
• Safe absorption of up to 40 A peak current from an electrically fast transient
• Low capacitance of 0.09 pF pin-pin without compromising signal integrity
• Low leakage current with a maximum value of 20 nA.

Use of individual TVS diodes provides greater protection for the higher speed USB port, USB 3.2, with lower capacitance components which minimally impacts data transmission capacity.  
 
Figure 2: Recommended USB 2.0 and USB 3.2 interface protection components

 
Figure 3. 4-channel TVS diode array with a Zener diode for transient voltage protection

Protection for High-Speed USB 3.2 and USB 4.0 Interfaces with the Power Delivery Revisions

The USB 3.2 Gen 2x1 and higher versions require the use of the Type-C connector. As shown in Figure 1, the Type-C connector is a high-density connector. As a result, the Type-C connector can be susceptible to resistive shorts between contacts due to dust and dirt that can enter the connector. With up to 100 W on the power pins, the potential for damage to the connector and the associated circuitry is always present. Protect the USB Type-C connector from heat associated with the resistive fault using a digital temperature indicator on the Configuration Channel (CC) line as shown in Figure 4. With the digital temperature indicator on the CC line, it can provide accurate protection without impacting the 100 W capacity of the power delivery line, the VBUS line. Refer to the USB Type-C standard for more details on implementing this important thermal protection feature.² 
 

Figure 4. Recommended USB 3.2 and USB 4.0 Type-C protection components

For protection against transients, consider using various versions of TVS diode arrays that address specific requirements. Select a TVS diode array that has the lowest capacitance for the SuperSpeed lines. Selecting TVS diode arrays with low leakage current helps keep power consumption low, particularly for the VBUS lines. If the product will have application in the automotive industry, select TVS diode arrays that are AEC-Q101 qualified components (Automotive Electronics Council Failure Mechanism Based Stress Test Qualification for Discrete Semiconductors).³

Protection for HDMI, DisplayPort, and eSATA Interfaces

An identical protection scheme is recommended for the High-Definition Multimedia Interface (HDMI), DisplayPort, and eSATA interface ports; thus, these three interfaces are considered together. HDMI combines high definition video and digital audio from a display controller to either a video display device or an audio device.⁴  HDMI is known as the de-facto high definition television standard. The HDMI interface has been incorporated in products since 2004. It is now at version 2.1 and can transmit data at a maximum rate of 48 Gbps.

First introduced in 2006, DisplayPort is designed to transmit video data from a video source to a display device such as a PC monitor or smart TV. This interface replaces the VGA standard and can transmit audio and video simultaneously. This interface is also compatible with the HDMI interface. Version 2.0 will include a 77 Gbps data rate.  The Video Electronics Standards Association maintains the DisplayPort standard.⁵

Originally developed in a parallel format by IBM for the IBM AT PC, the Serial Advanced Technology Attachment (SATA) interface is now the industry standard interface for disk drives.⁶ The external SATA (eSATA) standard evolved in 2004 to create a robust connection for external hard drive connectivity.

Protecting these three interfaces, shown in Figure 5, from damaging transients can require a single component type, a four-line TVS diode array. Figure 6 shows the configuration of the 4-line TVS diode array. TVS diode arrays such as a 4-line array offer:

• Ultra-low capacitance of 0.2 pF which has an insignificant impact on the transmission eye diagram
• ESD protection up to ±20 kV via either air or direct contact transmission
• 25 nA leakage current for minimum power consumption
• SOD 883 packaging to conserve PC board space and reduce trace layout complexity.

 
Figure 5. Recommended HDMI, DisplayPort, and eSATA protection components

 
Figure 6. TVS diode array for suppressing voltage transients on four high-speed data lines

Designing ports with the appropriate protection components ensure robust and reliable communication

Designing protection from external threats into interface circuits is essential for presenting a reliable product to the market. Fortunately, not many components are needed; and protection components exist that do not compromise the transmitted or received signal. Design engineers can save critical development time by taking advantage of a manufacturer’s expertise in the design and selection of protection components. The manufacturer’s application engineers can advise on circuit configurations and components that can prevent damage from external threats and avoid corruption of transmitted and received signals. The application engineers can also assist with recommendations for space-saving and cost-effective solutions. With ports protected from current overloads and voltage transients, the product will gain a reputation for high quality. 

References

1. USB-Implementers Forum website: Front Page | USB-IF. 

2. Universal Serial Bus Type-C Cable and Connector Specification. Revision 2.0. August 2019. USB Implementers Forum (USB-IF), Inc.  https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-revision-20-august-2019 

3. Automotive Electronics Council website: AECMain (aecouncil.com).

4. High-Definition Multimedia Interface website: HDMI Forum

5. Video Electronics Standard Association website: VESA - Interface Standards for The Display Industry

6. Serial ATA International Organization website:  Home | SATA-IO (sata-io.org).

Additional References 

To learn more, download the Circuit Protection Products Selection Guide, courtesy of Littelfuse, Inc. For additional information on the use of digital temperature indicators in USB connections, see the Littelfuse setP™ Design and Installation Guide.