Design Tips for Protecting High-Speed Interfaces
In this second installment of the "Protect Your Ports! Top Design Tips to Keep Your Communications Connected" series, we explore what protecting high-speed interfaces looks like, including USB, HDMI, DisplayPort, and eSATA.
Numerous communication circuits and protocols exist to serve a wide range of applications. Since these circuits transmit and receive data between separate devices, the ports of the interfaces are subject to external threats to their circuitry. These threats include current overloads and voltage transients from lightning, electrical fast transients (EFT), and electrostatic discharge (ESD).
These circuits require protection from the damage caused by these external threats, but the interface’s transmission protocol cannot be compromised. With protection schemes implemented, the communication circuitry must reliably transmit uncorrupted data; and, the receiver must accurately detect and decode information so that the original data is completely recovered.
This article is the second in a series on protecting communication interfaces. The first presented solutions for protecting the ports of power-over-Ethernet interfaces. This article presents electronics design engineers with recommendations for protecting high-speed interfaces without compromising transmission/reception performance or interfering with product size constraints.
Four communication protocols are considered:
- the Universal Serial Bus (USB) standards which continue to evolve with higher speed formats,
- the High Definition Multimedia Interface (HDMI),
- the DisplayPort interface,
- and the External Serial Advanced Technology Attachment (eSATA).
The purpose for these standards and their current maximum bandwidths are described in Table 1.
Table 1. Communication protocols, function, and maximum data rate
The USB port is ubiquitous on personal computers, computer peripherals, electronic test and measurement instruments, and numerous other products. The USB interface allows easy and fast connection between, computers, smart devices, and peripheral devices. It was first standardized in 1996 and has been evolving with higher speeds and allowing more power carrying capacity for charging battery-operated devices.
The USB-Implementers Forum (USB-IF) have upgraded the standard through four major revisions. 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.
Table 2 lists the versions from 2.0 to USB4 and shows how the maximum throughput of each version has substantially increased.
Table 2. The currently active versions of the USB interface and their maximum data transfer rates
The different data rates allow a USB port 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 of 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) revisions 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).
The USB connectors have also evolved to enable higher data rates and greater power availability. Figure 1 shows the pin configurations and relative connector size for the various connectors used for each USB version. Table 3 shows the maximum data rate that each connector can achieve.
Figure 1. USB connectors designed for the various USB standards
Table 3. Maximum data rates for USB connector types
Protecting a USB 2.0 Interface
The USB 2.0 interface consists of a VBUS power line and two data lines as shown in Figure 2a.
Figure 2. Recommended protection components for USB 2.0 and USB 3.2 interfaces
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 resettable fuse will reset and the circuit can continue to function.
A polymer positive temperature coefficient (PPTC) fuse is a resettable fuse whose resistance increases significantly due to heat generated by an overload current. The internal structure of the PPTC fuse changes during an overload to cause an increase in resistance. When the device cools, the low resistance structure is restored. These fuses are designed for low voltage circuits where the maximum voltage rating is commonly 24 V.
Other features of PPTC fuses are:
- 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 ratings from 100 mA to 9 A
- Fast time to trip
- 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, use a uni-directional transient voltage suppressor (TVS) diode array. Versions of this type of diode array provide:
- Capacity to safely absorb 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 over the air or via direct contact
- Maximum low leakage current of 0.5 µA in 5 V circuits
- A space-efficient 0201 surface mount package
Be sure to protect the data lines from voltage transients that can corrupt the transmission of data. Consider a 4-channel TVS diode array for data line protection.
Diode arrays such as the one shown in Figure 3 have the following capabilities:
- 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
- Minimal impact on the data lines with a capacitance of 0.3 pF per pin to ground.
- Low leakage current of 10 nA for minimum loading on the circuit.
Thus, only three components are required to fully protect a USB 2.0 port.
Figure 3. 4-channel TVS diode array with a Zener diode for transient voltage protection
Protecting a USB 3.2 Interface
As shown in Figure 2b above, 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 as discussed for the USB 2.0 interface from overcurrent and overvoltage events. To protect the six data lines from voltage transients, consider a discrete TVS diode array on each port.
Individual TVS diode arrays can have these capabilities:
- Safe absorption of up to 40 A peak current from an electrically fast transient
- ESD protection to ±18 kV over the air and ±12 kV from direct contact
- Low leakage current with a maximum value of 20 nA
- Low capacitance of 0.09 pF pin-pin which does not compromise signal integrity
Using individual TVS diodes provides greater protection of the higher speed USB port with lower capacitance components for minimal impact on the data transmission capacity.
Protecting the 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 use of the Type-C connector. As can be seen from 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.
Figure 4. Recommended protection components for USB 3.2 and USB 4.0 Type-C interfaces
With the digital temperature indicator on the CC line, it can provide accurate protection during any power conditions, from the lowest level such as 5W all the way up to the maximum capability of USB-C, 100W. Refer to the USB Type-C standard for more details on implementing this thermal protection feature.
For protection against transients, consider using different versions of TVS diode arrays. Select a TVS diode array for the SuperSpeed lines that has the lowest capacitance. Keep power consumption low by selecting TVS diode arrays with low leakage current, particularly for the VBUS lines.
If your product will have use 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).
Protecting HDMI, DisplayPort, and eSATA Interfaces
A similar protection scheme is recommended for the High Definition Multimedia Interface (HDMI), DisplayPort, and eSATA interface ports, so 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. 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.
The DisplayPort interface is designed to transmit video data from a video source to a display device such as a PC monitor. This interface, which can transmit audio and video simultaneously, replaces the VGA standard. DisplayPort was first introduced in 2006. Version 2.0, with a target data rate of 77 Gbps is expected to be completed later this year. This interface is compatible with the HDMI interface. The Video Electronics Standards Association maintains the DisplayPort standard.
The Serial Advanced Technology Attachment (SATA) interface, originally developed in a parallel format by IBM for the IBM AT PC, defines an interface, which is now the industry standard interface for disk drives. 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 5. Recommended protection for HDMI, DisplayPort, and eSATA interfaces
Figure 6 shows the configuration of the 4-line TVS diode array.
Figure 6. TVS diode array for suppressing voltage transients on four high-speed data lines
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
- 25 nA leakage current for minimum power consumption
- ESD protection up to ±20 kV either via air or direct contact transmission
- SOD 883 packaging to conserve PC board space and reduce trace layout complexity.
Protecting Your Ports Enhances Product Robustness and Reliability
Protecting transmission interfaces involves selection of components that provide protection for the circuit without compromising the transmitted signals. Fortunately, not many components are required. However, there is a wide range of components to consider.
Take advantage of a manufacturer’s expertise when designing and selecting protection components to save precious development time. The manufacturer can help with advising on cost-effective solutions. Protecting your design against current overloads and voltage transients will result in a robust, reliable design that will enhance the reputation of your product in the market and reduce in-warranty service costs.
To learn more, download the following guides, courtesy of Littelfuse, Inc.
- Circuit Protection Products Selection Guide
- Littelfuse setP™ Design and Installation Guide
- ESD Protection Design Guide
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