Design Challenges Posed by USB 3.2

Double data rates and no expenditure required to replace USB Type-C cables should mean that USB 3.2 is a dream come true. It mostly is, but the fast, convenient USB upgrade does present some design challenges.

The latest USB specification, USB 3.2, doubles the data transfer speed of USB 3.1 to open up the potential for more use cases at a cost-effective price point. It also offers universality, with data able to be quickly transferred from host to device, smartphone to TV for example, and for faster charging of devices.

The specification, introduced in September 2017, doubles the data transfer speed of the USB 3.1 by using both 10Gbps lanes simultaneously to produce a transfer rate of 20Gbps. This transfer rate is achieved over existing USB Type-C cable, certified for SuperSpeed USB use at 10Gbps, which means that no new cables are needed. Although the two lanes run in tandem, using extra wires inside the USB cable, a hub update is required to transition seamlessly from single- to dual-lane operation. On the plus side, USB 3.2 works with USB connectors, and no upgrades are needed to devices such as laptops or phones. USB 3.2 equipment is needed at each end of the cable to realize the extra speed, however.

Figure 1: Nexperia offers its SuperSpeed protection diodes in a compact DSN0603-2 (SOD962-2) package.

Data Boost with USB 3.2
USB 3.2 focuses on electrical parameters, defining the data width and maximum data transfer speed, explains Stefan Seider, product manager, Nexperia.

This month, the company announced that it has optimized its TrEOS ESD protection diode series for use with USB Type-C interfaces. TrEOS technology was introduced to safeguard 10Gbps+ data line systems on all three fronts: robustness, signal integrity, and system protection. Typically, says the company, there are trade-offs, resulting in only two out of the three core parameters receiving adequate protection.

The company offers two new families to accommodate the optional capacitor at the Rx inputs that are introduced by the USB 3.2 standard. One group has high surge robustness and is designed to be positioned between the connector and the capacitor, and the second family has a low trigger voltage that is designed to be placed between capacitor and the system chip.

“The main changes from USB 3.1 to USB 3.2,” says Seider, “is that it is now possible to use all data lines from the Type-C cable, doubling the transmission rate from 10 to 20Gbps.” Alternative modes were available before USB3.2 was introduced, for example to connect a mobile phone over a Type-C cable to a TV, to show a video shot on a smartphone.

USB Type-C is the standard for charging and transferring data. It focuses on mechanical areas, such as the connector and the cable. USB 3.1 and USB 3.2 are interface standards. USB 3.2 in particular addresses electrical parameters, defining data width, maximum speed, and protocols to transfer data.

USB Type-C includes extra pins which, explains Seider, allow the host and device to negotiate charging voltage and charging current between them. This allows for faster charging but is not without its problems, as surges can occur.

Surge Protection
One challenge for designers of USB 3.2 is higher surge possibilities. “With Type-C it is possible to buy not-so-good cable,” says Seider. “Cable is cable, right? But if the cable breaks from wear or bending the power supply can connect to data lines. In the past, this was not much of an issue as Vbus meant 5V and 500mA. However, with Type-C, that is up to 20V and up to 5A (100W).”

Another design challenge presented by the latest specification is coupling capacitors. Conventionally, there is one pair between transmitter and connector each, but USB 3.2 introduces an optional capacitor between connector and receiver, which will decouple DC and couple the reverse AC signals. Although this provides DC protection, a protection diode between connector and this capacitor still has to be robust against surges, points out Seider. “Protection devices behind the capacitor stay optimized for very low voltages, so if a customer has an extremely sensitive transceiver—and we are seeing more and more of them—they can use one of these specialized diodes, with a low trigger voltage, very fast switching and [which is] optimized to protect the most sensitive chips we have encountered so far,” continues Seider.

Minimizing EMI in high-speed data connections is going to be the next challenge for the industry. The interference is hard to predict, and it may be detected in the late stage of design, or—even worse—after the product has been launched onto the market. Nexperia addresses this with common mode filters integrated into its protection devices.

The company has addressed the challenges of higher surge and higher voltages of USB 3.2. The latest ESD protection diodes support every protection strategy around the new USB 3.2 Rx capacitor, says Seider, to protect sensitive transceivers against possible fault conditions.

Explaining the company’s approach to ESD protection, Seider explains why it uses external devices. “Modern electronic devices typically have on chip ESD protection, but it is pretty weak because making on chip ESD protection in advanced silicon is very expensive,” says Seider. “What we do is provide external diodes, which protect against environmental conditions; the internal ESD protection is only to protect the system or transceiver chip against assembly. Assembly is done in a very controlled environment, so the requirements here are quite small.”

TrEOS ESD protection technology uses active silicon-controlled rectification to deliver capacitance as low as 0.1pF and low clamping, with high robustness against surge and ESD pulses (up to 20A 8/20µs, which is claimed to be the highest in the industry today). Devices can withstand up to 30kV discharge, which exceeds the levels stipulated in IEC 61000-4-2, level 4, points out the company.

As USB 3.2 is designed for small, compact devices, the protection diodes are packaged in a compact 0603 form factor, in a DSN0603-2 (SOD962-2) package which has no bond wires for fast inductance protection.

The company has developed dongles (Figure 2) that can loop between the device and a computer for a quick, functional check of ESD protection. They can also be used as a reference design, “a starting point for a typical Type-C application,” says Seider.

Figure 2: To deal with the higher surges and higher voltages of USB 3.2, Nexperia provides demonstration dongles for a functional check or as a reference design for developers.

Caroline Hayes has been a journalist covering the electronics sector for more than 20 years. She has worked on several European titles, reporting on a variety of industries, including communications, broadcast and automotive.


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