In 2013 the USB Implementers Forum released two new interfaces: USB 3.1 and USB Type-C. Together they offer many advantages over earlier USB interfaces, but those advantages will require careful design to actualize.
Specifically, products enabling USB 3.1 Type-C must take special care to provide ESD protection on the various high speed and high power lines incorporated in these new interfaces. At the same time, designs must ensure bandwidth is not sacrificed. The Silicon Enhanced ESD Diode Array (SESD) product line from Littelfuse offers a range of solutions uniquely suited to the challenges of this high-speed serial interface design. The SESD product line delivers maximum protection with minimal added system cost, complexity and attenuation.
The USB 3.1 interface allows for data transfers up to 10 Gigabits per second, and via the Power Delivery 2.0 specification is capable of power transmission up to 100 Watts. These speeds are capable of supporting high-end SSDs, RAID arrays and even high resolution video displays. The high power capability means USB 3.1 is suitable for powering most cell phones, laptops and other peripherals. To make connections more convenient, the Type-C interface eliminates the keyed interconnect of past USB interfaces. However, as with earlier USB interfaces, Type-C supports a variety of cable lengths. USB 3.1 and USB Type-C can be used independently, but consumers will see the most benefit from products that embrace the features of both interfaces in a single port. This constellation of high speed and high power delivery along with simple ubiquitous cabling sounds like a dream come true for technology aficionados, but it means interface designers will need to take extra care with their implementations.
The key feature of USB 3.1 is its high data transfer rate. Allowing for data transfers up to 10 Gbps puts USB 3.1 on par with the first generation Thunderbolt controllers. In technology demonstrations USB 3.1 has already proven it can handle the high speed data transfers SSDs support.
To understand how this high speed is achieved it is useful to look at the USB Type-C pin definition. Referencing Figure 1, and working from the outside edge inward, the first pins on the outside edges of the connector are four ground pins. Next to the ground pins are four twisted differential pairs, acting as two each of transmitting and receiving pairs or lanes. These differential pairs constitute the primary high speed bus. Each of these lanes is capable of 5 Gbps however only one set of each of the transmitting and receiving pairs is enabled, resulting in the overall 10 Gbps maximum bandwidth.
It isn’t just the cable and connectors themselves that must support this high speed interface however. Data is traversing an entire network from client PCB, through a connector, down a cable, through another connector and across the host PCB. The high speed data lines are highly susceptible to ESD events, particularly on the disabled or non-performing transmit and receive pairs. Although it is the summation of this complete system’s parasitics that will dictate the maximum possible bandwidth data can be transmitted and received, the primary factor of concern is capacitance. The SESD solutions from Littelfuse offer ultra low capacitance ESD protection in either unidirectional or bidirectional configurations to offer a design choice without sacrificing ESD protection and signal integrity.
The USB 3.1 specification requires that bits stream as either a 0 or a 1, without crossing zero volts. As such, unidirectional protection is the preferred solution. Bidirectional protection is a lower capacitance, and therefore a potentially higher bandwidth option. However, it is important to note the bidirectional protection format can expose the non-performing high speed lanes to potential over voltage threats. Littelfuse offers a convenient SESD unidirectional array (part number RF3077-000) packaged in a four channel configuration. This solution is specifically targeted to provide optimal protection to high speed USB 3.1 lanes while only adding 0.2 pF of capacitance to the overall design budget. Another option is to use single channel unidirectional solutions available in either industry standard 0201 or 0402 surface mount configurations. Littelfuse offers bi directional ESD protection in discrete SOD-882 and 0201 DFN formats for customers who value the flexibility of routing
Working further inward on the connector, the next pins encountered are the four Vbus (bus power) pins. The USB 3.1 interface utilizes the USB Power Delivery 2.0 specification to support 5 power profiles. The most capable of these, Power Profile 5 allows for 20 V at 5 A or 100 W of power over a single cable. This high power capability is an attractive benefit for consumer adoption of USB 3.1 Type-C enabling faster charging of more and bigger devices. These power pins bring with them their own unique ESD concerns. One such concern is the small initial surge experienced when charging an empty battery pack. This surge occurs as both voltage and current spikes and consideration should be given to accounting for the additional over voltage (typically as much as 50% above the rated voltage). Littelfuse offers the SPHV24-01ETG in the industry standard 0402 package or the SP24-01WTG-HV in the 0201 standard package format. Both of these chips are designed to offer ESD protection through USB Power Profile 5 (100 W power delivery). They also offer a complete line of ESD protection chips for Vbus applications intending to utilize the lower power profiles.
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Continuing to work towards the center of the USB Type-C connector, following the Vbus lines are the Configuration Channel (CC1 and CC2) and Sideband Use (SBU1 and SBU2) lines. These lines constitute a lower speed bus which enables USB 3.1 Type-C to utilize alternate protocols such as Thunderbolt or Display Port. The USB Type-C specification dictates that plug orientation is determined by a pull down resistor on one of the two configuration channel lines. The CC1 and CC2 lines also enable lower speed communication, for example, to send Vendor Defined Messages (VDM) between the host and client. Although transmission over the configuration channel pins is lower bandwidth, these pins are still exposed to potential ESD threats. The sideband use channels are also used in lower bandwidth scenarios, such as for host to client display port session negotiation. Since the configuration channel and sideband use pins don’t require the high bandwidth capabilities of the high speed bus lines the configuration channel and sideband use pins will benefit from medium-range capacitance, unidirectional ESD protection. Littelfuse offers the SP1003-01ETG and SP1006-01UTG solutions for these pins.
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At the center of the USB Type-C connector are 2 differential pairs constituting a legacy USB 2.0 bus. However, unlike traditional USB 2.0, the bus at the center of USB Type-C is a bidirectional implementation. This high speed bus requires its own low capacitance, bidirectional ESD protection such as that offered by part numbers SP3022-01WTG or SP3022-01ETG from Littelfuse.
USB has a proven track record with consumers ensuring USB 3.1 Type-C adoption is certain. Already devices are making their way to store shelves which implement this standard. Many of these devices are mobile and customers expect a robust product that can withstand the ravages of the pocket, the desk and the car seat. Furthermore, they expect to connect devices with cables that can be up to 3 meters long and they’ve been promised a convenient connector that can be inserted in either right side up or upside down configurations. These conditions make designing for ESD protection all the more critical. Littelfuse offers a complete line of ESD solutions to enable the next generation of high speed and high power devices.