by Tsung-Ta Tasi
Thunderbolt is the connector standard published by Intel as the universal bus between computers and other devices. The generations 1 and 2 are integrated with Mini Display Port, while the newer generation 3, Thunderbolt 3, was first published in 2015 at the Computex Taipei with Mini Display Port giving way to USB Type C. USB Type C is the most extensively used interface among all electronic products. It is so widely used that users have more and more demands for its use.
Thunderbolt 3 is an extremely fast transmission port featuring up to 40Gbit/s of transmission, which is enough to satisfy emerging applications that are demanding for transmission rate, such as ultra-high definition (UHD) TV, monitor and solid-state drive (SSD). To realize such a high transmission rate, advanced-level semiconductor manufacturing techniques are required for Thunderbolt 3 and that inevitably leads to the significantly reduced durability of Thunderbolt 3 for ESD(electrostatic discharge).
Thunderbolt 3 is currently established on the physical port of USB Type C. This interface not only features transmission rate of up to 40Gbit/s, but also features boost charge with power supply up to 100W. Since Thunderbolt 3 is an ultra-fast transmission port and a port on system that are exposed to users to insert and pull out, the most common application for users is plug-and-play and unplug-and-turn-off. On the other hand, this hot plug is often the culprit of electronic system malfunction or even damage to the USB Type C control elements, as many instantaneous noises, such as electrostatic discharge, are introduced through this hot plug. During a hot plug, the signal cable at the port side is already live and the already live cable will discharge when in contact with system. This phenomenon is equivalent to the ESD that may cause serious damage to the system, and in general this is called direct discharge. In the system ESD test today, more and more clients ask to test products using direct-pin injection, as to simulate the ESD events that the system is subject to during the use at the user side.
In addition to the IEC61000-4-2 compliance, some of the brand-name clients go even further to ask that the USB Type C connector on their products has to be tested with ESD bombardment at ±8kV through direct-pin injection without compromising the system. Therefore, it is absolutely necessary to have ESD protection elements on the USB Type C interface to prevent the interference of ESD events with data transmissions. Figure 1 shows the direct-pin injection test setup.
Figure 1: Direct-pin injection test setup
Electronic products have been wide used in the world. However, the probability that these products are subject to electrical over-stress (EOS) at remote places increases dramatically due to poor infrastructure or extreme weathers, thus resulting in high shop visit rate. EOS usually comes from interference from power supply (AC/DC), power noises or hot swap. The EOS tests are based on IEC61000-4-5 with the test voltage waveform of 1.2/50uS and current waveform of 8/20uS. Apart from the ESD direct-pin injection test established in house, some clients ask for additional EOS direct-pin injection test. Figure 2 shows the EOS direct-pin injection test setup.
Figure 2: EOS direct-pin injection test setup
As far as the high-speed Thunderbolt 3 interface is concerned, the following shall be taken into consideration for the selection of ESD/EOS protection elements:
1. To ensure the signal integrity during the Thunderbolt 3 high-speed signal transmission, ESD protection elements with low parasitic capacitance shall be selected. The parasitic capacitance is recommended to be lower than 0.2pF;
2. The protection elements shall have good ESD resistance to resist at least the 8kV ESD bombardment in the contact mode specified in IEC 61000-4-2;
3. The most important parameter is the ESD clamping voltage. For an ESD protection element to provide effective protection for a system, not only the element itself has to survive high enough ESD bombardment, but also it has to consider whether the clamping voltage is low enough to keep the ESD energy at lower voltage in order to prevent damage to the internal circuitry of system. That is why the clamping voltage is the most important parameter with which to determine the protective performance of ESD protection elements for system circuitry;
4. This interface also features boost charging and support for 3 stages of voltage, 5V/12V/20V; for wired charging, hot plug generates frequent ESD/EOS issues on power cable and that is why it is necessary to have a better external ESD/EOS protection design in a system.
Amazing Microelectronic has state-of-the-art ESD protection design technology and launches the ESD protection device, model AZ5B8S-01F, specifically for the protection need of Thunderbolt 3. To keep the parasitic capacitance of protection device away from the high-speed transmission of Thunderbolt3 differential signaling, the parasitic capacitance of AZ5B8S-01F is designed as lower than 0.2pF as proven in 40Gbit/s eye diagram test. Figure 3 provides the 40Gbit/s eye diagram test result of AZ5B8S-01F.
Figure 3 40Gbit/s eye diagram test result of AZ5B8S-01F
Most importantly, AZ5B8S-01F features extremely low ESD clamping voltage, which helps Thunderbolt 3 interface survive ±8kV ESD bombardment via direct-pin injection. Figure 4 provides the current vs. voltage curve of AZ5B8S-01F using TLP measurement. With the 8kV ESD bombardment in the IEC 61000-4-2 contact mode (with equivalent TLP current of approximately 16A), the clamping voltage is as low as 5.5V, effectively preventing data errors, crashes or even damage of system products during ESD test.
Figure 4 ESD clamping voltage characteristics curve of AZ5B8S-01F
Today where electronic products are becoming light-weight and compact, the printed circuit board (PCB) in the products has to be smaller and smaller. However, the circuitry becomes more complicated as the demand for powerful functions in products never stops, which makes PCB a piece of prime property as well as a headache for product design. The AZ5B8S-01F is designed using ultra-small DFN0603P2Y packaging with a size of merely 0.6mm x 0.3mm and thickness of 0.3mm. This is used for the protection of the 4 differential pairs (TX and RX) in Thunderbolt 3. In addition, a single AZ1045-08F is enough to protect the remaining signal lines (D+/D-/CC/SBU). What is special is that the pins on AZ1045-08F are arranged in a staggered way to allow for a feed-through design in PCB layout and, thus, eliminate the troubles during trace layout. This not only helps reduce the PCB layout workload during the product design stage, but also shrinks down the PCB size for less system costs. This port also allows boost charging. An ESD protection device is required at the power supply side and the appropriate ESD protection device (AZ3105-01F/AZ4512-01F/AZ4520-01F) is selected for protection based on the design voltage. Now this port is well protected against ESD/EOS threats. Figure 5 shows a complete ESD/EOS protection circuit solution for Thunderbolt 3 interface.
Figure 5 ESD/EOS protection circuit solution for Thunderbolt 3 interface