One of the hottest topics regarding interface technology today is the universal serial bus (USB) Type-C connector, popular for its reversibility, higher data transfer, power delivery and additional protocols. While there is much excitement over the new standard, the reality is that the USB Type-A connector is still prominent and is being designed into end equipment today. When designing for USB host ports, you should consider two major areas of protection: overcurrent protection and electrostatic discharge (ESD) protection.
Per section 126.96.36.199.1 of the USB 2.0 specification, “All host and self-powered hubs must implement overcurrent protection for safety reasons.” You can accomplish this by using a fuse or current-limit switch. Moreover, a current-limit switch is the preferred solution over a fuse because it allows you to allocate less power per port during design. In a transient event, the load switch limits the output current to a safe level by operating in constant current mode. A fuse will allow the current to rise above the maximum current level until it shuts off. Choosing a current-limit switch allows you to select a smaller DC/DC converter and inductor because the switch more accurately limits the current during an overcurrent event. The current-limit switch also allows for decreased Vdroop in the 5V Vbus during transients versus a fuse implementation.
When designing USB ports, you should be aware of the high risk for ESD strikes and consider using ESD protection to prevent damage. For example, USB controllers and transceivers often handle ESD ratings based on the human body model. This rating is the oldest and most commonly used to detect ESD sensitivity. The HBM rating is intended for chip-level ESD protection and does not guarantee that the system will be protected from higher level ESD strikes. The International Electrotechnical Commission (IEC) 61000-4-2 standard tests for higher levels of ESD energy when compared to the HBM rating, as shown in Figure 1. For system-level protection, consider selecting the more robust IEC 61000-4-2 standard and using a transient voltage suppressor (TVS) diode.
In USB 2.0 applications, system-level ESD protection should be considered for VBUS and the data lines. VBUS requires a large capacitor for handling power transients like hot-plugging, and therefore can pass IEC 61000-4-2 by means of the capacitor. However, the data lines require a different approach. The high-speed lines operate at a maximum data rate of 480 Mbps, which means a large capacitor cannot be added to protect from ESD. You will need a low-capacitance TVS diode in order to decrease the effect on signal integrity.
Figure 1: ESD test comparison
Traditionally, multiple devices met the protection requirements for a USB host: a current-limited load switch for Vbus plus one or more ESD protection devices for the data pins. The TPD3S014 combines a current-limit switch and two channels of ESD protection to make a single-chip USB host-port solution as shown in Figure 2. The TPD3S014 and TPD3S044 allow 0.5A and 1.5A of continuous current, respectively, in a space-saving 2.9 mm by 2.8 mm DBV package.
The current limits in this family feature reverse-current blocking and are Underwriters’ Laboratories (UL)-recognized components (UL2367). They also provide IEC 61000-4-2 (Level 4) ESD protection for the data pins. These devices simplify your designs by reducing the number of devices and shrinking the overall footprint of the printed circuit board (PCB) to ensure optimal USB host-port protection.
Figure 2: USB host-port protection solution
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