Protecting USB From Power Surges

The Universal Serial Bus (USB) has become the connection scheme of choice for interconnecting different devices, many of them providing human interface support. Support for many different types of peripheral, which can be self- or bus-powered, calls for stringent protection for not just the target device, but also the bus itself.
For example, overcurrent protection for USB-powered devices is needed, both by standards such as UL60950, as well as by the USB specification itself. Also required is good electrostatic discharge (ESD) support to ensure that devices and the host are not adversely affected by the spikes caused by plugging in devices, or the user brushing against exposed pins in the USB connector. This article looks at the requirements for protecting the power and data lines in USB and covers devices such as the TE Circuit Protection Poly switch, theSTMicroelectronicsUSBLC6, the Texas InstrumentsTPDD4EUSB30, the NXP SemiconductorsIP4234CZ6, the Littelfuse1206Lfamily and the Bourns MF families, among others.
Protecting USB From Power Surges
The design of USB peripherals poses some challenges for the engineer. The USB specification in its latest version allows for very high data rates, which puts an onus on aspects of systems design such as signal integrity. At the same time, the ability to plug and unplug devices from a live system – providing the user with an easy means to add and remove human-interface and other devices – means that the delicate electronics behind the connector need to be protected from the sudden spikes of electricity that can result from pins making or breaking contact with the host system. Designers also need to consider the effects of electrostatic discharge (ESD) caused by users touching the pins of the USB connector or cable.
To support high data rates and overall high performance, IC vendors have moved to semiconductor processes based on finer and finer geometries. The result of the shrink in process geometry is a decrease in ESD robustness, demanding that designers implement protection measures close to the connector.
The USB specifications provide a number of options for designers to use when implementing both hosts and devices. From a power perspective, USB ports can be configured in two ways. They can be self-powered ports or bus- powered ports. This distinction affects not just motherboards and dedicated hubs. Many user-interface devices, such as keyboards, may contain low-speed USB ports to make it easier for the user to connect mice and other pointing devices to the upstream system.

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