Designing a USB Power Distribution System
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Joined: Sep 2010
14-10-2010, 11:51 AM
The USB specification (version 1.0) defines the power distribution requirements for selfpowered hubs (including hosts) and bus-powered hubs. This application report explains how the TPS2014 and TPS2015 power distribution switches provide the power switching and overcurrent protection needed to meet the USB requirements and how they are superior to other devices commonly used to accomplish the same functions.
Overcurrent protection devices such as fuses and PTC resistors (also called polyfuses or polyswitches) have slow trip times, high on-resistance, and lack the necessary circuitry for USB-required fault reporting. The faster trip times of the TPS2014 and TPS2015 power distribution allow designers to design hubs that can operate through faults. The TPS2014 and TPS2015 have low on-resistance and internal fault-reporting circuitry that help the designer to meet voltage regulation and fault notification requirements. Because the TPS2014 and TPS2015 are also power switches, the designer of self-powered hubs has the flexibility to turn off power to output ports. Unlike a normal MOSFET, the TPS2014 and TPS2015 have controlled rise and fall times to provide the needed inrush current limiting required for the bus-powered hub power switch.
The universal serial bus (USB) interface is a 12-Mb/s, multiplexed serial bus designed for low to medium speed PC peripherals (for example, keyboards, printers, scanners). The four-line USB interface is conceived for dynamic attach-detach (hot-plug-unplug) of peripheral devices. Two lines are provided for differential data and two lines are provided for 5-V power distribution. USB data is a 3.3-V level signal, but power is distributed at 5 V to allow for voltage drops in cases where power is distributed through more than one hub. Each function must provide its own regulated 3.3 V from the 5-V input or its own internal power supply. The USB specification defines the following five classes of devices, each differentiated by power
sourcing and sinking requirements:
• Bus-powered hubs
• Self-powered hubs
• Low power, bus-powered functions
• High power, bus-powered functions
Self-powered and bus-powered hubs distribute data and power to downstream functions. This application report describes power distribution solutions for both types of hubs.
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