Circuit & System Protection


Protecting USB OTG ports

14 May 2008 Circuit & System Protection

After the success of the USB 2.0 standard, the USB Implementers Forum developed a complementary standard called USB OTG (on the go).

USB OTG was developed to allow peripheral devices to communicate directly with each other without going through a traditional PC host. This smart communication standard allows direct communication between peripheral devices and a multitude of new products, which require over-current protection on the USB OTG enabled ports.

USB 2.0 vs. USB OTG

USB 2.0 traditionally consisted of a host/periphery topology where a PC was the host and the peripheral communicated only through the host device (see Figure 1). USB OTG was introduced to supplement USB 2.0 to allow existing mobile devices to communicate in a point-to-point manner without the traditional host (PC).

Figure 1. Example of a USB 2.0 host/ periphery topology
Figure 1. Example of a USB 2.0 host/ periphery topology

Under USB 2.0, portable computing devices such as PDAs, cellphones and digital cameras connect to a PC as a USB peripheral whereas USB OTG allows such portable devices to connect directly with each other. USB OTG therefore adds host functions to peripheral devices.

Peripheral devices that are USB OTG enabled have the ability to be either a host or a peripheral (dual role devices) and dynamically switch between the two. USB OTG is not designed to eliminate the need for a PC; it is intended to complement the concept of the 'extended PC' where the PC is at the centre of the consumer's extended world of digital devices. USB OTG is designed to give limited host functions to peripherals in order to perform basic functions. A USB OTG topology is illustrated in Figure 2.

Figure 2. Example of a USB OTG host/periphery and periphery/periphery topology
Figure 2. Example of a USB OTG host/periphery and periphery/periphery topology

Over-current protection

In USB 2.0, over-current protection is a requirement and is specified for self-powered hubs and hosts. The standard uses an example of a PPTC as a possible over-current protection device. A Bourns Multifuse PPTC resettable fuse such as the MF-MSMD110 or MF-MSMF110 is a commonplace solution for many USB 2.0 compliant ports. The reason PPTC resettable fuses are specified is in case the aggregate current drawn by a gang of downstream facing ports exceeds a preset value of 5 A. In this example, the PPTC resettable fuse trips, removing or reducing the current from all the affected downstream facing ports.

However, in the new USB OTG standard, any peripheral device that is designed to act as a limited host (A-device) must be able to transmit and receive power. In such equipment, if the current rating per port of the A-device is greater than 100 mA, then the voltage regulation is required to be between 4,75 V and 5,25 V, and the A-device is required to meet USB 2.0 specification requirements for power providers. Therefore, it should require over-current protection. Again, a Multifuse PPTC resettable fuse such as the MF-MSMD110 or MF-MSMF110 is a possible solution.

Many peripheral devices are handheld equipment, such as mobile phones, PDAs and digital cameras. In such uses, a smaller device may be desirable such as the 1206-sized MF-NSMF110 or MF-NSMF150.

Multifuse MF-MSMF110 and MF-NSMF150

The Multifuse MF-MSMF110 is an ideal current limiting PPTC resettable fuse for USB 2.0 applications. With a maximum operating voltage of 6 V, it is suitable for the full voltage range of USB 2.0 circuits, 4,75 V to 5,25 V. In the case of an aggregate current draw by a gang of downstream facing ports exceeding 5 A, the PPTC trips, removing or reducing power from all affected downstream facing ports.

The MF-MSMF110 trips at 2,2 A and has an operating current of 1,10 A at room temperature. As USB OTG ports are being used in small portable equipment, mini-AB receptacles are also being used, where space is at a premium. The MF-NSMF150, a 1206 sized PPTC resettable fuse, was designed for these applications. It has a maximum operating voltage of 6 V, trips at 3 A and has an operating current of 1,50 A at room temperature.





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