Overcurrent and Overvoltage Protection for Flash USB Applications

USB flash memory sticks and their hosts are vulnerable to surges from electrostatic discharge (ESD) on the data lines and overcurrent surges from connection to the V+ line. In these applications, circuit protection devices, such as those shown in Figure 1, are commonly used to help protect against damage caused by overcurrent and overvoltage conditions.

 

 

ESD Protection

A common problem in flash memory applications occurs when users interface with electronic circuits and devices after electrical charges build up on the devices, circuits and even on users themselves. Typically, charges are generated on devices simply by the user handling them. In this instance, when the device that bears these charges is inserted into an electronic apparatus or its USB connector, the charges cause current to flow to the application through the terminals of the connector or through other components of the system. The charges can cause damage to the circuits or circuit elements on the memory stick and can ultimately result in failure of the electronic apparatus itself.

 

Overcurrent Protection

USB specifications call for resettable overcurrent protection on the interface and specifically reference polymeric positive temperature coefficient (PPTC) devices as an appropriate technology. These devices are commonly used to help protect against overcurrent events that occur on the USB power line and are suitable for plug-and-play applications. As USB communication rates increase and circuitry gets smaller and more sensitive, protection against circuit transients becomes more critical.

Tyco Electronics’ PolySwitch™ picoSMD035F device can often be used with the latest generation of high-frequency data ports, I/O ports and storage devices. The picoSMD035F device helps provide overcurrent protection using less than half the board space required by previous offerings, providing a significant advantage in space-constrained board designs. Rated at 6V, the picoSMD035F device provides a hold current of 0.35A, a trip current of 0.75A and a maximum resistance of 1.4Ω.

The SMD product family is available in a broad range of configurations and sizes to help designers meet their specific circuit protection needs. PolySwitch devices help meet safety agency standards, are full-edge terminated with nickel-gold finish for superior solderability and are RoHS-compliant.

Like fuses, PPTC devices help protect circuitry from damage caused by overcurrent events. Unlike fuses, however, they reset when the fault is removed and the power is cycled. In addition to resettable functionality, PolySwitch devices offer other critical design advantages in portable applications when compared with fuses, including low resistance, fast time-to-trip, low tripped power dissipation and inherent resistance to nuisance tripping. They also provide a lower cost solution than silicon equivalents.

 

 

Overvoltage Protection

The USB port is also susceptible to damaging overvoltage transients, including ESD pulses. At the much higher data rates of USB 2.0, IEEE 1394, and digital visual interface (DVI), the parasitic impedance of traditional protection devices can distort and deteriorate signal integrity.

The new USB 2.0 protocol allows for data transfer rates of up to 480Mbps and supports plug-and-play hot swappable installation and operation. These higher data rates have made low capacitance ESD protection of the bus even more critical to maintaining data integrity rates between devices.

PESD devices help shunt ESD away from sensitive circuitry and provide the low capacitance required for use in high data rate circuits. The PESD device’s low trigger voltage and low clamping voltage also help protect sensitive electronic components.

As shown in Figure 2, a PolySwitch device on the Vbus and a PESD device on the D+/D-, help protect both the host and guest application from damage caused by such events.

Figure 3 shows a typical circuit protection design utilizing Tyco Electronics’ PolySwitch picoSMD035F device and PESD device. This figure shows how circuit protection devices are applied between the host USB controller and the USB port to protect host USB applications.

 

 

Summary

Tyco Electronics’ picoSMD035F devices and PESD0603-240 devices can be used in a coordinated protection scheme to help protect flash USB applications from damage caused by both overcurrent and overvoltage surges.