The evolution of USB protocols, such as USB Charger and USB 3.0 has increased the demand for more robust and reliable circuit protection techniques. The new USB protocols have also increased data-rate speeds and, to ensure data integrity between devices, have made low-capacitance ESD protection of the bus more important than ever.
The evolution of USB protocols, such as USB Charger and USB 3.0 has increased the demand for more robust and reliable circuit protection techniques.
The new USB protocols have also increased data-rate speeds and, to ensure data integrity between devices, have made low-capacitance ESD protection of the bus more important than ever.
Higher Current, Higher Risk
Although power via USB has been in use for some time, the new standard dramatically increases the amount of current that can flow to a USB device – to a new maximum current rating of 1.5A (instead of 0.5A for USB 1.0 and 2.0). These higher current applications require more reliable and robust circuit protection to help prevent damage caused by overvoltage transients and overcurrent conditions. Overvoltage transients are generally caused by ESD (electrostatic discharge) and may occur on both the power bus as well as the data lines. Although modern ICs are protected up to 2000V, a human body can easily build up static charge that ranges up to 25,000V. In I/O port protection applications, a very low capacitance ESD device with fast clamping and recovery response is required on the data lines. Overcurrent conditions can also affect the power bus. The circuit protection requirement here is the ability to handle inrush currents (up to 1.5A per port for USB charging) reliably, and limit current faults below 5A in 60 seconds in the event of a fault (UL 60950 Table 2C).
Protecting Host/Hub Chargers
In host/hub charger applications, the portable device may not cause the voltage to drop below 2.0V under normal operating conditions. It must limit the amount of current drawn from a host/hub charger to 1.5A at low speed or full speed or to 0.9A at high speed. A coordinated circuit protection strategy for protecting host or hub chargers is shown in Figure 1. The PolySwitch™ device helps protect against overcurrent damage on the VBUS line, and PESD devices are used to shunt ESD away from sensitive circuitry on the data lines. PESD devices provide low capacitance compared to traditional MLV (multilayer varistor) or TVS (transient voltage suppression) diode technology. Their low-trigger voltage and low-clamping voltage also helps protect sensitive electronic components.
PolyZen™ Device Helps Protect Portable Electronics
Multiple interfaces and charging systems can expose portable devices to damage caused by the use of incorrect chargers, improperly regulated third-party chargers, or hot connect/ disconnect events. Although typical USB supplies provide regulated lines at 5V +/- 5%, the voltage at these lines may, under certain circumstances, exceed 5.25V, resulting in damage to the system or unprotected peripherals. As shown in Figure 2, the PolyZen device incorporates a stable Zener diode for crisp voltage clamping and a resistively nonlinear PPTC (polymeric positive temperature coefficient) layer. The PPTC layer responds to either diode heating or overcurrent events by transitioning from a low to high resistance state. In the event of a sustained high-power overvoltage condition, the tripped PPTC element limits current and generates a voltage drop to help protect both the Zener and the follow-on electronics – effectively increasing the diode’s power handling capability.
For traditional USB 2.0 host and hub charger applications, TE Connectivity Electronics recommends the PolySwitch miniSMDC110 device for single port connections, the PolySwitch miniSMDC150 device for 2-port connections and the PolySwitch nanoSMDC200S device for 3-port connections. To help protect traditional 0.5A USB peripherals from overvoltage surges on VBUS, TE Connectivity Electronics recommends PolyZen ZEN056V130A24LS and ZEN065V130A24LS devices. For new USB charging applications, including dedicated chargers, hosts and hubs, TE Connectivity recommends PolySwitch nanoSMDC200S devices for overcurrent protection on each port. To support new 1.5A charging on the device side, PolyZen ZEN056V230A16LS devices on the VBUS line are suggested. PESD0603-240 devices, on all D+ and D- line devices and host/hub, independent of USB 2.0 or USB charging, are also recommended.