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16 Circuit Protection for High-Speed Serial Interfaces As crucial as these advantages are to users, it's equally essential for designers to consider the safety issues associated with power and temperature management at the cable-to-device connection. Without proper circuit design, including temperature monitoring, cables and connectors can accumulate contaminants inside the connection, causing them to heat up quickly, easily damaging or destroying the cable and the mobile device. Basics of USB-C Wired Charging Three individual items must work together in any charging system—the device being charged, the cable, and the charger. USB-C cables have one or more symmetrical (and, therefore, reversible) 24-pin connectors. USB-C chargers have an AC plug on one end (for plugging into a wall outlet) and either a cable with a USB-C connector on the other (for plugging into the device to be charged) or a USB-C output port for plugging in a USB-C cable. From a safety standpoint, USB-C cables must be capable of carrying appropriate voltages and currents. For chargers with captive or fixed cables, the cables must handle the charger's maximum voltage output. Cables with USB-C plugs on each end must be capable of driving 21V and at least 3A. Cables that incorporate special e-marker integrated circuits can carry 5A of current. Any device placed in the path of power, especially protection devices, must also manage these levels of voltage and current. Sources of USB-C Charging Damage USB-C connectors' pin pitch is 0.5mm—much tighter than the 2.5mm pin pitch in USB Type-A connectors. This tighter pitch significantly increases the risk of a fault that could cause a thermal event. When connector pins become deformed or dust, metal particles, hair, or other debris gets stuck in a USB-C cable connector, a resistive fault can be created from the power line to ground. These resistive faults can cause a dangerous temperature rise while increasing current only minimally. Damage to both cables and devices, even fire, has been reported (Figure 1). The traditional approach to protecting USB cables from overheating involves locating either a polymeric positive temperature coefficient (PPTC) device or a mini-breaker (also known as a thermal cutoff) on the V BUS power line. The chosen device would be placed on a printed circuit board inside the connector to sense the resistive fault's temperature rise. However, using this approach when attempting to protect against up to 100W can challenge design engineers. One of the drawbacks of adding a protection device in the path of power is that its resistance, even if it's just milliohms of resistance, can contribute to significant power loss, making it more difficult for manufacturers to meet mandatory efficiency requirements. Figure 1: Resistive faults in USB-C connectors can damage both cables and the devices being charged. (Source: Littelfuse) From a safety standpoint, USB-C cables must be capable of carrying appropriate voltages and currents. " "