Fast Role Swap, Linear ORing with TPS25947 and LM73100 in USB Type-C systems

1 Introduction

USB Type-C interface is quickly becoming one of the most popular ways of connecting electronic devices. From speakers to laptops, a single USB Type-C port can deliver power and offer high-speed data transfer. However, USB Type-C faces its own set of design challenges such as the process of Fast Role Swap (FRS). Its power-protection devices also need to qualify for the unique design specifications while protecting the port from overvoltage, short circuit, and reverse current. The TPS25947 efuse and LM73100 ideal diode can offer power protection while meeting the USB-Type C requirements for robust high power delivery, high-speed data transfer, and FRS.

Figure 1-1 USB Type-C Power Protection Block

Figure 1-1 shows that two protections devices are needed per port to protect the system at all times especially during FRS.

2 Fast Role Swap (FRS) Explained

Fast Role Swap (FRS) Explained

FRS describes the process when the device transitions from a power sink to a power source and vice versa. A laptop docking station, for example, requires robust FRS design and protection to ensure uninterrupted power supply to its accessories when external power disconnects. To further put FRS into perspective, imagine the setup in Figure 2-1: a laptop and its docking station connected through a single USB Type-C. The docking station initially charges the laptop and powers the other accessories through external power source. When the external power is removed, the laptop needs to quickly transition from a power sink to a power source to continuously supply power for its connected accessories. There are two major challenges in FRS:

The switching time must be less than 150 µs. Denoted by tSrcFRSwap, the time is defined to be from Vbus dropping below 5 V to the new source beginning to supply power at 5 V.
The Vbus voltage immediately before FRS is uncertain. With external power, the docking station charges the laptop at about 20 V. Once FRS begins, the system needs to promptly switch to the new source within the 5-V specification.

While meeting FRS requirements with fast charging and linear ORing, TPS25947 and LM73100 also feature adjustable overvoltage and undervoltage protection that ensures a smooth FRS within the 5-V specification.

Figure 2-1 FRS for a Laptop Docking Station

3 Fast Charging Enables FRS

In addition to voltage protections, TPS25947 uses fast charging to control inrush current to meet the FRS time requirement. Conventionally, inrush current control is implemented with adjustable slew rate control. By controlling the slew rate, efuse devices can prevent the initial voltage spike caused by inrush current at startup. However, conventional adjustable slew rate control is too slow for the FRS time requirements. By integrating a variable frequency charge pump and internal control logic, TPS25947 is able to detect switching and quickly boost the frequency to rapidly charge the gate of the FET.

Figure 3-1 FRS Test

For this test in Figure 3-1, Vout was initially supplied at 20 V by an external source, indicating that the end device is initially a power sink. Once the external supply is disconnected, Vout quickly drops and the end device switches to a power source to match a constant 5-V output. As shown by Figure 3-1, TPS25947 is able to FRS in ~30 µs.