Designing with the Microchip USB2422/MJ 2-Port USB 2.0 Hub Controller
The ever-growing ecosystem of USB peripherals demands robust and reliable expansion solutions. The Microchip USB2422/MJ, a low-power, 2-port USB 2.0 hub controller, serves as a critical component for designers looking to add downstream USB connectivity to a wide array of systems. Its integration simplifies the process of extending a single USB host port into two, supporting a mix of low-speed, full-speed, and high-speed (480 Mbps) downstream devices. This article outlines key considerations for effectively implementing this hub IC in a new design.
A primary advantage of the USB2422 is its high level of integration, which significantly reduces the external component count and simplifies the board layout. The controller requires only a single 24 MHz crystal or oscillator and a handful of passive components to become fully operational. This integration is crucial for minimizing both the physical footprint and the overall Bill of Materials (BOM) cost, making it an ideal choice for space-constrained and cost-sensitive applications such as embedded systems, desktop peripherals, and industrial equipment.
Power management is a cornerstone of the design process. The USB2422 offers flexible power configuration options to suit various application needs. It can be powered from a single 3.3V supply, deriving all internal voltages from its integrated regulators. A key feature is the ability to configure each downstream port for either bus-powered or self-powered operation via external components. For bus-powered applications, where all power is drawn from the upstream host, the hub includes integrated downstream power switches with comprehensive over-current (OC) detection. Each switch is individually controllable and provides a digital OC status output, allowing the host to swiftly respond to fault conditions and ensure system safety.
PCB layout is paramount for maintaining signal integrity, especially at high-speed USB 2.0 data rates. To ensure robust performance, designers must adhere to strict impedance control for the differential data pairs (D+ and D-). It is critical to route the USB differential pairs with 90Ω controlled impedance,
keeping traces as short and direct as possible. These pairs should be routed on a single layer, avoiding vias, and must be length-matched to within 5 mils to prevent signal skew. A solid, continuous ground plane beneath the signal layers is essential to provide a clear return path and shield the high-speed signals from noise. Proper decoupling, with capacitors placed as close as possible to the VDD pins, is also mandatory for stable operation.
Beyond the hardware, the USB2422's functionality can be tailored through configuration via external EEPROM or strapping resistors. While the hub operates with a default configuration out of reset, an attached serial EEPROM can be used to load a customized Vendor ID (VID), Product ID (PID), and other specific descriptors. For simpler changes, such as disabling battery charging support or setting the downstream port mapping, a series of built-in strapping options allow for configuration through GPIO pull-ups or pull-downs at startup, eliminating the need for an EEPROM in many cases.
In conclusion, the Microchip USB2422/MJ provides a compact and efficient solution for adding dual USB 2.0 ports to a design. Success hinges on careful attention to power architecture, meticulous high-speed PCB layout practices, and leveraging its flexible configuration options to meet the specific needs of the end application.
Keywords: USB 2.0 Hub, Signal Integrity, Power Management, PCB Layout, High-Speed Design
