BCM5345KPBG Gigabit Ethernet Switch: Datasheet, Pinout, and Application Circuit Design
The BCM5345KPBG from Broadcom (now part of Avago) is a highly integrated, low-power, single-chip solution that functions as a 5-port Gigabit Ethernet switch. It is designed for use in demanding networking applications, including SOHO (Small Office/Home Office) equipment, network-attached storage (NAS), digital home media players, and embedded systems requiring robust and reliable LAN connectivity.
Datasheet Overview and Key Features
The datasheet for the BCM5345KPBG highlights its advanced feature set. At its core, the IC integrates five 10/100/1000 Mbps Ethernet transceivers, a high-performance non-blocking switch fabric, and the necessary MAC units. Key characteristics include:
High Integration: The chip incorporates the switch fabric, buffer memory, and five PHYs into a single 128-pin package, significantly reducing the bill of materials (BOM) and board space.
Low Power Consumption: Built with advanced low-power process technology, it is ideal for energy-conscious applications.
Advanced Management Features: It supports IEEE 802.1Q VLAN for network segmentation, Quality of Service (QoS) prioritization, and IGMP snooping for efficient multicast traffic management.
Flexible Interface Options: It offers multiple MII/RGMII interfaces for connecting to an external host CPU or other network processors.
Pinout Configuration
The BCM5345KPBG is housed in a 128-pin Pb-free LQFP (Low-Profile Quad Flat Package). Its pinout is logically grouped to simplify PCB layout:
Power Pins (VDD): Multiple pins for core (1.2V), digital (2.5V, 3.3V), and analog (3.3V) power supplies. Proper decoupling with capacitors near each pin is critical for stable operation.
Port Interfaces (Ports 0-4): Each of the five Gigabit Ethernet ports has dedicated pins for the differential transmit (TXP/TXN) and receive (RXP/RXN) pairs. These must be routed as controlled impedance differential pairs to the RJ45 magnetics module.
LED Indicators: Pins for link/activity and speed status LEDs for each port.
Management Interface: Includes pins for MDC (Management Data Clock) and MDIO (Management Data I/O) for communicating with the internal management registers.
CPU/Host Interface: MII/RGMII pins for connection to an external CPU.
Clock Input: A single 25MHz crystal or oscillator input provides the reference clock for the internal PLL.
Application Circuit Design
Designing with the BCM5345KPBG requires careful attention to several key areas to ensure signal integrity and compliance with Ethernet standards.
1. Power Supply Decoupling: Use a multi-layer PCB with dedicated power and ground planes. Place 0.1µF and 10µF decoupling capacitors as close as possible to each VDD pin to filter high and low-frequency noise.
2. Clock Circuit: A 25MHz crystal should be placed very close to the XI and XO pins, with load capacitors as specified in the datasheet. Alternatively, a 25MHz oscillator can be used for greater accuracy.
3. Magnetics and RJ45 Connectors: Each of the five ports requires an external Gigabit Ethernet magnetics module, which provides isolation and signal conditioning. The differential pairs from the chip to the magnetics must be routed with 100Ω differential impedance, kept symmetrical, and isolated from noisy signals like power supplies or clocks.
4. Management and Configuration: The MDC/MDIO lines, often connected to a host CPU, are relatively low-speed but should still be routed neatly. Pull-up or pull-down resistors may be required on configuration strap pins that determine the device's boot-up behavior (e.g., default PHY address, LED mode).
5. Thermal Management: Although a low-power device, ensuring adequate copper pour for the thermal pad on the bottom of the LQFP package is essential for heat dissipation.
The BCM5345KPBG stands out as a highly integrated and power-efficient solution for implementing a complete 5-port Gigabit Ethernet switch. Its combination of integrated PHYs, advanced management features, and a simple host interface makes it a compelling choice for designers looking to add reliable, high-speed networking to their products without excessive complexity or cost. Careful PCB layout, especially for the GHz-speed differential pairs and power integrity, is the key to a successful implementation.
Keywords: Gigabit Ethernet Switch, BCM5345KPBG, PHY Integration, RGMII Interface, Application Circuit Design.
