If you have ever opened a budget 5-port desktop switch from brands like TP-Link, D-Link, or Tenda, chances are you've seen a Realtek chip inside. The Realtek RTL8305NB-CG is the industry standard for cost-effective, unmanaged 10/100M Fast Ethernet switching.
Due to its high integration (embedded PHYs), low power consumption, and simple QFN package, it is also a favorite among hobbyists and engineers designing IoT gateways, port multipliers, or embedded network interfaces. This guide explores the technical details needed to design with the RTL8305NB.
Table of Contents
- RTL8305NB-CG Datasheet & Specifications
- Reference Design & Application Circuit
- QFN48 Pinout & PCB Layout
- Comparison: RTL8305NB vs. RTL8309N
- Conclusion
RTL8305NB-CG Datasheet & Specifications
The RTL8305NB-CG is a single-chip 5-port 10/100M Ethernet Switch Controller.
- RTL8305: Series Name.
- NB: Generation/Version (Low power optimization).
- CG: Package type (QFN) and Green (RoHS).
Key Specifications Table
| Feature | Specification |
|---|---|
| Port Count | 5 Ports (Integrated PHYs) |
| Speed | 10/100 Mbps (Auto-Negotiation) |
| Fabric Bandwidth | 1 Gbps (Non-blocking) |
| MAC Address Table | 2K Entries |
| Buffer Memory | Integrated Packet Buffer |
| Package | 48-Pin QFN (6x6 mm) |
| Input Voltage | 3.3V (Internal Regulator for Core) |
Green Ethernet (IEEE 802.3az) Features
One of the key selling points of the "NB" version is its support for IEEE 802.3az Energy Efficient Ethernet (EEE).
- Cable Length Detection: The chip detects the length of the connected Ethernet cable and adjusts the signal strength accordingly, saving power on short runs.
- Link-Down Power Saving: Ports that are not connected enter a deep sleep mode, reducing power consumption to micro-watts.
Price Analysis & Availability
This is one of the most affordable switch chips on the market.
Note: Despite its low cost, authentic chips are essential for stability. [Check Stock for RTL8305NB-CG at Aichiplink] to source genuine Realtek components.
Reference Design & Application Circuit
Designing a switch with the RTL8305NB is surprisingly simple due to its high integration.
Power Supply & Crystal Oscillator
- Power: The chip typically runs on a single 3.3V supply. It has an internal LDO (Low Dropout Regulator) to generate the lower core voltage (typically 1.0V or 1.1V), reducing the BOM (Bill of Materials) cost.
- Clock: A single 25 MHz crystal or oscillator is required across the XI/XO pins. The internal PLL generates the necessary 100MHz clocks for the PHYs.
EEPROM Configuration (Optional)
For basic unmanaged switching, no external memory is needed. However, connecting a small 24C02 I2C EEPROM allows you to configure:
- VLANs: Port-based VLANs.
- QoS: Priority settings.
- Flow Control: Enable/Disable 802.3x flow control.
- LED Modes: Customizing blink patterns.
QFN48 Pinout & PCB Layout
The QFN48 (Quad Flat No-lead) package is compact (6x6mm) but requires careful PCB layout.
Magnetics & MDI Interface
The RTL8305NB integrates 5 transceivers. These pins connect directly to the Ethernet Magnetics (Transformers).
- Connection: Pins are grouped by port (e.g., TX+/TX-, RX+/RX-).
- Isolation: You must use 1:1 isolation transformers (magnetics) before the RJ45 jacks. Many designs use "MagJacks" (RJ45 with built-in magnetics) to save space.
- Layout Tip: Keep the differential pairs (TX and RX) length-matched and impedance-controlled (100Ω) to prevent packet loss.
Comparison: RTL8305NB vs. RTL8309N
Engineers often confuse these two similar parts.
| Feature | RTL8305NB-CG | RTL8309N-CG |
|---|---|---|
| Port Count | 5 Ports | 8 Ports |
| Package | QFN-48 | QFN-64 |
| Application | Small desktop switches, IoT | 8-port desktop switches |
| Power | Ultra-Low | Low |
Verdict: Use the RTL8305NB for compact, 5-port designs. If you need 8 ports, move to the RTL8309N.
Conclusion
The Realtek RTL8305NB-CG enables the creation of incredibly compact, energy-efficient, and low-cost Ethernet switches. Whether you are building a custom IoT backplane or repairing a home router, this chip offers the perfect balance of simplicity and performance.
Sourcing Realtek Components Low-cost chips are often targeted by counterfeiters. Ensure your production quality by sourcing from trusted suppliers. Visit Aichiplink.com to search for RTL8305NB-CG and other Realtek networking ICs.
Written by Jack Elliott from AIChipLink.
AIChipLink, one of the fastest-growing global independent electronic components distributors in the world, offers millions of products from thousands of manufacturers, and many of our in-stock parts is available to ship same day.
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