W5500 PCB layout


I am using the information on this link Reference Schematic | WIZnet Document System to create a PCB for my application. I have a couple of questions:

  1. Do traces from connector to magnetic and from magnetic to W5500 have to be impedance controlled? I have to use external magnetic.
  2. Do damping resistors R21 R22 R23 R24 affect the impedance of the traces?

Hi , I’m Alan

  1. Yes. It should be designed with an impedance of 100 ohms.
  2. Technically, it will affect impedance, but this resistance is damping resistance for EMC, so I don’t think you need to care about the impedance of the trace.

If you have any additional questions, please ask.

Thank you Alan, I have a few more questions:

  1. I noticed the receiving pair is terminated with two 150R resistors R16, R17. I believe they are not Bob Smith termination resistors. What are the functions of these resistors? Are they optional?

  2. On the W5500 side, why the transmitting pair is terminated to 3.3V and receiving pair to GND? And why the center tap of the transmitting pair is connected to 3.3V?

  3. Are C9 and C10 there to form RC filters with the termination resistors?

Yes, hello.
I’m answering.

  1. I think it will depend on the trans and RJ45 choices. As you said, it’s not a Bob-Smith circuit. Termination resistance, which is used to reduce noise in the RX. Noise is reduced when a resistor is fitted, but the signal may become a little lumpy. You will need to consider and design it for each circuit. It is not an essential element, as it does not depend on a typical circuit.

  2. The W5500 has no internal bias voltage at the transmitting end and an internal bias voltage at the receiving end. Therefore, the transmitter must apply bias voltage to an external circuit. If you search for Ethernet Voltage Mode and Current Mode, there is a lot of information.

  3. Yes. It also acts as an RC Filter. Filter. The primary function is EMC.

Thanks Alan.
Another question: what is the power consumption of the termination resistor R1, R2, R6, R7? R1 is connected to 3.3V, so the power will be 3.3*3.3/49.9 = 0.218W.
However, on the evaluation board, 0402 resistors are used, which can only dissipate 0.031W.

You don’t have to worry about that.
The communication line is not directly connected to the GND via a resistor. Because it is connected to the IC side, the current is used only as much as necessary.