KSZ8081RNA PHY Design

Question

I am designing a circuit using the KSZ8081RNAIA Ethernet PHY, and I have a question regarding the use of pin 23 (LED pin). The datasheet can be found here.

As I understand it, pin 23 is a strap-in pin where the digital state gets latched at chip reset, after which it functions as an output pin. I plan to pull this pin high at chip reset and then use it to control a two-color LED. I’ve attached the circuit for reference.

In my design, at chip reset, the PNP transistor sets the pin to around 2.65V to configure it, and afterward, this pin drives the dual LED as an output.

Do you see any potential issues with this circuit design? Are there any ways to improve it?

Thanks.

Answer 1

I'm not sure why you'd use a 150V transistor to switch an LED in a 3.3V circuit.. the gain characteristics are inferior to lower voltage common transistors.

I think the pin voltage is likely not going to meet the 2.0V minimum spec, and it may well be unreliable, especially at temperature extremes when the red LED drops less voltage. The red LED drops around 1.8V at low current at room temperature (see the datasheet) and that will pull down the pin somewhat below 2.0V nominally.

Needless to say, this is much worse than it not working at all.

Maybe something like this:

^{simulate this circuit – Schematic created using CircuitLab}