Hi,
I'm using the following devices as a PoE system:
- LT4321IUF#PBF: PoE Ideal Diode Bridge Controller
- LT4294IDD#PBF: IEEE 802.3at PD Interface Controller
- LT8645SEV#PBF: 8A, 65V Synchronous Step-Down Regulator
And I have a problem with the behavior of T2P signal.
When only(with no AUX) the PoE is connected to the system, everything works as it should, you can see in the following pictures the negotiation between the PSE and the PD (it is PoE++ Type 4 Class 8 - 71.3W):
- The yellow signal is OUTP voltage (input) and the green signal is T2P.
- You can see that after stabilization the T2P shows a duty cycle of 25% as written in the datasheet.
The problem appears when I connect an external power supply 15V, which is connected to the AUX pin on the LT4294, I don't get the desired T2P signal.
- The yellow signal is PWRGD on LT4294 and the green signal is T2P, you can see in picture that T2P does not behave as expected, in addition to that, it seems that there are iterations of negotiations that cause cyclical behavior of the T2P:
- Closer look of PWRGD and T2P:
- The yellow signal is OUTP voltage that seems like trying to start negotiation and the green signal is T2P:
-T2P expected behavior for AUX connection:
Im adding a picture of the design, hope it can help:
There are 2 PUs on T2P and PWRGD just for an option, only one of them assembled.
I would like to know what you think can cause the issue and is it normal function?
I hope I was able to illustrate the problem, I will be happy to provide more information if needed.
Thanks,
Daniel
