Dear Sirs.,
I'm asking for support for the TPS23754 IC that we are trying to implement in one of our designs.
We are implementing a solution to get 24VDC from POE+ (802.3at) 30W to our design, that is an Audio Amplifier with IP capabilities to receive audio streams.
We work with the Evaluation Board TPS23754EVM-384 at the start to check the IC functionality. Due to this board is designed to get 12VDC/2.5A we modify a bit:
- Change output capacitors C11 and C12 with 35V voltage
- Change D9 and D10 to 24V version
- Change R23 to 90.9k to get Vout 23.2V
- T1 transformer was modified changing from parallel to series the coils at the secondary side.
All this changes were tested and worked well, connecting our audio amplifier board as a load of the evaluation board. Only a bit of heat on the Mosfets of secondary side but not a huge problem.
After validating this design we went to implement the new layout design integrating the POE+ with the amplifier and IP board following layout guidelines by TI.
And here is where we are having the problems. The power supply didn't start. All the POE start-up Sequence seems correct but when the mosfets start to switch they power off after a 2ms (sounds like ZZZZ), and is trying again a few times, after that, a small wait and again the same. The output can't get the 24V, only 9-10V and after 2ms the output voltage goes down.
It will be helpful any tip to work in the correct direction, because we spent to many time now trying to solve this.
We don't know if it's due to limit current, incorrect configuration of some parameter, Layout design...
For info, the most important differences with the EVAL board/24V are:
- we have not implement the O-ring part. We attach PPD and APD pins to VSS and GND respectively as datasheet indicates.
- The T1 transformer is custom made according to the spec. and to get the 24V/1.25A at secondary side.
- RJ45 Is a different model, also compatible with POE+ (PULSE JK0-0177NL).
I attach the schematic of the final design of the POE+ part, some osc. captures of drain mosfets and auxiliary output to supply Vc.
Can include the layout design if will necessary or take more osc. captures or whatever you need.
Best regards and thank you in advance,
