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TPS2375: Large voltage drop accros PD controller and low signature resistance

Cristian Monea
Expert 1795 points
Part Number: TPS2375


Hello! I designed a PoE 802.3af solution using:
- on PSE side: Silvertel AG7100 as a 48V isolated source, LTC4263 as PSE controller, regular Ethernet jack;
- on PD side: TPS2375PW as PD controller (class 1 PoE), MAX5033CASA as DC-DC converer to provide 12V/500mA, WE 7499211121A PoE jack.

Below are my schematics.

I have two problems with the PD device:


1. The voltage before TPS2375PW is 46.8V (between POE_48V and POE_GND), but the problem is that after the PD controller the voltage is 30V (between POE_48V and GND). Why is this voltage drop happening? Is it normal? Is there something wrong in my schematic?


2. The PSE controller provides power only if I disconnect C7. With C7 connected it alerts me using LED flashes that it sees a low signature resistance (which means it is between 300ohm and 15kohm). Does the value of C7 disturb TPS2375PW?

 

  • 0
    TI__Genius 9230 points
    Hi Cristian,

    I have notified our PD experty and will get back to you, shortly.

    Thanks,
    Thomas A.
  • 0 in reply to Thomas Amlee
    Expert 1795 points

    I connected the PG output of the PD chip to the ON/!OFF input of the converter and the converter generates 12V, but the problem is that the voltage does not maintain. The statement is valid when I remove the DC converter input capacitor; I tried a 22uF, 47uF, 22uF, 100uF capacitor, but every time I connect the capacitor, the PSE (LTC4263) does not supply power, instead it indicates low signature resistance by flashing the LED with a specific pattern.

    These are my oscilloscope captures:

    1. Voltage across D1 diode:

    2. PG output:

    Is the problem related with the PD circuit? How can it be resolved? What capacitor should I use?

  • 0 in reply to Cristian Monea
    Expert 1795 points
    I've tested the PD module with a commercial PSE and it worked. It seems that the problem is related with my PSE.