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TPS737: ENABLE PIN - VIN PROBLEM. SO STRANGE AND DIFFUCULT TO FINDOUT.

Sezgin YAGBASAN
Prodigy 20 points
Part Number: TPS737

Hello,

I desgined TPS73733DCQR like before. But this time LAN8720A ic can not work well. The board have 4 layer. 1st layer is GND plane (all components is in 1st layer),  4th layer 3.3V power plane which is regulater with TPS73733.

TPS73733 have Enable pin which is enabling via MCU. MCU gets its power from another 3.3V LDO.

the problem LAN8720A can not  work with these conditions. 

1. if I give VIN 3.6V or higer voltage, Vout is 3.3V, it is ok but LAN8720 is Does not work.

2. If I connect EN pin to Vin, every thing is OK.

3. If I give VIN 3.5v or 3.3V every thing OK. 

Board Current Consuption is about 175mA. 

So what is Wrong??

TPS73733 is limitting the Current in 200mA?? or What is Wrong with enable pin?? 

Please help.

  • 0
    TI__Genius 13265 points
    Hi Sezgin,
    Can you explain further how the LDO appears to not work correctly? Does the LDO give the correct output voltage when enable is high or is the output voltage incorrect? What is the incorrect output voltage, if so?

    It looks like the LAN8720A has multiple power rails - it could be possible this is a power sequencing problem with the LAN8720A device. Have you changed the order at which the power rails are rising/falling?

    Regards,
    Mike
  • 0 in reply to Michael Hartshorne
    Prodigy 20 points

    Hi Michael,

    LDO gives always correct output voltage. LAN8720A and other ICs gets its power from TPS73733. only one MCU (STM8) have different power source that it enables TPS73733.

    IF I connect ENABLE to VIN via external jumper, LAN8720A and other STM32 MCU and other ICs works fine. (The VIN voltage 4.2V)

    IF I remove the VIN jumper from ENABLE pin, LAN8720A not works correctly, namely not connect the LAN DHCP. This case enable pin high via STM8 mcu and it is 3.3V.

    IF I reduce VIN to 3.3V with STM8 Enable, LAN8720A is works fine! 

    I thing the reason about CURRENT. Something must limiting the LAN8720A current. 

    TPS73733 output voltage allways stable, I monitored with Oscilloscope. 

    Waiting your comments.

    Regards,

    Sezgin YAGBASAN