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LM2733 Schottky diode voltage recommendation

Pial
Expert 7280 points
Other Parts Discussed in Thread: LM2733

Dear team,

Could you please help me figuring out if a 40V schottky diode is enough for a LM2733 boost 28Vin -> 37Vout to 39Vout ?

From the datasheet, it seems that it's ok, but the EVM app note mention a 5V margin ? (  paragraph 10)

Also, on a side note, I am wondering how much margin we should take regarding the internal mosfet (40V abs max) ringing ?

Datasheet plots a 40Vout efficiency, does this mean that the mosfet is actually capable of supporting ~45/~50V ringing value ?

Thanks a lot for your help!

All the best,

PA

  • 0
    TI__Expert 6860 points
    Hi Pial ,

    THe maximum input voltage allowed on LM2733 is 14V so i am hoping that you would be using a different supply to bias the input of LM2733 for 28V to 39V boost .

    For the diode use any 45V(Like BYS10-45 etc) or higher rated diode . Further when you are designing for higher Vout , special attention is needed to be paid on minimizing high di/dt loop to avoid high ringing . If you still see higher ringing , you may use snubber to dampen it out .
    I have designed LM2733 with 40V out(LED driver application) and have not seen any issue .

    Can you please let us know the specification of the output current and application you intend to use this device .

    --Ambreeesh
  • 0 in reply to Ambreesh Tripathi
    TI__Expert 7280 points

    Hello Ambreesh,

    Thank you for your fast answer.

    There is a 3.3Vbias on the design.

    This is for a constant voltage 36~39Vout, 60mA design.

    Today, a 40V diode (MBR140P) is selected and there is some failure in field. How do you explain it ? Following numerous simulations (transient, resistors tolerance etc.), it seems the output is never going higher than 39V.

    Thanks again!

    Best regards,

    PA

  • 0 in reply to Pial
    TI__Expert 6860 points

    Hi Paul,

    When you say field failure , what exactly is the failure . Do you see the device failure ? Is their short at SW to GND in the failed part ? Also you may want to see the actual SW node waveform in the PCB and verify for no  excessive ringing which may be the cause of the failure .

    ----Ambreesh

  • 0 in reply to Ambreesh Tripathi
    TI__Expert 7280 points

    Hello Ambreesh,

    In fact this is not a device failure but the 40V diode failure, but I'd like to understand why since the output voltage never goes above 40V..

    Thanks!

    Best regards,

    PA

  • 0 in reply to Pial
    TI__Intellectual 2225 points
    Hi Pial,

    I think that having a 40V rated diode when the output voltage is 39V is too close and does not leave enough margin. I would recommend trying with a 65V or higher rated diode and seeing if the failures still occur. Switch node ringing and other parasitics can cause the voltage to easily exceed 40V for a very short time.

    Perry