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LM5050 - can't get the FET to turn off

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: LM5069

Hello,

We are doing some breadboarding with the LM5050 for an ORing application.  We wired it up with back to back FET's as we need to shut off the input from the load and prevent a second supply from back-feeding the input.  However, everything we've tried to turn the GATE off, just doesn't work.  We have 36V coming in, and when it is in the on state, the voltage on the GATE is around 40V, and in the off state, the GATE is still at 36V.  We'd expect it to go to zero.  

We bought an EVM to see if we had done something wrong.  We are seeing the same exact measurements with the EVM.  In the off state, the GATE is still at 36V.  We have applied a load to both circuits, but that doesn't change it.  On the EVM, we don't have back to back FET's, as a note.  

Is this expected behavior?  Are we doing something wrong?

Thanks for your help

  • 0
    TI__Mastermind 31580 points

    Hi Jamal

    The LM5050 is support by another team on the POE and ORing fet forum, I will move the post and get one of apps engineers from that team to respond.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    TI__Genius 12621 points

    Hi Jamal,

    The LM5050 unforunately cannot be used with back-back FETs. The reason is because:

    1. There is an enternal zener diode clamp from IN to GATE. So charge will make its way to the GATE and keep the MOSFETs on.

    2. The LM5050 pulls GATE to IN rather than to ground.

    If you need a circuit to shut off the MOSFET, I would recommend a hot swap controller instead.

    To understand the purpose and benefit of the LM5050's OFF pin, check out this brief E2E thread which also discusses an LM5050 + LM5069 design which adds several protection features:

    Thanks!

    Alex

  • 0 in reply to Alex Triano
    Genius 3245 points
    Hi Alex,

    Appreciate the response. There are multiple e2e posts saying the LM5050 can support back to back FET's, which took us down the wrong path.
    Here is one such example...
    e2e.ti.com/.../297508

    Would your recommend the TPS24xx family instead?

    Thanks
  • 0 in reply to Jamaal_Charles
    TI__Genius 12621 points
    Thanks for spiking that out Jamaal.

    We understand there are existing threads on this subject and we need to clean up any lingering misinformation asap.

    In the meantime, the TPS24xx cannot be used for 36V applications, only 12V ones due to its VIN range.

    Given the application requirements mentioned earlier (must shut off reverse current to prevent backfeeding, and needs to disconnect input from load), we can only recommend a 2x IC solution.

    Either the LM5050 + LM5069 design linked earlier, or the LM5069 + an OPAMP as discussed in this application note:

    www.ti.com/.../snva673.pdf

    Thanks,
    Alex