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LM3429BKBSTEVAL: OVLO: Shutdown or Regulate?

Samuel Jaffe
Mastermind 25590 points
Part Number: LM3429BKBSTEVAL

Hi E2E,

My customer is using the LM3429BKBSTEVAL with Rov2=332k and Rov1=5620 to achieve a voltage of about 74V and a hysteresis of 6.6V.

If the voltage rises above 74V, does the IC regulate the LED current to maintain 74V or does the IC shut down?  Their tests show the IC trying to regulate the current to maintain the output voltage of 74V but the datasheet (pg 18) says the GATE pin is immediately pulled low.  Which is it?

Thanks,

-Sam

  • 0
    TI__Mastermind 25500 points

    Hello Sam,

    When the output voltage reaches 74V it will stop switching until the output discharges to 74V minus the 6.6V hysteresis, so it will start switching again when it falls to 67.4V. It will continue to do so until the open circuit is corrected or the device is shut down. So the average output voltage while open will actually hover around the middle, or just a bit above 70V. Does that answer your question or did I misread?

    Thanks,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Mastermind 25590 points
    Clint,

    Yes, that's what we expect to happen. Thank you for the confirmation. It seems, however, that the customer is seeing something different - namely that it appears to regulate to 74V. I've asked for more info on test setup and measurement. I'll update when I receive more info.

    Thanks,
    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25590 points
    Clint,

    The customer is using a resistive load. Could this be causing what we're seeing?

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25500 points

    Hello Sam,

    You mean they are increasing the resistance until they hit OVP? If so yes, that would affect it some. The operation would still be the same except the output voltage sawtooth waveform would be at a higher frequency than if there were an open at the output. Can you probe the OVP pin to see if it is functioning correctly? Does it actually hit the threshold and just stay there? What is your input voltage? I'm wondering if something is just breaking down at that level. The voltage the diode and FET will see is Vin + Vout. Are they rated high enough? Can you tell me exactly what modifications they have made to the EVM?

    Thanks,

    Clint