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LM3409HV: short circuit performance

Brian Angiel
Expert 4070 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

I'm using LM3409HV in an application where it is connected to a power supply that provides 48VDC and is capable or sourcing 100's of amps. In the event of a short circuit, my current short-circuit response circuit can take 10-100ms to shut the device down.

I will be running various short circuit tests, but can you help me understand what to expect from LM3409HV in those 10-100ms while the short is present and before the device is shut down?

Thanks,

Brian

  • 0
    TI__Mastermind 28655 points

    Hello Brian,

    The LM3409 regulates peak current so it should be fine unless the on-time requirement becomes less than 211 ns.  A true short, output voltage below 1.24V, will keep the off-timer from charging to reset the turn-on.  This defaults to 300 us, maximum off-time.  If your output can be a partial short still allowing the off-time to reset then a few calculations need to be made to determine if the output current can walk up due to minimum on-time.  High input voltage and high switching frequency are two things that make this more likely.  The inductor value and saturation current also come into play for how long it takes to rise as well as saturate the inductor.  There are things that can be done to prevent this if it is a problem.

    What is your output voltage, switching frequency, Roff and Coff values, inductor value and anything else of importance?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Expert 4070 points

    Thanks, Irwin. What about the front end--if the input supply is shorted to ground? Is there anything in the device to prevent it from frying if all that current starts rushing into the device before the shut-off circuit kicks in?

  • 0 in reply to Brian Angiel
    TI__Mastermind 28655 points

    Hello Brian,

    If the input supply is shorted to ground it will just shut down.  I would assume the input supply shorted to ground puts the current in the short.  Maybe you are asking a different question?

    If you mean the LM3409 input/power stage fails shorted it probably will fry with 100's of amps sourcing the LM3409 and power stage.

    Best Regards,