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LMZ31704: LMZ31704

TAN SB
Intellectual 400 points
Part Number: LMZ31704
Other Parts Discussed in Thread: LMZ31710, LMZ31707

Hi,

For LMZ31704 the max Output Current is spec at 4A,  the Current Limit Threshold is spec at 9A if the ILIM left open, does that mean the part can be loaded up to 9A before the OCP function kicks in?

Thanks,

SB

  • 0
    TI__Genius 17905 points

    Hello again SB,

    The peak current limit will be 9A, but that does not translate to the load current directly because there will be inductor ripple (a function of Vin, Vout, switching frequency) on top of the load current.

    Kris

  • 0 in reply to Kris F
    Intellectual 400 points

    Hi Kris,

    If I tied the ILIM pin to AGND to lower the Current Threshold to 6A, will it lower the max Output Current?

    I am trying to determine an appropriate current rating for a resettable fuse that we plan to add to PVIN. Which current spec is more appropriate, Iout or Ilim?

    Thanks,

    SB

  • 0 in reply to TAN SB
    TI__Genius 17905 points

    Yes, lowering the current limit threshold will lower the max output current the device will be able to output before tripping OCP.

    If you want the fuse to trip to be a redundant protection to the device's internal current protection, then you would want the fuse rating to be higher than the device's current limit such that it will not trip before the LMZ31704's current protection trips.

  • 0 in reply to Kris F
    Intellectual 400 points

    Hi Kris,

    The datasheet mentioned that connecting ILIM pin to AGND will reduce the Current Limit Threshold by approximately 3A, does that mean the IOUT will be limited to 1A, in the case of LMZ31704?

    Is there a chart or a formula (for LMZ31704, LMZ31707 and LMZ31710) that I can use to correlate the IOUT and Current Limit Threshold?

    Thanks,

    SB

  • 0 in reply to TAN SB
    TI__Genius 17905 points

    Hi SB,

    Please help me understand what you are trying to achieve. Are you using LMZ31704 well below 4A rated output current?

    The max Iout (solely based on current limit) will be approximately current limit Ilim minus half the inductor ripple Ipp:

    max Iout = Ilim - Ipp/2                  where Ipp = (Vin-Vout)/L * Duty / Fsw.

    For this calculation, you can use the basic Duty Cycle = Vout/Vin, and the inductor value is L=1uH.

    For a design with Vin = 12V, Vout = 3.3V, Fsw = 750kHz, and tying ILIM pin low to set the current limit to 6A, the calculated Ipp = 3.19A, and max Iout is 4.4A.

    As you can see from the equation above, a lower frequency will increase the inductor ripple (since inductor is fixed) and reduce the max output current allowed before tripping current limit.

    I hope that helps...