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LM74670-Q1: Low Current Solutions?

Daniel Hirsh
Prodigy 30 points
Part Number: LM74670-Q1


I'm looking to use the LM74670-Q1 in a very low current use case (300 nA - 1 mA).

The problem is that the intrinsic body diode in MOSFETs doesn't generate the minimum 0.48V forward voltage that the LM74670-Q1 requires when currents are that low (or just barely at 25C, but not when it gets warmer).

Is there a solution for low current cases such as mine?  

For example, can the LM74670-Q1 work with IGBTs?  Since IGBTs don't have intrinsic body diodes, it might be possible to add a discrete diode with more suitable properties.  And/or is there a better option?  

Thank you.

  • 0
    TI__Genius 16305 points

    Hi Daniel,

    For very low currents <1mA, using a schottky diode would be more beneficial than LM74670-Q1.

    Power dissipation on diode would be very less and this would be a very cost effective solution.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 30 points

    Thank you, Kari.  However:

    1.  A quick calculation of a sample NMOS shows a power dissipation of 68 uW (including leakage current) compared to 300 uW for a sample Schottky diode, so the NMOS combined with the LM74670-Q1 still provides the lower power dissipation.

    2.  While ultra low power is important, the attempted use of the LM74670-Q1 is to recover voltage headroom.  Having a voltage drop of 15 mV compared to 280 mV is a very big deal for this application.

    So I'm still interested in knowing whether there is a way to get the LM74670-Q1 to work. 

    Is my idea to use an IGBT with an external diode feasible?  Is there a better way?

  • 0 in reply to Daniel Hirsh
    TI__Genius 16305 points

    Hi Daniel,

    Can you please share the operating voltage range at the input of LM74670-Q1, this will help to respond better.

    Note that, the IC itself requires few uA of current that must be drawn by the load for the IC to work properly.

    nA is definetly will be an issue, may not have enough charge pump strength.

    Regards

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 30 points

    Roughly 2.5 - 4 V.

    The nA figure I gave above has to do with the load, not what the supply can provide.  Pulling current to charge the capacitor for the charge pump is fine.

    As I stated in my initial message, the problem is that the intrinsic body diode in MOSFETs can't generate enough forward voltage to trigger the charge pump.

    That's why I was asking whether using an IGBT with an external diode was feasible.  Although if you know of a better solution, I'm open to ideas.

  • 0 in reply to Daniel Hirsh
    TI__Genius 16305 points

    Hi

    In this IC, charge pump current has to flow through load and then return back to supply on the ground path of the load.

    That is the reason for LM74670 not  able to work in this scenario.

    IGBT may be able to generate enough drop, but still the above problem remains.

    Regards,

    Kari.