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LM74720-Q1: System quiescent current estimation

Kazuki Kuramochi
Genius 10485 points
Part Number: LM74720-Q1

Tool/software:

Hello Expert,

I'd like to confirm about the estimation method of system quiescent current.

As far as I confirmed at E2E, the meaning of Iq(Total System Quiescent Current) is  GND pin to GND current during EN=2V.

On the other hands, following current won't be appear at GND pin of IC but it will be sink from VBATT.

  • I(PD_SRC): It is source current to FET gate so it will be sink from VBATT but it doesn't goes to GND pin.
  • Boost efficiency loss: It will be converted to heat generation so it will be sink from VBATT but it doesn't goes to GND pin.

Therefore, I think we cannot directly use Iq to System quiescent current estimation(Sink current at VBATT).
Then, could you please tell me how to estimate System quiescent current when we use LM74720-Q1?
Also, if there is any wrong understanding, please let us know.

Best regards,
Kazuki Kuramochi

  • 0
    TI__Mastermind 46390 points

    Hi Kazuki-san,

    System Iq is measured during steady state that is when PD_SRC has already been supplied, and all the FETs are turned-on. Yes, the boost will still keep operating and all of those factors have been accounted to calculate the mentioned Iq in the datasheet.

    This Iq is the average value and not an instantaneous value which means everything is accounted.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    TI__Genius 10485 points

    Hi Shiven-san,


    I want to make sure about your answer.

    At following E2E thread, Praveen-san said customer should measure current flow beween IC GND pin and GND.

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1232803/lm74720-q1-iq-test-for-lm74720?tisearch=e2e-sitesearch&keymatch=LM74720-Q1%20Iq#

    I think that this current flow don't contain loss current from Boost converter efficiency, source current from PD and etc...
    This is because Boost efficiency will be converter from input current to heat generation and source current of PD will through from input to PD pin.

    However, you said "the boost will still keep operating and all of those factors have been accounted to calculate the mentioned Iq in the datasheet".


    Then, could I confirm the definition of Iq "Total System Quiescent Current" again?


    I suppose that source current from PD pin in actual application should be less than 1uA.
    This is because, PD maximum supply current is 50uA but actual sink current from FET gate should be very low(less than 1uA...).

    So I suppose that to estimate system Iq, I think Iq on datasheet is enough.
    However, I want to make sure about whether my understanding is correct or not.

    Best regards,
    Kazuki Kuramochi

  • +1 in reply to Kazuki Kuramochi
    TI__Mastermind 46390 points

    Hi Kazuki,

    The Iq in the datasheet is an average value. The boost current and PD current is instantaneous. 

    The boost circuit will work in a hysteresis like this and only consumes current in rising edge. This helps in saving Iq. 

    Same is true for PD pin, once it is fully enhanced, it doesnt consume current.

    Hence the Iq mentioned in datasheet is enough. Definition still hold true that Iq is the current flowing out of GND pin.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    TI__Genius 10485 points

    Hi Shiven-san,


    I understand that boost converter and I(PD_SRC) want cause significant impact for Iq if LM74720-Q1 in stable operation.

    Best regards,
    Kazuki Kuramochi