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Original question:

XTR300: Power sequencing for negative supply.

LM27761: Startup inrush current

Erika Silva
Prodigy 10 points
Part Number: LM27761

Tool/software:

Hi,

I'm looking for some clarification on the startup characteristics of the LM27761. We are using this component in a design powered by a current restricted 5V5 supply (limited to about 40mA).

Do you have information on the scale and duration of the inrush current for the LM27761 when power is initially applied? And can this behavior be controlled by adjusting timing around the power supply enable?

Thank you,

Erika

  • 0
    TI__Genius 17630 points

    Hello Erika,

    thanks for using E2E.

    The inrush current duration depends on the desired output voltage, the applied load and the used capacitors. A smaller output cap, lower output voltage and without applied load will result in a faster power up duration. You could use the LM27761 PSpice Transient Model in the free PSpice for TI simulation tool to simulate your startup conditions and to get a rough feeling for the inrush current. There is also an EVM available for device evaluation.

    But I'm afraid that the 40mA input current will be exceeded during the first phase of start-up (at least for ca. 300us).

    You could try to add a big enough bulk capacitance at the input to buffer the inrush current, but this cap also needs to get charged at some point in time.

    I think a slow ramp up of the power supply will not help to fully control the inrush current and it can be problematic for proper LM27761 startup.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    Prodigy 10 points

    Hi Andreas, this is helpful context. Our 40mA current restriction is due to thermal limitations, so if the first phase of startup is limited to 300us, we may be able to handle the brief overload. I'll continue with the analysis you recommended.

    Thanks,

    Erika

  • 0
    TI__Genius 17630 points

    Hello Erika,

    thanks for your feedback.

    Let me know if you have further questions.

    Best regards,

    Andreas.