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LM5122-Q1: Issue with LM5122 2-Phase Boost Converter: Input Dropping to 5V, Output Retains Charge

Vinod Justin
Prodigy 105 points
Part Number: LM5122-Q1
Other Parts Discussed in Thread: LM5122

Tool/software:

Hi Team,

I’ve designed a 2-phase interleaved boost converter using the LM5122QMHX/NOPB.
Input: 12V
Output: 36V @ 6A to 8A (no load during testing)

I'm facing a strange issue during power-up on one of the boards:

  • When I apply 12V to the input, the voltage immediately drops to ~5V.

  • The board starts sinking current even with no load connected.

  • I initially suspected a short, but it seems transient — the short exists for a brief time during startup, then disappears.

  • After disconnecting the power, I still see 12V across the output bulk capacitor, indicating the output is charged and holding.

  • I tried discharging the output cap before the next power cycle, but the issue persists.

  • This behavior is repeatable — each power-up results in the same symptoms.

Has anyone experienced similar symptoms with the LM5122? Could this be related to soft-start, UVLO settings, or a damaged FET?

Any guidance or debug suggestions would be appreciated.

Thanks in advance!

  • 0
    TI__Mastermind 30390 points

    Hi Vinod,

    Thanks for using the e2e forum.
    All the point mentioned make it sound like you have large output bulk capacitors.
    A boost topology has a direct connection from input supply to output through the inductor and body diode of the high side MOSFET.
    This means if you connect the 12V power supply, both input and output capacitors are charged up to this 12V level, even if the controller itself is still in shutdown mode.
    This creates a large inrush current spike.

    This behavior is expected for a boost topology and cannot be avoided with the LM5122.
    To avoid this, you would either need to ramp up the power supply slowly, so the caps are charging slowly as well, or integrate a current limiter externally.

    Please let me know if you have additional questions on this.

    Best regards,
    Niklas