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LM3489: LM3489 100% duty cycle operation with TPS65988

Max Guan
Intellectual 1350 points
Part Number: LM3489
Other Parts Discussed in Thread: TPS65988

Hi Team,

My customer reported an issue to me that follow the TPS65988 EVM schematic design they find the LM3489 did not work in 100% duty cycle which lead to larger voltage drop in case a 20V PDO is negotiated. 

Would you please review the schematic and help to suggest whetherwe could implement the 100% requirement?

Thanks for your help.

Max

  • 0
    TI__Mastermind 22299 points

    Hi Max,

    Reviewing your schematic, it looks like the output capacitors are ceramic.

    Hysteretic converters like the LM3489 need output capacitor ESR for proper stable operation.

    As mentioned in section 8.2.2.2 of the datasheet, ceramic capacitors have voltage 90 degrees out-of-phase and will result in lower switching frequency and increased output ripple.

    If you're using ceramics then you need to introduce a series resistor to generate ESR. Alternatively you can use tantalum or aluminum capacitors instead of ceramics.

    This should help your design the most. Afterwards you can adjust the feedback resistors to get the correct VOUT.

    Hope this helps, let me know if you have further questions.

    -Orlando