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LM25118: How lm25118 shifts from buck to buck -boost?

Matthew Chen
Intellectual 1340 points
Part Number: LM25118

Hi team,

For lm25118, there are two mode, buck and buck-boost. In what condition it will shift from buck to buck -boost? Does it related to the load current?

In the datasheet, it simply explain that it will shift to buck-boost when D = 75%

In a circuit where Vout should be12V/3A , when Vin = 15V , the output is about 10.8V/3A (stability with load ), the HO duty cycle is about 80%; Lo is low , and the Rt=12k; the Vout will decrease from 12V/3A to10.8V/3A when Vin from 18V down to 14V. What's the reason for that?

Thanks a lot.

Matthew Chen

  • 0
    TI__Guru 53451 points

    Hi Matthew,

    no the load current does not have an impact on the duty cycle and also on the shift from buck to  buck-boost.

    May you can share the schematic for an review.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Intellectual 1340 points

    Hi Stefan,

    1. From the datasheet, I see when the duty circle of Buck reaches to 75%, than the boost duty circle begins to increase, is D(buck)>75% the only requirement for the chip to shift to buck-boost? If not, what are the detail conditions that the chip will shift from Buck to Buck-mode? 

    2. In figure, it's obviously that the shift point of the Duty is much higher the 75%, what the reason for that?

    Thanks a lot

    Matthew Chen

  • +1 in reply to Matthew Chen
    TI__Guru 53451 points

    HI Matthew,

    as this is not a parameter from the parameter section this is not be specified accurately.

    So you might read the text more that way: Once the duty cycle reaches ~ 75%, the ...

    From the graph above i would say this was 78% for that specific device which was used for the measurement.

    But as this is depending on Device, Temperature, .... this can also shift.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Guru 53451 points

    Hi Matthew,

    I have not seen an update on this thread for the last 15 days.

    If the issue is solved it would be great to press the is solved button.

    If there is still something open, just let me know with the new details.

    Best regards,

     Stefan