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LM25118: Problem in buck-boost mode.

Roma Zhukov
Prodigy 40 points
Part Number: LM25118

Hello! I designed buck-boost converter on LM25118 IC with parameters: Vin=16..24V, Vout=12...24V, Iout=3A.

If Vin>Vout, Iout=2A (Buck mode) converter works fine.

If Vin<=Vout, Iout=2A (Boost and Buck-Boost modes) converter only works in buck mode. I have Vout=18V with Vin=24V. If there is no load Vout = 24V.

I tried change values of most components, but no result. 

Can I solve this problem? Thanks. 

  • 0
    Prodigy 40 points

    Help, please. I need ship pcb to client.

  • 0 in reply to Roma Zhukov
    TI__Guru 63025 points

    Hello Roma,

    Please check if the behavior changes when you connect R55 to FB (short pins 1 and 2 on MAX5161).

    Please check the behavior with R46 removed.

    You mention that the output voltage is 24V when the input voltage is smaller than the output voltage, but only when there is no load. How exactly does the output voltage change when you slowly apply more load?

    Scope plots of the output voltage, the input voltage, the switch signals, the COMP signal as well as the RAMP signal are helpful, if the above suggestions do not help.

  • 0 in reply to Brigitte
    TI__Guru 63025 points

    Hello Roma,

    As there is no more information from you, I expect that this issue is solved and I will close this thread. If this is not the case, please add another post below or create a new thread if this one is locked already.