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LM3488Q-Q1: SEPIC issues : output voltage and switching frequency unexpected under certain conditions

Gaetan Andrieu
Prodigy 20 points
Part Number: LM3488Q-Q1
Other Parts Discussed in Thread: LM3488

Hi!

I designed a SEPIC converter based on the LM3488Q-Q1 chip but I have an issue on my SEPIC : when the input voltage is lower than Vout_order and Rload is high (a few hundreds of ohms), frequency’s gate oscillation is around the nominal frequency (~200kHz) but when Rload is low (<100 ohms), frequency's gate oscillation is around few dozen of kHz and voltage output can’t reach nominal order value (5V). When input voltage is higher than Vout_order, the frequency transistor's gate oscillation is independent of the charge output on SEPIC works perfectly as expetced.

In short : when my output load is too low and my input voltage is lower than Vout_order, it's impossible to reach the final output voltage and I remark that the frequency fixed by Rfadj is not respected in that condition too.

I can give more details on my test procedure, schematics and captured waveforms in private if necessary for understand and resolve my issue.

Thank in advance for your help,

Gaetan

  • 0
    TI__Genius 13095 points

    Hi Gaetan,

    We will look into this and come back to you soon.

    Best Regards,

    Feng Ji

  • 0
    TI__Mastermind 21040 points

    Hi Gaetan,

    Thanks for using the e2e problem.
    Just for my understanding, you are using LM3488 in SEPIC topology, but the output voltage cannot be supply when operating with low Vin, correct?

    Could you give the values for 
    - Input voltage range
    - Output voltage (5V?)
    - maximum load current

    Also, there might be a misunderstanding with the load conditions. You mentioned you have "high load" at a few hundreds of ohms, and "low load" at <100 ohms.
    With Ohm law ( I = V / R ), the load is higher the smaller the load resistor becomes.
    E.g., if you have a 50 Ohm resistor at Vout of 5V, there will be 100mA load. At 500 Ohm resistance, there will be 10mA load.
    Can you clarify once more at what loads the issue appears?

    A schematic of your design would be helpful as well. I only the circuit around the LM3488 and the power stage. You do not have to share the full application.
    If you cannot share this public, feel free to send it to me via a private message in the forum.

    Thank you and best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 20 points

    Hi Niklas,

    Yes, this is the main issue.

    Load = Pure resistor (in this case) and it's inversely proportional to the current drawn by the load.

    I give you in private message all informations you need to understand my issue with a test report with schematics and captured waveforms.

    Thank you Niklas,

    Gaetan

  • 0 in reply to Gaetan Andrieu
    TI__Mastermind 21040 points

    Hi Gaetan,

    Thanks for providing me more data.
    I received your private message and will review your design and waveforms.

    I will get back to you with my insights within the next two days.

    Thanks and best regards,
    Niklas