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LM3488: Average output current limiting

Zheng Hao Chin
Prodigy 50 points
Part Number: LM3488
Other Parts Discussed in Thread: LM5155

Tool/software:

I am hoping to use the LM3488 in a boost converter for charging my batteries at 30V. Unfortunately, I only have a source that can output 20V at 3A, and the battery might draw more than that when charging. I understand that the LM3488 has a current limit feature. However, it seems like this current limit is for the maximum switched current, and not for the average current output to the load.

My questions are hence the following:

1) Is my understanding of the current limit feature correct? Or is the current limit actually for the average current output to the load.

2) If my understanding is correct, is there a part that allows me to set an output current limit, with the over current behavior being that the converter clamps the current draw, rather than shutting down.

Thank you!

  • 0
    TI__Mastermind 30640 points

    Hi Zheng,

    Thanks for using the e2e forum.
    Your understanding of the current limit feature is correct. It only regulated the peak inductor current, not the average current.
    It is also meant as a protection feature which is not intended for regulating the current.

    In theory, you could calculate the expected inductor ripple current and set the device current limit accordingly, but it is possible that the regulation will not be stable, as the feedback regulation loop gets overwritten by the current protection feature.
    Also you cannot change the softstart time with LM3488, so the startup comes with additional problems of high inrush currents.
    Therefore, this approach is not recommended.

    The best solution should be adding an external regulation loop to regulate constant current at the output and feed this loop into the COMP pin.
    You can also consider the LM5155 device for this, which is more modern and has an adjustable softstart time to handle inrush current problems during startup.
    https://www.ti.com/product/LM5155

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

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 50 points

    The LM5155 also does current limiting in the same manner (peak inductor current limiting) as the LM3488 right? That’s my understanding from the data sheet but do correct me if I am wrong. 

  • 0 in reply to Zheng Hao Chin
    TI__Mastermind 30640 points

    Hi Zheng,

    Exactly, LM5155 uses the same peak current limiter as LM3488.
    The external loop should work with LM3488 as well, I just recommended LM5155 as this device is more modern and comes with improved additional features such as the external softstart previously mentioned.

    Best regards,
    Niklas