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Original question:

LM3478: Minimum load

LM3488: LM3488 SEPIC - Minimum load/skipping pulse

Bruno Andrade
Intellectual 520 points
Part Number: LM3488
Other Parts Discussed in Thread: LM5118, LM25118, LM5122, TPS61088, TPS54302

Hi,

This is my circuit (I have another one reg. set to 3.8V/2.5A having the same problem):

I'm having the pulse-skipping behavior when no load or a low load:

The input voltage will vary from 3-25v all the time.

To design the regulator, I've used the AN-1484(https://www.ti.com/lit/an/snva168e/snva168e.pdf) step-by-step. I don't know if I missed something, but I did not find a way to calculate the minimum load for the given component values.

What is the exact formula to make sure my circuit do not enter in pulse-skipping mode down to 1mA in any condition of input voltage?

Thank you!

  • 0
    Intellectual 520 points

    The images were not showing, corrected.

  • 0 in reply to Bruno Andrade
    TI__Guru 61135 points

    Hello Bruno,

    There is a minimum energy delivered to the output in each switching cycle. The diode does not allow negative current, so if the output voltage is getting higher than the target level, the device has to stop switching.

    If you need a converter to switch down to almost no load current, you need to use either a dummy load or synchronous conversion to allow negative current from the output to the input.

  • 0 in reply to Brigitte
    Intellectual 520 points

    Hi Brigitte, thank you for your response.

    Before moving to a synchronous part, I would like to know if there is a formula that dictates the relationship between the output load and the required on-time for a given frequency in the SEPIC configuration. Perhaps another part which has a lower minimum on-time could be sufficient for me. Would you help me?

    Best Regards,

  • 0 in reply to Bruno Andrade
    TI__Guru 61135 points

    Hello Bruno,

    A possibility would be to reduce the frequency on the given design. You would need to adjust the compensation for the lower frequency, but dependent on the minimum load you have, it is possible that with a lower frequency the on-time is small enough.

  • 0 in reply to Brigitte
    Intellectual 520 points

    Hi Brigitte,

    Indeed lowering the switching frequency helped with the minimum load, but not the necessary.

    I'm considering LM5118 (or LM25118). The only problem that I would have is that the minimum Vin at startup is 5V, and my input is 3V.

    With this device I wouldn't have this problem with the minimum load, right?

    Is it possible to connect Vin to 3-25v supply normally, provide 5V (with a small external charge-pump) at startup on VCC pin and disconnect this 5V after the startup?

    Best Regards,

  • 0 in reply to Bruno Andrade
    Intellectual 520 points

    Brigitte,

    After a more careful review of the LM5118, I've noticed that it is not a synchronous controller and I think I'll have the same behavior, now I'm certifying that LM5122 will fit using a similar approach as I described above for the VCC will work...

  • 0 in reply to Bruno Andrade
    TI__Guru 61135 points

    Hello Bruno,

    you are right, there will be the same problem with this type of controllers. Please have a look at these components:

    There you might find the part you need to synchronize your controller.

    I will forward your post to the other team as I do not know these devices in detail.

  • 0 in reply to Brigitte
    TI__Mastermind 31750 points

    Hello Bruno and Brigitte,

    I see that this post is lingering.

    I believe that the synchronous rectifier controllers found at the html link in the previous post can be considered as "ideal-diode emulators".
    In other words, they are intended to make the MOSFET behave as a normal diode with almost zero forward voltage drop.

    But they intrinsically do not allow reverse conduction (which is not an ideal diode behavior) so I do not believe any of them will provide the "synchronous conversion" function you are looking for here.  I'm sorry, our group does not have a device that does this.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 520 points

    Hi Ulrich and Brigitte,

    Indeed I haven't found a good solution for the project. There are some parts from TI that would be a perfect fit, if it wasn't for the price and availability.

    So to maintain the product at a certain cost, I decided to change a little the architecture, so it doesn't need a buck-boost anymore, but a buck, and a boost, then I had a lot of options to choose. The parts chosen are the TPS54302 and the TPS61088.

    Thank you for your support.

    Regards,