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LM2611: Stopping switching

Part Number: LM2611
Other Parts Discussed in Thread: TPS28225

Hi,

My customer is evaluating LM2611 on their board and switching stops as below.

Schematic is below.

Do you know why switching stops and VOUT rises then?

Best Regards,

Kuramochi

  • Hello Kuramochi,

    Please clarify if this is at no or light load condition and as soon as the load is increased, the part switches permanently.

    If yes, the device detects a too high output voltage and needs to stop until the load discharges the output again to the target level. The ripple is about 1% of the target output voltage as far as I can see.

  • Hi Brigitte-san,

    Thank you for your advice.

    As you expect, this is light load.

    Is it possible to improve this behavior without increasing load?

    Increasing load is difficult because this is battery application.

    Best Regards,

    Kuramochi

  • Hello Kuramochi,

    There is a minimum amount of energy going from the input to the output when the device is switching, so the device needs to stop at a certain point because the diode does not allow backward current.

    If the permanent switching is absolutely necessary, I can only think about increasing the load or try to exchange the diode with a FET and add a sync-FET driver. But I do not know these devices very well and do not know if there are some that would fit this application. Let me try to get the thread to the expert for these.

  • Hello Kuramochi,

    I see Brigette has been assisting with the LM2611 and a question or possible solution regarding driving sync FETs is mentioned. Replacing the diode with a synchronous FET and drive circuit can address the ability to conduct negative current. But I am not sure how the driver will interface with the internal power switch arrangement in the LM2611.

    The TPS28225 and TPS 28226 are synchronous FET drivers with a single control input which can work will VDD as low as 4.5V up to 8V. So it may be compatible with the LM2611 depending on the input voltage range.

    I am not sure if the driver PWM input can be driven or interfaced with the LM2611 SW node however.

    Regards,