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LM25011: High duty and no load start up

K. Sasaki14
Guru 10200 points
Part Number: LM25011

Our customer using case is Vin=34V, Vout=26V, Iout=0.6A, Fsw=500kHz.

When Vout start up at no load condition, Vin voltage becomes flat several period by inrush current(probably) and high duty operation is done because Vout rise up.

In this time, BST capacitor charge is not maintained and LM25011 cannot normal switching. So Vout doesn't rise up to setting voltage.

This is removed by setting longer soft start time. 

And it is described on datasheet about decreasing FB resistor value to provide minimum 1mA load.

Is there possibility that LM25011 or external components are damaged when LM25011 continue operation with the condition that there is no enough charge on BST capacitor?

Do you have any idea about solution other than soft start time or min load current?

Is there soft start capacitor min value and max value?

Can LM25011 keep sufficient BST capacitor voltage for normal switching under any conditions if there is min 1mA load?

Best Regards,
Kohei Sasaki

  • 0
    TI__Mastermind 25590 points

    Kohei,

    I don't have any evidence that what you described has damaged the LM25011. Insufficient BST cap voltage means the high-side FET may not turn on fully but this will just increase the heat dissipation in the FET. If the IC has not overheated, it is most likely not damaged.

    Soft start cap should be no smaller than 1nF. There is no upper limit to SS capacitance as long as you consider your application. Does your load accept this slow rise?

    Our tests show LM25011 maintaining sufficient BST cap voltage with a load of 1mA.

    These are the two best solutions for your this part.

    -Sam