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LM25011: Pre-bias start up

K. Sasaki14
Guru 10200 points
Part Number: LM25011

Can LM25011 support pre-bias start up?

According to our customer pre-bias start up test, Vout doesn't rise up to set point by FB resistor because there was high duty no load operation period and enough BST capacitor charge was not maintained.

Is there any solution for this phenomenon?

Best Regards,
Kohei Sasaki

  • 0
    TI__Mastermind 25590 points

    Kohei Sasaki,

    Section 7.3.5 of the datasheet explains, "In applications with relatively high output voltage and low minimum load current, the internal pre-charge device of the LM25011 may not pull the SW pin sufficiently low during the off-time to maintain the voltage on the bootstrap capacitor. If the bootstrap capacitor (CBST) discharges during the long off-times, and the regulator will cycle on and off at a low frequency. Decreasing the values of the feedback resistors RFB1 and RFB2 to provide a minimum load of typically 1mA at nominal VOUT will increase the minimum switching frequency and maintain sufficient bootstrap capacitor voltage."

    -Sam

  • 0 in reply to Samuel Jaffe
    Guru 10200 points

    Sam-san,

    Thank you for your reply.

    Is this explanation that it relates to pre-bias start up too?

    Can LM25011 support pre-bias start up if there is 1mA minimum load at nominal Vout?

    Best Regards,

    Kohei Sasaki

  • 0 in reply to K. Sasaki14
    TI__Mastermind 25590 points

    Sasaki-san,

    Our tests show that increasing the minimum load to 1mA helps start up into a pre-biased condition if the output is not externally maintained. The minimum load will reduce the output voltage to allow the IC to start up.

    Is your load maintained by external circuitry on the output? If so, this may not help. You may need a synchronous IC which can set the SW node low to charge the bst cap.


    -Sam

  • 0 in reply to Samuel Jaffe
    Guru 10200 points
    Sam-san,

    Output voltage decreases by 1mA load and boot cap is charged by decreasing output voltage.
    For pre-bias start up support, output voltage should be decreased.
    Is this correct?


    Best Regards,
    Kohei Sasaki
  • 0 in reply to K. Sasaki14
    TI__Mastermind 25590 points
    Sasaki-san,

    Yes, a higher output voltage at startup means the part will have a more difficult time starting up. If adding 1mA to the output drops the output voltage, the 1mA will help.

    -Sam