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LMR14206 unstable output with no load

Magnus Ek
Prodigy 20 points
Other Parts Discussed in Thread: LMR14206, LM2736

Hello,

We have problem with a design with LMR14206.

Settings:  Output 14.0V, Input from 18V to 28V.

With 150mA load everything is ok, but with no load and input 18.5V or lower the output is unstable.

See attached schematic, output and layout photo.

Rgds,

Magnus

  • 0
    TI__Genius 15810 points
    Hello,
    At no loads, the boot cap might not be able to charge up completely and not be able to turn on the FETs properly. A minimum load would help. If a dummy load is not possible, you could reduce the feedback resistor values by a factor of 10 to increase the current flow there.
    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Prodigy 20 points

    Hello,
    Thanks for your answer. 

    The solution with reducing the feedback resistor did not work.      The solution with dummy load of 1kohm gave us a better result, now stable down to Vin = 15V.  It is not an optimal solution for us. Do you have any other recommendations?

    Regards,
    Magnus

  • 0 in reply to Magnus Ek
    TI__Mastermind 19851 points

    Hello Magnus,

    What is the input voltage range that you need to support? Looking at the datasheet spec for the maximum duty cycle, it is 0.87 typical and 0.81 min. This means that in order to get 14V regulated output, minimum Vin is 14V/0.81 = 17.28V.  Given that there loss terms in the conversion, 18V minimum Vin is probably as low as you can get.

    Another solution (in addition to what Akshay suggested) is to derive the boot voltage through a simple external circuit. May be something similar to figure 13 in the LM2736 datasheet (), using a zener and a diode.  

    I would suggest to first verify if the boot voltage is really the problem. You can do that by supplying external 5V to the boot pin (through a diode):

    Cheers, 
    Denislav