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LT3999 light load operation

Hi 

When using LT3999 with a light load, the output voltage became higher than the set value.

I also tried it with LTspice and I saw a similar waveform.

Is there a way to  regulate the output voltage at light loads?

Best Regards,

Hgtk

  • Hello,

    please check page 11. If you need a more stable Vout the use of a LDO might be necessary.

    br

    Markus

  • Hello Markus-san

    Thank you for your quick reply.

    I understand your answer but please let me know about the output voltage at light load.

    I evaluated with almost the same circuit as datasheet sheet1 and found the following result.

    Vin:12V

    Vout:35V

    Iout:no load

    Fsw:500kHz

    Turns ratio  Np:Ns=1:1.08

    The output of this circuit is calculated to be about 13V so this result is very high.

    Are there any way to get an output voltage about 13V at no load condition?

    Best Regards,

    Hgtk

    
    

    I would like to know the 



    Is there a way to  regulate the output voltage at light loads?
  • Hello,

    the LT3999 is a push-pull stage where the Dutycycle is fixed and can only be modified with the RDC pin from the primary side. There is no feedback loop to accomodate for load/line variations from the output.

    So you might 'attach' either a reistive load for operating at a default load as minimum load or use a zener on the output to limit the possible Vout increase.

    The intended low noise applications are mainly Signalchains consisting of several opamps/ADCs which offer a decent amount of power consumption, hence your problem will not occur.

    kind regards

    Markus

  • Hello 

    Thank you for your reply.

    I will try it.

    Best Regards,

    Hgtk

  • Hello Markus-san

    I have an additional question about LT3999.

    I am evaluating a ± output circuit and found a following large ringing waveform at SW pin.

    CH1:10V/div

    Fsw:500kHz

    output current:50mA

    I use almost the same circuit as below.

    Would you please tell me the how to reduce this ringing? 

    Best Regards,

    Hgtk