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LM25085-Q1: DC-DC

ELVIS SUN
Prodigy 20 points
Part Number: LM25085-Q1
Other Parts Discussed in Thread: LM25085, LM25141-Q1, LM5146-Q1

Dear sIR/Mam,

   i used  LM28085-Q1 to build one DC-DC from 18V-25V to 15V/3.5A, but find the output voltage will go rise with the rise of Vin, can not be stable.

  i checked the FB pin voltage, find the FB pin voltage is really higher a little at 22V input compared to 18V  input, the P-P value of FB is around 100mV. waveforms and circuirt are as below. would you please help and give me some advice on this issue?

       

  • +1
    TI__Guru 73072 points

    Hi Elvis,

    The injected ripple amplitude may be a little high here. Note that this COT controller regulates based on the valley of the ripple voltage (when the FB voltage decays to reach the reference voltage, the high-side FEt is turned on). Thus, the output voltage will run higher than the setpoint, especially at higher Vin when the ripple amplitude increases.Looks like your circuit is stable, so you can afford to increase either the ripple resistance or cap value of the type-3 injection circuit.

    Regards,

    Tim

  • +1 in reply to Timothy Hegarty
    Prodigy 20 points

    Hi Timothy, 

    yes if increase the RC, the output voltage rise will be better. so it seems the LM25085 controller can't be used in wide range input location if the demand accuracy of output voltage is strict basing on the working principle. thanks for your help and advice.

  • 0 in reply to ELVIS SUN
    TI__Guru 73072 points

    Yes, it's important to kep the injected ripple amplitude reasonable for best line regulation. If output accuracy is important, consider a voltage-mode or current-mode controller, e.g. LM5146-Q1, LM25141-Q1.

    Regards,

    Tim