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LM5017: LM5017

kushal banerjee
Prodigy 210 points
Part Number: LM5017

Hi,

I am using LM5017 DC-DC converter IC.
Maximum Input voltage can be 92V. (Currently we have checked upto 32V)
OUtput current required = 300mA.
Inductor used - 470uH and 220uH.

Our observation - The output voltage is fluctuating with error of 100mV. The FB voltage as per datasheet should not be greater than 1.25V, but we observed voltage of 1.3V 
How can we make our output voltage constant?

  • 0
    TI__Mastermind 40020 points
    Hello Kushal,

    When you say fluctuation are you referring to an oscillation? or do you mean there is movement of Vout with load?

    If the latter, the the reason is, because you have a voltage at the feedback VFB= Vref+Ripple. If you have a lot of ripple presented at the feedback node, there will be an error in the output voltage. I suggest your probe to measure the ripple voltage and reduce to ~25mV if you measure a voltage ripple greater that this. This will help minimize the voltage.

    if you are see oscillation, you maybe unstable because you do not have enough ripple presented at the feedback node. Suggest looking at the datasheet as it discusses ripple voltage on the feedback node?

    If you are still having issues, it would be good to look at the schematic.



    Hope this helps?

    Thanks.


    David.
  • 0 in reply to David Baba
    Prodigy 210 points

    Hi David,

    Thanks for you suggestions, it really helped.
    Our issue is solved by reducing the Inductor value, more closer to the calculated value.

    We had the fluctuations. 

    I have one more doubt on our implemented solution.
    The design of the switching inductor suggests to select the inductor value equal or greater than the calculated value. But when I did that, why our output fluctuated?
    For example, calculated value came equal or greater than 338uH but we used 470uH. Later when 470uH was changed to 330uH, then the fluctuations were removed. 

  • 0 in reply to kushal banerjee
    TI__Mastermind 40020 points

    Hello Kushal,

    The LM5017 is a COT Regulator and needs a certain amount of ripple to be stable.  I suspect that increasing LBuck has reduced the amount of ripple voltage at the feedback node.  It is recommended that a ripple of 25 to 50mV be prsent at the feedback node.  Suggest using a lower Lbuck or changing the ripple injection circuit to increasing ripple at the feedback node.  I belie the fluctuation is caused by instability which can be confirmed by taking a scope measurement at the switch node.

    Hope this helps?

    David.