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LM27313 oscillating output

Uri Bendelac
Prodigy 60 points
Other Parts Discussed in Thread: LM27313, LM2733, LM2731

I have successfully been using a LM27313 with 6V input, converting to 13.5V, using 10uH inductor (VLP8040T-100M), 10uF ceramic capacitor on input, 2x10uF ceramic capacitors on output, 14.7K&150K feedback resistors with 120p in parallel to the 150K and it works fine (0 to 100mA output current). I recently had to change input voltage to 12V (in practice had 12.4V), and the output without load now has large oscillations between 30V decaying to 13.5V and then switching back to 30V, every 55msec. The only change was the increase in input voltage.

Does anyone have any idea why?

  • 0
    TI__Intellectual 2225 points

    Hi Uri,

    I think that you are running into the minimum on-time limitation of the LM27313.  When the input voltage becomes very close to the output voltage, a boost converter needs to switch on for a very short amount of time to maintain the output voltage.  if this required on-time is less than the minimum on-time of the boost controller, then the output voltage will start to climb out of regulation.

    If you need to have such a small difference between input and output, then you will probably need to select a part with a lower switching frequency.  Have you considered the LM2731 or LM2733?  They both have a 600kHz version which will be less prone to this issue (however it will still occur, just at a higher voltage). They are also pin compatible with the LM27313.

    Another workaround for this issue is to just shut the converter off (using a separate comparator) as the input voltage gets too high using an external comparator. Then the diode will just allow input voltage to flow to the output.

    Perry

  • 0 in reply to Perry Tsao96839
    Prodigy 60 points

    Hi Perry,

    Thanks for your reply, that explains my problem. I would rather stick with the LM27313, as it is cheaper than the alternatives you suggested. I could raise the output voltage beyond 13.5V, for example, to where there are no oscillations anymore. However, I looked again in the LM27313 datasheet and did not find any mention as to this minimum on time you mentioned (there is just a specification for the maximum duty cycle, not minimum). Did I miss it? Can you provide some specifications regarding the minimum voltage difference I need to maintain on output to guarantee stable operation?

    Thanks,

    Uri

  • 0 in reply to Uri Bendelac
    TI__Intellectual 2225 points
    Hi Uri,
    You are right that the minimum duty cycle it is not in the datasheet. Unfortunately this is not a parameter that is tested in production, so I cannot share any additional specifications on the minimum minimum voltage difference. I think the best way to characterize it would be to test it in your system at minimum and maximum temperature and then add 10% margin to account for part variation.
    Perry