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LM5013: The output is not monotonic when powered off

Hailiang Zhang
Genius 11020 points
Part Number: LM5013

Tool/software:

Hi Team,

Customer use LM5013 in a Switch project. The input is 52V, output is 12.5V. Customer report that the Vout is not monotonous when powered off.
I wonder if this has something to do with the PWM duty cycle limit and VOUT protection response. Please help to check internally whether there is similar debugging experience.

Test waveforms as attached:
CH1=Vout, CH2=EN, CH3=Vin

Schematic

Regards,
Hailiang

  • 0
    TI__Guru 73502 points

    Hi Hailiang,

    I suspect the input voltage during turn off is bouncing near UVLO, causing Vout to turn on/off in tandem (otherwise known as motor-boating). Note there is a huge time constant on the EN divider, which results in a delay and Vin likely reaching the converter UVLO setting. Try removing the cap. You can also test it by pulling EN low.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Genius 11020 points

    Hi Tim,

    Removing the cap or pulling EN low work.

    But judging from the waveform, when the first restart occurs, Vin = 13V, Ven = 4.8V, at this time Vin > Vout (12.5V). I want to know why the output voltage drops and why it restarts? Why are the restart intervals different?

    Regards，
    Hailiang

  • 0 in reply to Hailiang Zhang
    TI__Guru 73502 points

    Hi Hailiang,

    Just looking at the schematic, 51pF is quite low for the type-3 coupling cap and may have some discharge due to the 49.9kΩ feedback resistor, complicating the FB voltage behavior.

    Also, there's no need for the cap across the upper feedback resistor (this is only for when the output ripple is triangular and needs to be coupled to FB with higher gain than that of the FB divider).

    10uF is definitely too high on the EN divider. There shouldn't be much noise on the EN node.

    Regards,

    Tim