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LM5118-Q1: Output Oscillation in Transition Mode

Lorenz Golob
Prodigy 10 points
Part Number: LM5118-Q1
Other Parts Discussed in Thread: LM5118

Tool/software:

Hello!


I have a setup with the following parameters:

Vout Vin Imax
8.9V 9-24V 2A

All component values have been verified with the LM5118 Quick Start Component Calculator to ensure plenty of phase margin in both Buck and Buck-Boost mode.
The regulator performs just fine for input voltages above 13V and voltages below 11V.

However, for a narrow region of input voltages between 11.5V and 12.5V (with a slight hysteresis), I am observing a large ripple or oscillation on the output voltage for load currents between ~100mA and 400mA.
The frequency of this observed signal increases with load current until it disappears entirely at around ~400mA.

There seems to be some sort of oscillation in the transition region from buck to buck/boost mode.




Is this a stability issue or related to the board layout?


  • 0
    TI__Guru 58641 points

    Hi Lorenz,

    can you measure the response on the output voltage on a load step response.

    This can give an indication about the loop stability. You can measure this close the the supply levels where it gets unstable.

    I also think that the layout can have a big impact here. Doing a review of the layout would be recommended as well.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hi Stefan,

    the output voltage remains stable during load transients.
    I assembled a second (identical) board with the same compensation network and can not reliably reproduce the initially observed behavior.

    I will continue to test the design in detail and will report back if the issue comes up again.

    Best regards,
    Lorenz

  • 0 in reply to Lorenz Golob
    TI__Guru 58641 points

    Hi Lorenz,

    can you check the boards for proper soldering - in case of bad connections a internal GND shift can have impact like this.
    (also the thermal pad)

    Best regards,

     Stefan