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LM25085: Current oscillation

Taemin Ahn
Intellectual 795 points
Part Number: LM25085
Other Parts Discussed in Thread: LM5085

Hi Team

I have seen a current oscillation while using the LM25085.


Input voltage 17V ~ 20V Output 500mA load is applied.

I added capacitors to GND in parallel between R37 and Q2 and the oscillation disappeared.

Vin's bypass capacitor looks away from the pin. This is supposed to be a reinforced effect.

If yes, please explain in detail why this is happening.

Thanks,

Taemin

  • 0
    TI__Mastermind 33050 points
    Hello Taemin,

    It looks like you have a current limit noise issue?

    If I understand your post correctly, you are saying adding a cap from Vin to Gnd helps your issue? If this is the case, it is very likely due to Current limit. If no cap is used, you will get noise on Vin of the device and this will corrupt the current sense signal, engaging current limit.

    By placing a cap at Vin to the LM5085, there is likely to be less noise and the current limit will engaged at a point that is accurate to the Rsense value used.

    Also it looks like you have a LC filter at the input, this need to be Damped. Suggest placing a Cdamp of 4x the value of Cin you currently have and place a one ohm in series with the Cdamp. Call this Rdamp. This will likely resolve your issue also?

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Intellectual 795 points

    Hi Team

    Thank you for your help.

    This circuit diagram is used as the motor power supply.
    When the motor turns or stops, it consumes a large current.
    At this point, the LM25085 is broken.
    However, changing C2 (Cvcc) to 0.47uF does not break it.
    What are some speculations about this phenomenon?