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LP5907: LP5907SNX_3.3V output oscillation(~360KHz),any clue

Part Number: LP5907

Gentlemen

     we use the several pcs LP5907SNX_3.3V at our module, while find the LP5907SNX_3.3V output got oscillation, the frequency is about 360KHz. the output cap PN CL03A105MP3NSNC from samsung, voltage rate 10V.

  3.8V_SW is the input power, inital set the 3.8V_SW = 3.5V, the ourput ripple is ~20mV

when  set the 3.8V_SW = 3.8V, the ourput ripple is 80mV high

when  set the 3.8V_SW = 4.0V, the ourput ripple is 100mV high

 what caused the oscilllation and which the higher input power could cause the high osicalltion, any clue

Apprieciate the support and help.

Input power =3.5V, output ripple 20mV

Input power =3.8V, output ripple 80mV

Input power =4.0V, output ripple 100mV

  • Hello,

    Typically the output capacitor is the primary source of oscillation problems. The cap. you mentioned looks like it should be OK, but, if you add additional capacitors, does it go away? Is there any ripple on the input?

    Regards,
    Mike
  • Mike
    we put one more cap the ripple do become smaller.

    the queston is why the oscillation at output is right round 360KHz?

    why the higher input lead to higher ripple ( when 3.6V input, output have 20mV ripple, and when 3.8V input, the output have 70~80mV ripple, when 4V input the output have 80~100mV ripple)?

    for regulator, we thought whatever input, the output ripple is similar/ not change too much.

    Thanks
    Liu
  • Liu,

    There may be input oscillations occurring, which will also show oscillations at the output. I do not know why it would be at 360 kHz, I suspect there might be a resonance frequency on the input that creates this. Increasing VIN would lower the effective input capacitance, which would also cause a change in the oscillation. Can you check the input to see if there is an oscillation, and what the magnitude is, if so?

    Regards,
    Mike
  • Mike

       Thanks for the feedback.
       for the input we do check it before, the 3.8V is from a boost, which frequency is 2.5MHz, and the 3.8V is much clean, no ocsilation.

    Thanks

    Jizhong

  • Hi, Mike

      how is the converter frequency of the LDO, my understanding is the LDO internal have negitive feedback loop to make output power stable

    how is the frequency of the internal negative feedback loop.

    Thanks

    Jizhong

  • Hi Jizhong,

    There is no particular unique frequency response at 300 kHz. The loop gain of this device can vary significantly, depending on the load and the capacitor. It can be higher than 300 kHz. Can you show an image of the layout? Is the output capacitor close to the device?

    Regards,
    Mike