This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2611: 33KHz Oscillation in -5V power

Part Number: LM2611
Other Parts Discussed in Thread: ALLIGATOR

Hi team,

We are using the LM2611BMFX part in our design for converting 12V to -5V.

This power is going to the analog section in our design. We have seen noises in the Analog section & while analyzing this, we have observed that there is an oscillation/ripple of 33KHz(+/-60mVpK-pK) in the -5V power.

The regulator design is attached here. Please clarify, where is this 33KHz oscillation generated even though the switching frequency is 1MHz.

Regards,

Ahamed Faize

  • Hello Ahamed, 

    Thanks for reaching out. What is the load current in your system? If the load is very small, the converter might be operating in pulse-skipping mode (see datasheet, pg 12 section 7.4.1) and this might increase the ripple that you see on the Vout. Another thing to try would be to check the converter alone and probe the SW node and the Vout, in this case I would not recommend to use an alligator clip but use instead the tip and barrel (or pigtail) method to avoid having big ground loops (you can find more details in this article https://e2e.ti.com/blogs_/b/powerhouse/posts/how-you-measure-your-ripple-can-make-you-or-break-you ). Please let me know if I can help you furthermore. 

    Kind regards, 

    EM 

  • Hi EM,

    Thanks for your comments.

    The load current is between 100mA & 200mA. We have checked without loads (i.e.) Regulator is disconnected from loads. At that time, we have not observed the 33KHz oscillation. When we are increasing the loads, 33KHz oscillation comes.

    I will probe the switch node & VOUT of the regulator alone.

    Regards,

    Ahamed Faize

  • Hello Ahamed, 

    Thanks for your feedback. Please share your oscilloscope plots when you are ready. One more thing from my side, can you confirm what type of component is the one in the box labelled FL19? I would guess this is a ferrite bead, it might also be that your oscillations on the output are cause by the ferrite bead, did you by chance try to measure your circuit without this component? Let me know if further help is needed. 

    Kind regards, 

    EM