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LM5165: Extra pulses with higher input voltage

Michael Wolf
Prodigy 20 points
Part Number: LM5165

My system was observed to have a noise problem when the input voltage was raised from 12V to 24V.  After some looking around, I observed a switching anomaly in the 3.4V supply using an LM5165.

The following is a trace of the switch node, with Vin=12V.  The frequency was expected to be 400KHz based on Rt=51.1K (not 180KHz as observed), but that discrepancy is not a big problem.

The problem is that when Vin=24V, the single pulse divides into 3, creating much more noise in the system.

The following is the schematic:

What can I adjust to prevent this from happening?  Output is 3.4V at about 110mA.

Thanks for any help, Michael

  • 0
    TI__Expert 3835 points

    Hi Michael,

    Can you monitor the FB voltage and inductor current during this period. This looks to me like PFM mode waveforms. Try removing L5  as well.

    Regards,

    Ethan

  • 0 in reply to Ethan T
    TI__Expert 3835 points

    Hi Michael,

    Were you able to solve your issue? I am going to close the thread, but feel free to open it by replying to this message.

    Thank you,

    Ethan

  • 0 in reply to Ethan T
    Prodigy 20 points

    Hi Ethan,

    I rechecked my load and found it was only 80-85mA, so I reconfigured to the PFM mode by replacing R82 (Rt) and R96 (output cap series resistor) with 0 ohms.  This reduced my system noise problem by 80-90%, so I will work on tuning the inductor and output caps to try for a little more noise reduction in the PFM mode, rather than try for a lot of noise reduction in the COT mode.  The switch frequency reduced from about 180KHz to 80KHz, and there is a visible ring of about 1MHz during the off time.  Can an RC snubber be used to reduce this ring without lowering the efficiency too much?

    Thanks, Michael

  • 0 in reply to Michael Wolf
    TI__Expert 3835 points

    Hi Michael,

    Yes RC snubber will help in this situation. How are you monitoring the SW node? GND clips cause inductive loops that may show some artificial ringing.

    It will be difficult to say without evaluating the design, but since this is a lower current applicaiton, the snubber will be a more significant factor. I would try to optimize layout as well.

    Regards,

    Ethan