LT3045 output isolation with LC-filter or ferrite beads

I have several chips powered by LT3045 that requies power-line isolation. Usually we can use Pi-type LC filters or ferrite beads with some capacitors on each chip's power-in port. As LC circuit can resonate on specific frequency, we usually put a large capacitor series with a small resistor to increase the ESR to do zero compensation.

But I noticed on LT3045 datasheet P16, it says "Therefore, it is very important to avoid adding extra impedance (ESR and ESL) outside the feedback loop."

I don't quite understand why we should avoid adding extra ESR outside the feedback loop. Do these LC filters or ferrite beads in my application makes LT3045 unstable and break the output?

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    •  Analog Employees 
    on Oct 28, 2020 6:00 PM 2 months ago

    I think you raise a good point.  I will double-check with the design group to see if they did not mean "... inside the feedback loop."  I did an experiment in the lab just now.  I did a load transient test with the standard LT3045 demo board and then I repeated the load transient test with a ferrite bead-capacitor filter added right at the output terminals.  My experiment showed that the stability indicated by the load transient test did not change much when I added the output filter.  I show the scope captures with conditions and schematic in the images below.

    Conditions:  LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 200mAy to 400mA to 200mA, VCOUT IOUT load trans_0.bmp

    LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 200mAy to 400mA to 200mA, VCOUT IOUT load trans_0.bmp

    Conditions:  LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 200mAy to 400mA to 200mA, filter out, VCOUT IOUT load trans_0.bmp

    LT3045EDD, DC2491A, 5VIN, 3.3VOUT at 200mAy to 400mA to 200mA, filter out, VCOUT IOUT load trans_0.bmp

    DC2491A-3-SCH_1.png

  • Thank you so much for your detailed experiment results. So it seems like a typo in the datasheet?

    That makes me relieved, thank you

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