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LM43603: LM43603: Poor voltage regulation under load

Part Number: LM43603

I have made 2 PCBs with the same schematic shown but different layouts. One circuit works fine and the other has poor output voltage regulation.

On the one that has poor regulation, under no-load it outputs the desired voltgae (5.75V), but even with the smallest amount of load (few mA), the output and feedback pin voltages drop. I can't get it to supply more than 50mA or so.

I have been through the datasheet calculations several times and the component values seem okay. I have also replaced all parts on the PCB more than once with no change to the regulation.

Increasing the switching frequency improves the load regulation (can draw up to 1A if the maximum switching frequency is used), however the output of the IC goes unstable after about 1.5MHz. In the case where it does draw about 1A or so, it should be drawing around 2A, so is still not regulating properly.

Layout seems to be the only major difference between my working and not working designs, but I didn't think that the layout on the not working circuit was so bad that it would have this impact...

Note: R100 is 100k on the schematic, but it is acutally 33k on the PCB (and is intended to be 33k by design).

  • Hi,

    In terms of the schematic, could you try the following:
    1) Reduce the inductor to 2.7uH. Also noted the inductor should have a rated current beyond 3A

    2) Add 2x22uF output capacitor. Total output capacitor should be at least 22uF after voltage derating. 

    3) Change C82 to 470pF instead of 1nF

    Here is a sample webench design for your condition as an example

     In terms of layout:
    1) One major thing that i see is that pin 10 on the PCB layout that is working is connected directly to the IC bottom. You want to connect the AGND pin directly just like the one on PCB layout working. 

    I hope that helps