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LM5085: lm5085 design doesn't work properly

Part Number: LM5085

hi,

I've a problem, I need a supply with programmable current limit. I've chosen for the LM5085 because it has that function built in. The problem though is that the current limit doesn't work in my design and I have no idea why. I have read the whole data sheet multiple times, but I don't  have a clue why this doesn't work.

  • Abdel,

    I'm not entirely sure what all your external circuitry is is doing, particularly the FB node op-amp make me somewhat nervous about your converter stability. 

    Is your output voltage correct?

    Also the current limit is for sensing the high side SW current to protect the FET, it was not designed for sensing output current.

    Be that as it may, from what I can understand you are sourcing the IADJ pin with a 100mV voltage source.

    The ISEN pin can go up to a 15V rail.

    Look at the block diagram(pg10) the current limit comparator triggers when the IADJ voltage is higher that the ISEN voltage so I suspect your ADJ voltage is always lower as it is sourced from a low voltage supply and not VIN as it usually is.

    You probably want to bump up that supply and/or lower the RADJ so you get reasonable voltage on the IADJ node for the comparator to work.

    Your use case is an outlier so I'm not sure how much help I can be, but let me know if you have further questions,

    -Orlando