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LM5118: Internal Compensation for variable output

Part Number: LM5118
Other Parts Discussed in Thread: LM5141

I previously designed a variable power supply (controlled via an EE-Pot by a micro) that operated from 44-56 VDC input, the variable output voltage range being 11-39V at 80W above 16V and capped at 5A below 16V; this used a switcher IC that had internal compensation - I picked such due to the ride range of the output.  I now need a similar design (same input voltage range) but the output varies from 5V to 24V, output being 85W for 12V and above and capped at ~8A below 12V.  I thought it would be wise to stick to an internal compensation switcher but have yet to find one that can handle these parameters, so I was looking for direction on this new design:

    • Is the internal compensation critical?
    • If not how do I pick design values for the compensation?  If I use Webench and try a few data points (say 5V, 12V, and 24V) the circuitry is vastly different

  • Hi Dan,

    Thanks for reaching out with your questions. Based on our requirements I think a buck controller is a better solution than the LM5118 which is a buck-boost controller. A buck controller fits better because the input voltage is always greater than your target output voltage. Something like the LM5141 fits this application.

    Based on the power level and the voltages I am not aware of a part that has internal compensation. Most controllers have programmable compensation due to the large operating range that the device can handle. The calculations for the compensation components can be found in the datasheet or the provided calculators. For a buck controller the compensation should be designed for the maximum input voltage, the lowest output voltage and at maximum load current.

    Let me know if you have any questions.

    Thanks,

    Garrett

  • Thx Garrett.  The problem with the final sentence is when the maximum current does not occur at the minimum voltage:

      • 24V @ 3.55A is max voltage and power
      • 12V @ 7.1A is same 85W as the above but max current
      • 9V @ 6A (and I'd probably be OK down at 4-5A)
      • 5V @ 5A (and I'd probably be OK at 3-4A)

    So this makes the compensation design needs a bit gray to me - I'd probably tweak it for 12V even though it's not the minimum voltage.  And there's obviously more than just the compensation - most notably the inductor is typically picked specifically for the application parameters.  I can look at an LM5141 design and see how different the 5V and 24V circuit values look.

  • Dan,

    In that case I would design the compensation for the 12V output and then plot the loop for the other outputs to make sure the control loop is stable.

    Here is the quick-start calculator to help with the loop calculations and component calculations.

    Thanks,

    Garrett