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LM5116: Variable output

Lorenzo Lutti
Intellectual 655 points
Part Number: LM5116

I would like to use an LM5116 for a software controlled, variable output DC-DC, let's say 60 Vin and 35...55 Vout. My idea was to change the output voltage divider ratio by using a digital potentiometer, reed relays or similar technique. In your opinion, is this possibile and/or advisable? Many components have to be selected based on the output voltage, and while in some cases (e.g. input and output capacitors) is quite easy to determine the worst-case scenario and select a one-size-fit-all solution, in other cases this is more difficult to do:

Output inductor, ramp capacitor: selected based on the minimum output voltage?
Input capacitors, output capacitors, soft start capacitor, MOSFETs: selected based on the maximum output voltage?
Error amplifier compensation: ???

The other components (current sense resistor, bootstrap capacitor etc) do not seem directly related to Vout.

Am I doing something horribly wrong?

Thank you!

L.

  • 0
    TI__Guru 57555 points

    The considerations you mentioned are correct.  Depending on how wide an output voltage range

    you need you may need very different components.  If you output voltage range is narrow, it might work OK.

    I would use Webench to do "what if" designs with your various output voltages and see if you can find a

    compromise on components. 

    Changing the output voltage can be done with relays, FETs or a DAC.

  • 0 in reply to Frank De Stasi
    Intellectual 655 points

    Frank De Stasi said:

    The considerations you mentioned are correct.  Depending on how wide an output voltage range

    you need you may need very different components.  If you output voltage range is narrow, it might work OK.

    I would use Webench to do "what if" designs with your various output voltages and see if you can find a

    compromise on components. 

    Changing the output voltage can be done with relays, FETs or a DAC.

    Frank, thank you for the answer. Unless I'm not looking in the right place it seems that the "what if" analysis doesn't allow you to change the output voltage. I've simulated the two operating points (35 Vout and 55 Vout) by creating two different designs, but I cannot force an output voltage to a design. It would be interesting to export the whole simulation model to PSPICE (this would give more room for experiments) but I don't think this is possible either at the moment.

    By the way, why do you dismiss the possibility to use a digital potentiometer for the voltage regulation? After all it is quite similar to a DAC in concept. Are there some parasitic effects that I'm not considering?

    Thank you again!

    L.