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LM3478 used as a negative boost

Joao Figueiredo
Intellectual 335 points
Other Parts Discussed in Thread: LM3478, LM5116

Hi there,

I'm developing a negative boost with the following features:

Vin (min and max): -12V

Vout: -80V

Iout: 0.1A

Studying different topologies and checking the Webench Tool designs, I came to two approaches: a negative boost smps or a flyback smps. I really prefer the first option (negative boost), once the dependency on the transformer (which occurs with the flyback) can be undesired when the manucfaturing occurs in big scale.

So, I came to the following design:

Now I need some help with the adaptation of this circuit into a negative boost. Reading some applications about negative boost, I saw the diode goes inverted and the MOSFET is a P-channel.

Well, if you can help me with this issue (concerning this LM3478 circuit or even suggesting me other topology) I would be thankful.

Regards,

João P. B. Figueiredo

  • 0
    TI__Intellectual 1775 points

     

     Hi João Paulo,

    A negative boost converter cannot be designed with a classic boost controller like the LM3478. Usually, it would be done using a buck controller as detailed in the application note below. 

     http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slyt516&fileType=pdf

    Ohe of the challenge is that the output voltage will be seen across the input of the IC and there are not so many part that can handle such a high voltage. The LM5116 might be a fit.

    Best Regards,

    Florent Boico

  • 0 in reply to Florent Boico
    Intellectual 335 points

    Hi Florent,

    Thanks for the explanation and also for the LM5116 recommendation.

    I'm studying the application note (about how to design a negative boost from a buck) but I really need some help with this implementation.

    Do you know if it's possible to get some reference design from this input using the Webench tool? In case it's not possible, do you have some design with a similar application (using LM5116) to show me?

    Regards,

    João P. B. Figueiredo

  • 0 in reply to Joao Figueiredo
    TI__Intellectual 1775 points

    Hi João Paulo,

    Unfortunately I don't think you'll find designs for it on Webench as the primary focus of these ICs is positive input to positive output.

    I myself do not have experience with designing such a system. As I see it the main issue is ensuring that all the circuits of the IC will be properly biased to keep the part safe and operationnal. Since the IC was not designed with this application in mind it will take some time to check each pin.

    The behavior of the negative boost converter should be the same as that of a positive boost converter so compensation should follow that of a standard current mode control or voltage mode control boost depending on what the actual IC chosen is using to regulate its standard application.

    Best Regards,

    Florent