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8-36V In, 12V @ 15A out Buck-Boost using LM5118

Niels Koll
Expert 1950 points
Other Parts Discussed in Thread: LM5118, LM5175

Hello,

I am in need of a buck-boost switching regulator with the following requirements:

  • 8-36V input voltage
  • 12V output voltage
  • 15A output current

Is it possible to use the LM5118 for this task?

I thought about using some FETs with Ron < 4mOhm. What is the max. allowed Qg for using a FET with the LM5118 in this application?
Which maximum switching frequency can I use?

Regards,
Niels

  • 0
    TI__Mastermind 21720 points
    Hello Niels,

    The input and output voltage range is suitable for the LM5118. The output current might make it hard to find an inductor that can handle that level of current at a reasonable size.

    Its important to note that a proper VCC capacitor should be chosen to help the VCC rail from dropping if large Qg FETs are used. Below is the equation, Qg is the total gate charge of all MOSFETs added together, fsw is the switching frequency and Ivcc is the required amount of VCC rail current.

    Ivcc = Qg*fsw

    Based on you output voltage spec, VOUT can be diode or'ed to the VCC pin and used as the internal bias rail. By doing this you will be able to supply as much current is needed to operate the selected FET.

    Let me know if you have any questions.

    -Garrett
  • 0 in reply to Garrett Roecker
    Expert 1950 points

    Hy Garrett,

    thanks for your answer.

    You are right, the currents through the inductor are incredibly high.
    Which regulator would you suggest to ease inductor selection?

    Niels

  • 0 in reply to Niels Koll
    TI__Mastermind 21720 points
    Niels,

    The LM5175 maybe a good option. This would also help to improve the efficiency over LM5118.

    -Garrett
  • 0 in reply to Garrett Roecker
    Expert 1950 points

    Garrett,

    thanks for your help.
    I just tried to use webbench with the LM5175, but the webbench solution is limited to 7A.
    So I will start reading the data sheet. ;-)

    Niels