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LM5157: LM5157 SEPIC

Luis Montiel Leos
Prodigy 10 points
Part Number: LM5157
Other Parts Discussed in Thread: LM5155

Hi ...

I am working on the design in SEPIC based on the LM5157 and I am experiencing some problems. Let me start by saying that the circuit is going to have an input between 7.0 V to 20.0 V and the intention is for it to have an output voltage capability of 15.0 V and a current of 3.0 A. Is this feasible for the device?

  • 0
    TI__Mastermind 25980 points

    Hi Luis,

    Thanks for using the e2e forum.
    The LM5157 device has an integrated FET which allows peak currents of 6.5A.
    Calculating the worst case conditions, (min Vin 7V, max load 3A,) there could be peak currents up to 7A due to inductor current ripple.
    These are just some quick calculation with our Power Stage Designer tool:
    https://www.ti.com/tool/POWERSTAGE-DESIGNER

    If the current ratings are too high for internal FET, I can recommend to use external FET alternative for this part, which is LM5155.
    This device has no such current limit restriction, and should be a good fit for your SEPIC design.

    You can also use the quickstart calculation tool for additional calculations:
    https://www.ti.com/tool/download/SNVR481

    Please let me know if you have additional questions on these devices.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 10 points

    Thanks for the response. I got an additional question.

    I built a prototype using the components generated by using the LM5157 SEPIC calculation sheet. I found that even by connecting a 1.0 A load the inductor current went really high.

    I really want to have some conversation with some specialist in TI about my findings. Can you provide some contact information in the area of Detroit Michigan USA?

    I appreciate your suggestion of use an external MOSFET driver, I will look into this alternative.

  • 0 in reply to Luis Montiel Leos
    TI__Mastermind 25980 points

    Hi Luis,

    Thanks for the update.
    It is not only FET capabilities, that limits the max load of the application.
    Please have a look if your inductors can support the peak current ratings and do not fall into saturation.
    This can lead to a snowball effect, that will result in very high current flows.

    Regarding a TI contact, I recommend to reach out to a local TI sales or marketing contact. They will supply you with further support and resources and can setup a meeting with our experts if required.

    Best regards,
    Niklas