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LM5022: LM5155: How to evaluate, design and fine tune a SEPIC topology?

AnBer
Mastermind 30110 points
Part Number: LM5022
Other Parts Discussed in Thread: LM5155, TIDA-020009

Team,

I have seen the below LM5022 TI design for a BUCK/BOOST based on SEPIC:
http://www.ti.com/tool/PMP40488

-Are we planning any SEPIC TI design based on LM5155?

-What would be the methodology to dimension the below system?
    Vin: 12V
    2V < Vout < 48V
    I out max < 0.5A
The design need to be flexible enough to accommodate this wide Vout range even if the efficiency is not optimum.

-Webench enables to simulate either BUCK or BOOST but not both. Also LM5155 has only a "Quick start calculator" for BOOST.
What would be the best approach to make a quick evaluation for the above specs?

Thanks in advance,

A.

  • 0
    TI__Mastermind 21720 points
    Hello AnBer,

    There is a plan to release and EVM with the LM5155 configured as a SEPIC. If you would like to see and example right now please look at TIDA-020009.

    If you have any other questions please contact me directly.

    Thanks,

    Garrett
  • 0
    TI__Intellectual 820 points

    Hi,

    Yes, there will certainly be more TI Designs uploaded that will feature the LM5155 in SEPIC applications. This part is new, so it will take some time to create new designs and application material.

    The challenge for your design will be to properly set the inductance and switching frequency. Verify the controller can provide the required FET on-time over all load/line conditions. This is shown in PSD. A starting point would be 500Khz and 20uH (coupled inductor), with a 18% - 80% duty cycle swing (2V - 48V). Size the inductance at Vin max, 48Vout, 100% ripple inductor ripple current and ~20% max load. The inductance for these conditions set the transition point when the converter goes from CCM to DCM operation at ~0.1A output current (20%). Max output power is 24W, so total losses will likely be in the 3W - 4W range, so be sure to properly size the power components (FET, rectifier, inductor) to handle these losses. 

    I recommend you download the Power Stage Designer Tool (PSD) and select the SEPIC topology. This will allow you to easily visualize waveforms and duty cycle, for specific input voltages, load current, inductances and switching frequencies.

    www.ti.com/.../powerstage-designer

  • 0 in reply to John Betten
    TI__Mastermind 30110 points

    Hi John,

    Thanks for the info you share. It is helpful to get started.

    A.