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LM5149-Q1: Output power limit

Ben
Intellectual 400 points
Part Number: LM5149-Q1
Other Parts Discussed in Thread: LM5149,

Tool/software:

Hi TI team.

The LM5149 reaches its power limit at a load of 20A.

- input : 54V

- output : 12V

I designed the LM5149-Q1 according to the attached quick design guide excel file.

QuickstartDesignGuide

I added R(49.9)-C(47pF) filter to the current sensing line(ISNS+ to VOUT).

How can I modify design to reach 20A load at 60Vmax condition?

Thank you

  • 0
    TI__Genius 11315 points

    Hi Ben,

    The current limit is set at max output current + Inductor ripple - thumb rules is to set ~140% of max current - therefore, try to set OCP at ~28A for 20A design. The recommended sense resistor will be 1.8mOhm. 

    The inductor value is also a bit too low in your design. 

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Intellectual 400 points

    Hi Taru

    What inductor value and switching frequency do you recommend for a 36~60V input range?

    Thank you.

  • 0 in reply to Ben
    TI__Genius 11315 points

    Hi Ben,

    Ben said:
    What inductor value

    The quick start calculator will give you the recommended values. Inductor value is chosen in such a way that the inductor ripple current is ~30-50% of output current. 

    Ben said:
    switching frequency

    Switching frequency impacts multiple aspects of the overall design. Broadly speaking Higher switching frequency gives a smaller solution size with
    better dynamic performance however, it comes at the cost of design efficiency due to increased switching losses. Again, the quick start calculator will give you an idea of the expected losses and efficiency for your design. 

    I see very high losses in your design (~6W and ~4W) of power loss in the FETs at full load. There are a couple of things you can do:

    1. Reduce the switching frequency (You will have to change your passives accordingly)

    2. Use FETs with lower Rdson. (especially the low side FET)

    Thanks,
    Best Regards,

    Taru