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Original question:

LM2678: LM2678 Problem

LM2678: 5.9V output with no load on a 5V Fixed Switcher

Stephane gautier
Prodigy 10 points
Part Number: LM2678


Hello, i have some experience with the LM2678 in almost all it's package versions,

my last design include a LM2678SD-5.0/NOPB which does not require Feeback resistors to program the output voltage.

Vin 18V ; Iout 1.5A,  Cin 2x15µF Tant+470nF MLCC ; CB 10nF MLCC , Cout 2x100µF Tant, Schottky 3A NRVBS330T3G, Inductor 33µH/3.7A MPX1D1040L330

The issue is we measure 5.9V Output with no load.

as soon as we connect the load, the voltage drops to a normal 5V. what could cause this extra 900mV ?

i have confirmed the part values with WE-Bench, all within tolerances, Sch/PCB below.

Thank you.

  • +1
    TI__Guru 74475 points

    Hello

    I noticed that the FB connection is taken close to the inductor output.

    It would be best if it were connected near C45 to avoid any noise coupling.

    You can cut the trace to the FB pin and connect a short wire to the + side of C45 

    as a test.

    Also, please try to avoid the use of thermal reliefs on the power components 

    and power planes.

    It may be that this device needs a minimum load.  You may need to experiment with that.

    Thanks

  • 0
    Prodigy 10 points

    Hello, thank you for your quick answer, i agree with all your suggestions, the next revision will benefit from that ;)
    here's what we found :
    - With Vin under 16V, the output shows a stable 5V, even with no load.
    - above 16V we had to put a "bleeder-like" resistor of 10Kohm across the output.
    it's now working as expected.
    Thank you again.