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LM2596 ADJ Vout drops with a Low Load

Laura Vorster
Prodigy 20 points
Other Parts Discussed in Thread: LM2596

I'm using the LM2596 ADJ with a VIn of 18V.  The schematic for my circuit is shown below. When loaded down with a load of greater than 35 ohms the chip regulates the output voltage, but under 35 ohms the output voltage drops. Why is it doing this?

k

  • 0
    TI__Guru 64385 points

    What is your input voltage range and load range?

    You may be running into current limit or duty cycle limits. 

    The layout is also very important.  Please send a snapshot of your layout.

    I am not sure what your top FB resistor value is?  10.27k ?  If so that would give a Vout of 13.8V.  I have attached a Webench design

    for this output and some assumed inputs and load.  Webench uses a much bigger inductor; you might want to try that.

    webench_design_433359_192_340580978.pdf

  • 0 in reply to Frank De Stasi
    Prodigy 20 points

    The input voltage range is 17-22V. The load range is 10mA to 1.2A. The output voltage(13.75) starts to sag at 343mA.

    There are two resistors being used for the FB resistor. The combined value is 10.27k. The inductor I'm using is a 33uH as called out in the TI  spec sheet for this chip. We're going to purchase the 120uH inductor to try that as you have suggested.

    The lime green wire that spans the proto-board from bottom to top is the input voltage line. The load in the picture is 1k.

    Laura

  • 0 in reply to Laura Vorster
    TI__Guru 64385 points

    I would hold off on buying the bigger inductor for now.  It looks like that your layout may be the issue.  A solder-less breadboard is not optimum for a DC/DC

    converter.  Please send your shipping address and a phone number to the following email:  simpleswitcherapps@ti.com and I will ship you a PCB that you can

    populate and test your regulator.  You may have to bend the leads a little and cut the leads of the through hole components short to fit on the PCB.