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LM2596S-ADJEVM: DC-DC Converter IC Circuit is Not Doing Anything.

DavidC482
Intellectual 280 points
Part Number: LM2596S-ADJEVM

Using the LM2596-adj switch-mode power regulator, I built a prototype circuit.  As it is very compact, parasitic/stray inductances should be minimal.  At least that's my thinking.  There is a single ground point and an output ripple filter.  Lbuck is an EMI suppressed 33uH inductor, but Lripple is a std ferrite core.

The entire circuit acts like a piece of wire.  Vout = Vin = 17V.  There is no bucking action going on.  The adjustment pot has no effect on the Vout.

Input current = 17mA.

Rout = 33ohms 

Output current should be 515mA, but that's not going to happen if the Input current is only 17mA.

I will admit that I did not take much care in handling the IC.  If it is static sensitive, it could be blown.

Any suggestions would be helpful.  If it were doing something, I could deal with it, but it's not doing anything except acting as a piece of wire.

Thanks,
Dave

  • 0
    TI__Guru 64515 points

    What is your target output voltage ?

    With the pot at mid point (25k) you would get an output of about 25V.

    In that case you will need mor input voltage.

    Otherwise I do not see anything wrong in the schematic.

  • 0 in reply to Frank De Stasi
    Intellectual 280 points

    Hi Frank,

    Thanks for taking the time to review the circuit.  The 50k pot allows for fine adjustment of whatever voltage is available.  The input voltage will be 17-18V and the output os expected to be in the range of:  1.2k*feedback current <  Vout < 17V-Vreg.

    It's possible I don't have the pot wired into the circuit as I've got shown in the schematic, but I've looked it over several times.

    Thanks,
    Dave

  • 0 in reply to DavidC482
    Intellectual 280 points

    All,

    Thanks for your support.  Problem solved.  Either the soldering heat or fabrication static ruined the chip.  Replaced the chip and now have variable output.  It's noisy, but that's to be expected with long leads on a pot and effectively not ground plane.    The feedback line is very jumpy while making adjustments, but settles down once the pot adjustments stop.  Will now see how long I can make the pot leads before it becomes too unstable.

    Thanks,
    Dave