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LM2990: Impedance between Input and GND very low when assembled on board

Hari Rathod
Prodigy 20 points
Part Number: LM2990

Tool/software:

I have used two LM2990 ICs on my design to convert -24V to -12V and -5V. 
After assembly, upon impedance measurement on supply lines, I observed that the impedance between input (-24V) and GND terminal is 0.2 ohms.

I have also checked removing the input capacitor of one the regulator, but still the same impedance. 
Please advise if this is normal or I need to look for any other problem.

  • 0
    TI__Mastermind 37025 points

    Hi Hari,

    Please make sure that your multimeter probes are connected properly.  Since this is a negative LDO you need your positive probe on GND and your negative probe on Vout.  Otherwise you may inadvertently be measuring the nonlinear impedance of the internal ESD diodes.

    Otherwise, no, you should not have 0.2 ohms from Vin to GND.  If checking the probe connection does not fix the issue then you may have inadvertently placed a solder short underneath the LDO. You may need to reflow the LDO IC and thoroughly clean the reflowed area with distilled water and isopropyl alcohol, then retest.  If there is too much solder underneath the LDO then you may need to remove the LDO, remove the solder from the PCB, clean the board, and resolder the LDO to the board.

    Thanks,

    Stephen