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LM3150 Disabled State

James Rosenthal
Prodigy 30 points
Other Parts Discussed in Thread: LM3150

Hello,


I have designed a 5V supply using the LM3150, from an input voltage of 20-42V.  The supply has been working correctly, however, when I drive the enable pin low, the output voltage seems to be approximately 0.6V.  Does anybody know the reason that this would occur, or if this is expected behavior?

Thank you,

James

  • 0
    TI__Genius 16025 points

    Hi James,

    This is not expected behavior. When the part is disabled the output voltage should reflect the lowest potential based on your input. Which usually is Ground. Check to see if you have your load operating correctly. My guess is that your output capacitor is not being discharged completely after you switch it off.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 30 points

    Hello Anston,

    Thank you for the response.  On the output of the supply I have 2150uF capacitance.  Even operating without a load, and starting from an input voltage of 0V, the output will still rise to around 300mV once I turn on the input voltage.  When I attach our system load, it rises to around 600mV.  Based on your comment, should I consider adding an output circuit to discharge the capacitors when the supply is disabled?

    Here is a picture of the design:

    Thank you,

    James

  • 0 in reply to James Rosenthal
    TI__Genius 16025 points

    Hi James,

    In the case where no load is attached, it may be that current from the Ron input leaves the IC from the SW or ILIM pins. For the purpose of test, temporarily remove the Ron resistor and reapply power with the part disabled. Under this condition, do you still see 0.3V in the absence of a load? If this resolves the problem consider the following solution:

    Add a 2N7002-7-F small signal MOSFET in series with the Ron line. Connect Drain to the bottom of the Ron resistor and connect source to the Ron input. Bias the gate with Enable. This should only supply Ron when the circuit is active and hopefully resolve the problem.

    The case of adding a load and experiencing 600mV out under non operating conditions is puzzling. Are there any other paths from Vin to the 5Vout that might contribute a leakage path to this? 

    Please let us know if this solution works.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 30 points

    Hell Anston,

    Thank you for the suggestions.  I found an alternate current path from another voltage source on our load that was causing the 0.6V on the output of the supply.  When I had tested no load, I had not given enough time to let the output capacitors fully discharge, so I was seeing a voltage of 0.3V.  This was actually degrading at a rate of around 1mV per 1.5-2 seconds, so I didn't recognize it at first.  When I later ran the same test with no load, and ensured the capacitors were discharged, the output voltage was approximately 0V.  

    In the end, it does turn out to be an issue with our load, and not the power supply design.  Thank you for your assistance as I looked into this issue.

    Best Regards,

    James