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LM5116: Negative Output Not Working

Ryan Bowe
Prodigy 20 points
Part Number: LM5116

Please see schematic and let me know if you see any issues.  I tried to use the Web bench to design this circuit but I'm getting 0.6V as the output. Not the -15v needed.  Thanks! -Ryan

  • +1
    TI__Guru 58371 points

    Hello Ryan

    Please check these items. 

    • SERVO_ENABLE_5V signal should be 5V between the EN pin and the AGND pin, not between the EN pin and the VOUT pin
    • Please check if hiccup mode protection is triggered. Please check the UVLO pin voltage
    • Polarities of many capacitors are wrong, but I guess there is no issue since they are all ceramic capacitors.
    • I cannot read the R224 value clearly. It should be 10.4k if R223 is 118k.

    - Eric Lee ( Applications Engineering) 

  • 0 in reply to EricLee
    Prodigy 20 points

    Thanks for the reply Eric. As I was checking UVLO voltage I saw that it was very close to the 1.2V threshold. Knowing we designed the voltage divider to a higher voltage I began checking things.  It turns out that the divider was meant to have +24v on one side and -15v on the other to create the UVLO voltage above 1.2V  Since -15V is not present when the board powers up our voltage divider wasn't really working and causing the chip to go into standby.  I've adjusted the resistor values and have the chip working.  Thank you for the quick response!  

  • 0 in reply to Ryan Bowe
    TI__Guru 58371 points

    Thanks for choosing TI. Please feel free to contact us if you have any questions.
    - Eric Lee (Applications Engineering)