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LM5116: Low and unstable output voltage

Drazen Zbukvic
Prodigy 30 points
Part Number: LM5116
Other Parts Discussed in Thread: TPS51125, TAS5162, TAS5518

Hello. I have an issue with LM5116 giving me as thread says low and unstable output voltage. I made my pcb following the schematic from http://www.ti.com/lit/ug/snva285a/snva285a.pdf  page 11.
No matter what input voltage I use the output voltage is 7.5V and if I short DEMB resistor then it rises to about 11v. If i connect at the output any load voltage drops down. For example if I attach 60 ohm resistor at the output, voltage drops down to 4V. I did try to check all connections on my pcb and I did double check all resistors, grounds etc and cant find the cause. I did discover that when I touch pin 5 of the IC or RAMP capacitor then it starts to works as it should. The fun part is that I need to touch it with my metal tweezers, if I touch it with anything non conductive it doesn't work. I suppose it react to body capacitance. So I tried to change that capacitor from 330pF to 100pF, 270pF, 480pF, 560pF and there is small difference in output voltage but never got it over 11V

  • 0
    TI__Expert 4555 points

    Hello Drazen,

    Are you using TI EVM or custom EVM? If you are using custom EVM, then please share your schematic and layout for review. Thanks!

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    Prodigy 30 points

    Hello Ankit,

    I made my own EVM using the schematic from www.ti.com/.../snva285a.pdf page 11








    On my PCB there are some other circuits because I wanted to test everything before final build so this is just prototype pcb.
    On the left bottom side you can see TPS51125, in left upper corner is TAS5518 and on the right is TAS5162 but they are separated by pin headers so that they cant influence on each other.

    Never the less I did measure voltage on some pins so it that can help here it is.


    Without touching ramp cap and without load, input voltage = 18V

    VIN=17.99 UVLO=0.922 EN=9.41 FB=0.670 VOUT=8.56 VCC=6.62 VCCX=6.53


    Without touching ramp cap and with 60 ohm, resistor load, input voltage = 18V
    VIN=17.91 UVLO=0.832 EN=1.184 FB=0.31 VOUT=3.042 VCC=6.03 VCCX=3.042


    Without touching ramp cap and without load, input voltage = 38V
    VIN=38.67 UVLO=1.118 EN=19.29 FB=0.651 VOUT=6.36 VCC=6.34 VCCX=6.36


    Without touching ramp cap and with 60 ohm, resistor load, input voltage = 38V
    VIN=38.54 UVLO=1.102 EN=19.11 FB=0.553 VOUT=5.462 VCC=5.441 VCCX=5.458


    If I touch ramp capacitor and attach 60ohm resistor this is what I get
    VIN=17.86 UVLO=17.54 EN=9.35 FB=1.183 VOUT=11.65 VCC=11.6 VCCX=11.64

    If I touch ramp capacitor while circuit is powered with 38V my output mosfets goes bad causing short circuit.

  • 0 in reply to Drazen Zbukvic
    Prodigy 30 points

    Hello I got it to work, I did beginners mistake!

    Instead of using 330pF as ramp capacitor also marked as (331) I used 33pF marked as (330), after changing that capacitor everything works as it should.

    Thank you for your support Ankit Gupta