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TPS65131-Q1: Having a problem with Inverting Buck-Boost

Ramazan Yaman
Prodigy 50 points
Part Number: TPS65131-Q1

Dear expert,

I have a problem with inverting Buck-Boost converter. I tried to implement the circuit which is explained in http://www.ti.com/lit/an/slva721a/slva721a.pdf and realized that the output voltage was around -12.5V. I used the feedback resistor values as R3= 1MOhm and R4= 121.2KOhm. The output voltage generated was not -10V, it was -12.5V. What could cause this problem? Note that other values such as output capacitor, input capacitor are all used according to the values given in the example which is given above.

Best regards,

Ramazan Yaman 

  • 0
    TI__Expert 6330 points
    Hi Ramazan,

    I have notified our expert regarding this topic. Please expect a response by 5/20/19.

    Thanks,
    Aaron
  • 0
    TI__Mastermind 18550 points
    Hello Ramazan,

    Are rest of the component values according to figure 8 in the datasheet? If they are, you may want to insert a 51.1K ohm resistor in series with feed forward capacitor C10 such that one end of the resistor is connected to C10 and the other end to the net VNEG (reference designator and net name per figure 8 in the datasheet). We have noticed that in some noisier designs, there is excessive noise coupling to FBN pin and that can cause the inverting converter to be out of regulation. Putting this series resistor will reduce that noise coupling and improve regulation.
    I can not think of anything else that will cause the output voltage to be -12.5V instead of -10V.
    I hope that this suggestion resolves the issue you are seeing.
    Kind Regards,
    Liaqat
  • 0 in reply to Liaqat Khan
    Prodigy 50 points

    Hello Liaqat

    The recommendation caused us to have -10V as output voltage instead of -12.5V, so problem was solved thanks to you. After that I tried to get -5V output voltage for an input voltage value of 5V. Up to 4V as the input voltage, an output voltage of -5V is generated without any noise. However, the circuit is out of regulation for input voltage values, which are greater than 4.1V. Looking at the scope, we see that for input voltages, which are greater than 4.1V, the output voltage goes up to -6.5V approxmately in 1.2ms and then the circuit cannot regulate itself. The output voltage swings from -2V to 0V, there is no -5V. Do you have any recommendation for this problem?

    Best regards,

    Ramazan Yaman