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TPS65131: Typical Application Help

Courtney Smith
Prodigy 10 points
Other Parts Discussed in Thread: TPS65131

Hello,

I am trying to use (3x) TPS65131 split-rail DC/DC Regulators in my senior design project to supply 6 different voltages off of my power board (+9V/-9V , +5V/-5V, and +3.3V/-3.3V).  I am using the typical application set-up as shown in the datasheet with the necessary adjustments (R1, R2, R3, R4, C9, and C10) needed to achieve the desired voltage outputs as explained in the datasheet. However, I am not including the transistor in my design, but I do not think that is my problem.  After calculating the R and C values for the corresponding voltages and using components that are fairly close (within 5%), I am not getting what I am trying to achieve.  For the +5V/-5V system, I am getting an output of 15V/-11V respectively...  For the 3.3V/-3.3V system, I get +5V/-3.2...  And for the  +9V/-9V system, I get an output of +8V/-11V.  Some of the outputs are close to what I need, however when I try to compensate the difference (tuning the resistor values) to get closer to my desired outputs I have to change the resistors drastically (change R1 from 1Mohm to 10Mohm) in order to reach +9V for example.  The main issue here is that when I change R1 to 10Mohm, the Positive Feedback Regulation exceeds 2V which is not in range as stated in the manual.  Also, When I connect a load to either the +9V or -9V output, the voltages on the outputs drop to +5V and -0.2V, respectfully.  I cannot seem to figure out why I am getting these inconsistent results.  What could I possibly be doing wrong? HELP!

I have attached my Altium schematic, including component values, with this post.

Thanks!

Courtney Smith

Split-Rail Converter Forum Post.pdf

  • 0
    TI__Expert 8010 points
    Hello Courtney,

    The TPS65131 converter consists of an asynchronous boost-converter to generate the positive rail and an inverting buck-boost converter to generate the negative rail.
    Because of the nature of every asynchronous boost converter (www.ti.com/.../slva061.pdf ) the output voltage is always bigger than the input voltage. In a simplified way the output voltage can be described as VOUT = VI / (1-D), with D as the duty cycle that can only be between 0 and 1. The minimum duty cycle of this part is at 12.5 % and that gives a minimum (idealized) output voltage of VOUT = 5 V/(1-0.125) ~ 5.7 V. Saying that you will not be able to generate +5V and +3.3V with this part with 5V on the input.
    The reason why you see +15V on the 5V rail is that the feedback divider always sees
    VFBP= R7/(R7+R6)*(VI – VF) = ¼ (5V – 0.5V)~1.1 V and that is less than the reference design. In turn the controller starts to regulate to a higher voltage as it thinks that the output voltage is too low. Similar happens with the 3.3 V rail. But here the voltage on the FB- Pin is already too high and therefore the converter does not charge up the voltage and you only see the input voltage (minus the voltage drop of the rectifier diode) on the output.
    Regarding the inverting buck-boost converter it seems to be quite unusual that you see the correct value on the -3.3 V rail although the resistors of this rail do not fit the equations as the voltage should be VNEG_3.3 = Vref (Rx4/Rx3) = 1.213 V(830 k/47.8 k) = -21 V. But for me the -5V and the -9V rail seems to be ok. Are you measuring the output voltages with a scope or a multimeter? Can you show me some scope plots that show the start up behavior of all the negative rails?

    I also noticed that you only have one big capacitor on the output, can you try to put 4x 4.7 uF as this can affect the stability of the device. And one more thing is it true that you put electrolytic capacitors on the all the rails? If so than C8 is connected in the wrong direction, did that not cause failures?
    Thank you,
    Best Regards.
    Ilona