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TPS65131: Help with component calculations

Andrew Coad
Expert 1625 points
Part Number: TPS65131

Hi,

I'm working through the equations in the data sheet to figure out inductor values and output capacitor for a 3.7V input and +/-15V outputs.

To make sure that I understand the parameters of the various equations I have been trying to calculate exactly the same values as the data sheet example.  For equation 5, the only way that I can come out with the same inductor value of 5.1uH is if delta I(L-P) is equal to 20% of 90% of max switch current of 1.95A. So this represents the worst case load such that the max switch current is somewhat less than the specified maximum. IPOS is not used in equation 5 and is unknown at this point.  The inverter inductor value also works out roughly the same using the same approach for delta I(L-N).

Using the same approach for my specific case, I get inductor values close to 6.8uH which I've trimmed back to 4.7uH on the grounds that my current load will be very light. So far so good.

However, for the output capacitor, IPOS and INEG are used in equations 7 and 8 respectively but these two values are unknown.  To get an output capacitor value of around 12uF for equation 7, you need an IPOS value of 218mV.  Where does this value come from?

If I calculate I(L-P) from delta I(L-P) as above and then retrofit that back into equation 3, I get an IPOS of 353mV which would represent the maximum load current such that the switch current does not exceed 90% of stated maximum. This all makes sense but, again, where does the value of 218mV for IPOS come from?

If I can figure out the example equations then I can figure out my particular case.  Any guidance will be welcome.

Kind regards,

AC

  • 0
    TI__Expert 4940 points

    Hi AC,

    I have notified our expert regarding this topic. Please expect a response by 6/10/20.

    Thanks,

    Costin

  • 0
    TI__Mastermind 18550 points

    Hello Andrew,

    I would recommend to use the inductor and capacitor values shown in the typical application circuit (L=4.7uH, Cout=22uF and Cin = 2 x 4.7uF) and that will cover a wide range of conditions from input voltage of 2.7V - 5.5V, output voltages up to +/-15V and light to heavy load. Please note that besides Cout calculation based on desired ripple, there are other factors such as loop stability that can be impacted if minimum Cout values calculated by equation 7 and 8 are used. Most validation of TPS65131 device was done using values shown in the typical application circuits so my recommendation to stick with these values.

    As for equation 7, you are correct that there is no IPOS value provided in the description. For calculating minimum C4=12uF, I used lowest possible switching frequency of 1250KHz, ripple of 10mv, VPOS of 10.5V, Vin of 2.7V and that yielded IPOS of 202mA. So the 12uF value was calculated for IPOS of approx. 200mA.

    I hope that this information is helpful.

    Kind Regards,

    Liaqat