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query about output current for buck boost IC

Hiren Thumar
Intellectual 470 points
Other Parts Discussed in Thread: TPS63020

Hello,

we have a two input power supply one is solar 5V Mppt and second is 3.7V battery but our system require peak current 3.7v@2A. Here we select buck-boost converter TPS63020.

As per datasheet, We have checked in Buck mode provide 4A and Boost mode provide 2A.  For buck mode, it works properly. We have query in boost mode. 

 Please check below calculation regarding max output current for boost mode.

Vout=3.7V; Vin=3.3V

D=3.7-3.3/3.7; D=0.097

Ipeak=(Iout/(n*(1-D)))+((Vin*D)/(2*f*l))

Iout=2A

n=90%

Vin=3.3V

f=2.5MHz

l=1uH

we put this value in boost mode equation.

Ipeak=2.82A

Does our understanding is correct and TPS63020 can works properly including component value.

Thanks & Regards,

Hiren

  • 0
    TI__Expert 8420 points
    The formulas are correct (chap.8.2.2.1 of the datasheet). I got slightly different numerical values for the same parameters's value:
    D=0.108
    Ipeak=2.56 A

    This peak current is the typical peak current in the inductor for a 2A load in boost mode. As stated in the datasheet the TPS63020 can operate at 2A load in boost mode with a 3.3V input voltage. You must choose and inductor that could withstand this peak current.

    You can find further details on design calculation for buck boost in this app note: www.ti.com/.../slva535a.pdf
  • 0 in reply to Sabrina
    Intellectual 470 points
    Hello Sabrina,

    Thanks for providing guidance. we have selected 1uH@3.8A inductor considering buck -boost mode for our 2A system requirement.

    Also we need suggestion for improved load transient performance.

    As per datasheet we found improved load transient performance in section 8.3.1. This circuit is use for decrease voltage fluctuation in 3.3V@2A system.
    But we required circuit for 3.7@2A system. so please suggest us resistor,capacitor values between Vout and FB pin.

    Thanks & Regards,

    Hiren
  • 0 in reply to Hiren Thumar
    TI__Expert 8420 points

    Yes you can improve the phase margin with the feedback network.

    Attached you can find a screenshot of the calculation I did in Mathcad to plot the phase and gain of the feedback network. You can use these calculations to play around with the values and see how each parameter influences on the phase. It can help you to find the right combination of components for you load profile.

    I can send you the Mathcad file if you want.

    .

  • 0 in reply to Sabrina
    Intellectual 470 points
    Hello Sabrina,

    Thanks for reply.
    Please suggest suitable resistor and capacitor value for 3.7@2A circuit. and also provide Mathcad file.

    Thanks & Regards,

    Hiren Thumar
  • 0 in reply to Hiren Thumar
    TI__Expert 8420 points

     These values strongly depend on your load profile and the desired transient behavior you need. 

    I would suggest you to start with the values of the datasheet R=68k and C=4.7pF, then increase or decrease the values of the components, according to your specifications for load transient behavior. 

    Normally increasing the value of the capacitor moves the crossover frequency down and therefor increases in phase margin and consequently the transient behavior. But you have to test it and see which values are the best for your circuit.

    When you want to modify components' values in the feedback divider, first use the mathcad file to have an idea on how they influence your phase margin. 

    I sent you the mathcad file by mail, as I could not post it here.

  • 0 in reply to Sabrina
    Intellectual 470 points
    Hi Sabrina,
    Thanks for suggestion.
    We received your math-cad file.
    We want to check it but we are facing some problems to open this file.
    Can you please suggest us the software(free version) which is useful open this file and finalize the value of R and C .
    Please guide us in mathcad.
    Thanks & regards,
    Hiren
  • 0 in reply to Hiren Thumar
    TI__Expert 8420 points

    I don't know any other free software to open this file.

    I already posted a picture of this Mathcad file to show you the calculations, so that you could understand how the parameters influence.

    Here is an overview on how the parameters influence you loop characteristics (For this case we can neglegate C2c and R2C)

    Either you could use any other software that supports formal calculations and complex numbers you use the equations I gave you. Or you could start your design with the same values given in the datasheet for the feedforward circuit

    For 3,7Vout  you could use R1=339kohm, R2=53kohm, C1c=4.7pF  and R1c=63kohm (graph you see above is given for these values)

    After measuring your circuit performance with these values first, I can help you optimize them if needed.