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TPS40200EVM-002 for low load application

Hi,

I tested this board for ~200mA load with a 22uH inductor. It worked fine. My question is how to improve the current/voltage ripples for low load conditions. I went through the step by step calculations. it seems that the inductor has to be very large to achieve the low ripple current, using the equation 15 on page 22 of the data sheet. Is this right?

My limitation is the inductor has to be less than 22uH.

So what else I need to change to improve the performs and stability.

My second question is if I have two of this switchers on the board (one for 5V and one for 3.3V). Is it true to synchronize the switching of the two is better?

Thanks,

F.Z.

  • Hi Fubin,

    Thank you for your detailed feedback of the problem. I will direct your question to the specialized forum that deals with this product family. This forum is exclusively for Simple Switcher products.

    Thank you,
    -Anston
  • EQ 15 is the equation you typically use to choose the inductor It only works if you are in CCM though. If you set Ipeak to be 20 to 40% of that low output current, you will get a large inductor value. Choosing a small inductor value will cause you to operate in DCM. If you absolutely need a small value inductor, then you will probably need to increase the output capacitance and the output cap ESR to lower the ripple. Synchronizing 180 degrees out of phase will reduce input ripple but not affect output ripple. On the EVM if you change Lout or Cout you will need to adjust the compensation as well.
  • Hi John,

    Thanks a lot for the detail explanation!

    My application and modifications are:

    Vin: 25-40V

    Vout: 3.3V or 5V

    Load: 50mA-200mA

    Lout: 22uH (2.8A and 174mohm)

    Cout: 22uF X 3 ceramic and 33uF tantalum and a few small decoupling caps. ---- This should push the Cout and Resr zero to way higher freq.

    PMOSFET: ZXMP10A17E6QTA from DIODES

    Also, I lower the cap C7 from 33pF to 27pF to increase the bandwidth of EA feedback loop. Are these changes fine? I think this should work but want to optimize it a little bit in terms of transient response and phase margin.

    The eq.27 in datasheet shows the gain should be -7.6dB for overall gain of 0dB@35kHz. My calculation got 4.55dB with Zfs ~182kohms. For the EVM-002 board configuration, I got -5.1dB with Zfs ~58kohms. What could I have missed here?

    Thanks again for the help,

    F.Z.