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LM43603 Output Capacitor Calculation

Pat
Intellectual 460 points

I'm calculating the minimum output capacitance of a a design using the datasheet guidelines, document SNVSA09B, Section 9.2.2.5 'Output Capacitor Selection'.

The resulting values I'm getting are in mF and even if I input the design example values (into Eq 19) I can't get even close to the 141uF result of the design example. I had another engineer here check to make sure I wasn't doing something really stupid but we both ended up with similar results.

Vin = 12V

Vout = 4V

Iload = 2.4A max

Resulting values:

L = 4.7uH

D = 0.33

Iripple = 430mA

Co = ??

Does anybody have any idea where I'm going wrong with these equations? There are a little different to what I'm typically used to seeing on TI datasheets for output capacitance. I've noticed a slight circular dependance in that the output voltage ripple is used to calculate the capacitor value (Eq 19) and the capacitor value is used to estimate the output voltage ripple (Eq 17 and Eq 18).

  • 0
    TI__Genius 15810 points

    Please give us your desired switching frequency and the desired max undershoot at the output during a load transient. 

    As such for your inputs of 12Vin to 4Vout at 2.4A, if I assume a 500kHz switching frequency and 3% undershoot, then my minimum output capacitance requirement is 110uF. 3% undershoot would mean an absolute value of deltaVout of 0.03*4 = 120mV.

  • 0 in reply to Akshay Mehta
    Intellectual 460 points

    Sorry I thought I put down my switching frequency, 1.3MHz, 3% undershoot is fine.

    It looks like the issue I was having was that I was incorrectly deriving a value for deltaVout, if I use your method (using the % ripple) then I get much more sensible numbers.

  • 0 in reply to Pat
    TI__Genius 15810 points

    Ok. You can also use Webench. The software uses the same equations as the datasheet.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Intellectual 460 points

    Thanks, I also used Webbench but  I needed to document the calculations verbosely.

    Related, I also struggled with the feedforward capacitor calculation in Webbench, if I change the default feedback resistors to 10K and 3.48K (instead of 100K and 348K - that is for a 3.9V output) it doesn't appear to recalculate the feedforward cap requirements and warns me that anything over 9.1pF is not recommended, the calculations meanwhile suggest a 1.5nF capacitor be used.

    Any idea on that?

  • 0 in reply to Pat
    TI__Genius 15810 points

    Webench, at this time, doesn't re-calculate any components for a manual change in a different component. They are working on that feature.

    It would be immensely helpful if you send the exact design condition and steps you followed that caused that message to appear. I will forward it to the folks working on Webench and have them look at it.

    Please refer to the application note (http://www.ti.com/lit/an/snva718/snva718.pdf) to re-calculate Cff based on the change in feedback resistors, output voltage and output cap values.