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TPS5420 output cap ESR limits

Jan Volkering
Intellectual 520 points
Other Parts Discussed in Thread: TPS5420

Hi,

I intend to use the TPS5420 to buck a 18-22V to 6V@1A

I checked both the datasheet and the EVM usersguide

Formula 7 and 8 in the datasheet are to determine the ESR-max value. Calculating back from the datasheet example I do not understand formula 7. What is FLC? Is that the LC corner frequency that should be kept between 3-30KHZ?

That frequency should also be squared in the formula right? 

If I simply follow the EVM component values (my output is 6V, EVM is 5V) the 100uF output cap is a special case requiring 80mOhm ESR.

Is it also allowed to take another cap, for instance a 100uF Sanyo Oscon with 30mOhm ESR and add a 40 - 50mOhm external series resistor adding up to 80mOhm ESR max?

  • 0
    TI__Guru**** 191445 points
    During that time we were attempting to "simplify" design equations. A big mistake. It has been almost 10 years ago, and I doubt that I can find my original notes on those equations. I would probably just let webench design your circuit for you.
  • 0 in reply to JohnTucker
    Intellectual 520 points

    John,

    ALso in Webench the stability seem to rely on the ESR of the output cap (100mOhm)

    I have a bunch of Sanyo Oscons 100uF with 30mOhm ESR.

    Would it be OK to add a 50-70mOhm resistor in series with the output cap to get the design stable?

  • 0 in reply to Jan Volkering
    TI__Guru**** 191445 points

    Yes, these parts are designed to work wit some amount of ESR.  The internal compensation is set up that way.  Normally for voltage mode control, the loop stability is independent of the output voltage.  But for this series of devices, the voltage divider that sets the output acts as a gain stage, so the amount of ESR varies with output voltage.  You can always add series resistance to the output capacitor.  The down side is the additional ripple on the output.  It may be possible that your 100 uF Oscon with 30 mohm is sufficient.  If the inductor is 56 uH, I estimate 9.2 kHz Fco with 45 degrees PM for 100uF and 30 mohm ESR.  Generally, 45 deg. PM is considered sufficient.  Upping the ESR to 50 mohm moves Fco to 9.4 kHz and PM = 52 deg.  You could alternately add a feed forward capacitor in parallel with the upper divider resistor.  Assuming 10 kohm upper resistor, you could try 470 pF.  Unfortunately, there is no SW tool to emulate this.  You would just have to try it, then measure the loop response.  I would suggest you get an EVM to experiment with.