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TPS54260: TPS54260 ripple current should be >150mA?

Sammy Peiren62
Intellectual 310 points
Part Number: TPS54260


Hello,

I'm using the TPS54260 to generate three rails in my system. The voltages are:

  • 5V-3V3 (300kHz)
  • 12V-5V (450kHz)
  • 12V-3V3 (675kHz)

I followed the design procedure as described in the datasheet, however for the 12-5V buck converter I'm a bit confused about the minimum ripple current requirement.

I have calculated the minimum inductor to be 47uH (choose 57uH) with equation 28. However, this seems to conflict with the minimum ripple current requirement. I only get 120mA with equation 29, but the datasheet mentions "Since the inductor ripple current is part of the PWM control system, the inductor ripple current should always be greater than 150 mA for dependable operation."

Equation 29 seems to suggest I should choose a lower inductor or a lower frequency, but this exactly the opposite of the requirements set by equation 28.

I have generated a design with the switcher pro software and it also suggests the same values as in my calculations.

What exactly is the consequence of choosing a ripple current <150mA?

Kind regards,

Sammy

  • 0
    TI__Expert 3870 points

    Hi Sammy,

    For your calculations, what load current (Iout) did you use?  You should always use the max rated current of the device for your calculations.  Using equations 28 and 29 and Iout of 2.5A, I get a min inductor value of 8.6uH and a current ripple of 750mA.  Please use our WEBENCH tool for design and simulation since the models are up to date and have been fully correlated with real silicon behavior.  WEBENCH recommends 10uH for your 12V to 5V design when selecting 450kHz as the operating frequency.

    Best Regards,

    -JP

  • 0
    TI__Genius 11085 points
    When you calculated using equation 28, what KIND value did you use, and what Iout value did you use?
    With ceramic output capacitors, you can use KIND=0.3, as suggested in the datasheet. The Iout value should be the max rated current of this IC, not the application's max output current.
    With these two values, the equation 28 yields Lomin = (12-5)/(2.5*0.3)*5/(12*450000)=8.6uH, then a 10uH should be a good choice.

    The suggested 150mA minimum ripple current is to maintain certain amount of signal to noise ratio in the control loop. Peak current mode relies on the peak current information to turn off the internal switch. If the ripple is too small, noise could interfere with loop stability.

    Regards,

    Yang