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LMR50410: FPM mode: actual switching frequency at low output currents

Francesco Anzalone
Prodigy 110 points
Part Number: LMR50410
Other Parts Discussed in Thread: LMZM23600

Hello,

I'd like to use LMR50410 in my system to provide 5V supply to all components.

My specification are:

  • Vin=24V
  • Vout=5V
  • Iout=0.1A

I want to use the scheme described in the typical application (section 9.2 of the datasheet) and I need to size the output inductor and capacitor.

In section 8.4.4, it is asserted that PFM mode is activated when minimum inductor peak current of 0.3A is reached. Since output current in my case is 0.1A, I'm assuming that LMR50410 will always work in PFM mode in my design, is it correct? If so, how is it possible to estimate the actual switching frequency in order to size output inductor and capacitor in the best way according to equations 9 and 12 in the datasheet?

More generally, is LMR50410 the best choice for such small output current?

Thank you

  • 0
    TI__Mastermind 19901 points

    Hello Francesco, 

    You should use the CCM switching frequency (700kHz) when sizing your inductor. 

    In your case, if you are only pulling 0.1A from the output, the part will shift into PFM mode and reduce the 700kHz switching frequency to save some power.

    If you want to check what the frequency would be as you move the load up and down, you could use webench to see the calculated PFM frequency for the particular condition. Here is a snapshot of where you would change the load current and hit calculate:

    If you don't want the frequency to shift down at light load, you could use the forced PWM option of the part. In that case, the switching frequency will be constant across the entire load. 

    If you need a simple 24V to 5V converter for 0.1A load, you could consider the LMZM23600 power module. It has a fixed 5V option so all you would need is an input capacitor and an output capacitor. It also has a MODE pin so you can pick between PFM and FPWM operation if you would like. 

    Cheers, 
    Denislav