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LM5575: Switching frequency

Anne Vilasini
Intellectual 295 points
Part Number: LM5575

Hi , 

I am using LM5575 in our project application for 48V to 12V buck operation . I am trying to measure the switching frequency of the device with minimum load at the output (25mA max).

However the switching frequency of the device is approximately 82.781kHz.Is the switching pulse supposed to look like this at low load conditions? 

  • 0
    TI__Mastermind 25590 points

    Anne,

    Yes this is a normal pulse-frequency modulation (PFM) waveform:

    1. SW goes high, inductor current ramps up

    2. SW goes low, inductor current ramps down to 0A

    3. SW goes high-impedance (both FETs off) which causes the ringing. At the end of condition 2, IL = 0A and VL = -VOUT (SW = 0V, VOUT = VOUT). This pushes some current through the inductor which resonates with the inductance L and the capacitance in the path through the off-FET back to the output capacitors.

    -Sam