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LMR16020: Abnormal switching frequency

William Zhang2
Intellectual 905 points
Part Number: LMR16020

Hi Team,

Customer report there is some abnormal switching frequency issue when using LMR16020.

They set the output 9.11V, and the load is the constant resistance mode of the electronic load, switching frequency is 620kHz.

When the input voltage is drop down to 10.4V, the switching frequency will change to 103kHz, and the output voltage will change to 8.95V.

The question is why will the switching frequency change when the input voltage drop down? What's the operation mode of LMR16020 at this time?

Thanks.

  • 0
    TI__Genius 10715 points

    Hello William,

    The converter is likely operating in dropout where the difference between input and output voltage is small. (See figures 7 and 8 in the Datasheet)

    The frequency must reduce in order to ensure that the BOOT capacitor can recharge in order to turn on the high side FET.

    Let me know if you have additional questions.

    Regards,

    Harrison Overturf 

  • 0 in reply to Harrison Overturf
    TI__Intellectual 905 points

    Hi Harrison,

    Can we restore the switching frequency to normal only by increasing the input voltage?

    What is the minimum requirement between input voltage and output voltage of LMR16020?

    Regards,

    William Zhang

  • 0 in reply to William Zhang2
    TI__Genius 10715 points

    Hi William,

    The switching frequency will return to normal when the input voltage is increased. 

    The minimum requirement between input and output voltage is given in the maximum duty cycle spec in section 6.6 (97% at 200kHz fsw), this gives the minimum off time (toff) of 150ns. 

    Use the following equation to estimate the minimum input voltage:

    toff = (1-D)*Tsw

    where D=Vo/(Vin*efficiency)

    Tsw = period of desired switching cycle 

    toff=150ns

    efficiency = ~90% (estimation) 

    I calculate minimum Vin of around 11.1V.

    Try increasing the input voltage and let me know whether the output voltage come back within regulation.

    Regards,

    Harrison Overturf