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LMR23625 Frequency fold back and other

K.Hamamoto
Guru 12155 points
Other Parts Discussed in Thread: LMR23625

HI 

I have some question about LMR23625.

1. Regarding the operation when the pulse width is hit to min on pulse.

    The pulse skip will be happen when the pulse width is hit to min on pulse. or   continue to keep the min pulse at PWM?

2.Regarding frequency fold-back.

   How is the frequency reduced ? Step by Step(e.g. 1/2 -> 1/4 -> 1/8)? or linearity?

   The following data is shown that the frequency is reduced at linearity.

 

3. Regarding the boot resistor at the series of Cboot.

    How much the Rboot should be? (Maximum)

Please let me know if you have any question.

Regards,

Koji Hamamoto

  • 0
    TI__Genius 15810 points
    The frequency is folded back only when the min on time pulse width is reached. At lower input voltages, the frequency would not be folded back.
    The foldback is mostly linear.
    The Rboot is generally used to slow down the switching rise and fall times. This helps with reducing the EMI generated, but hurts the efficiency as the switching losses increase. Are you trying to address an EMI problem? If not, you don't need Rboot.
  • 0 in reply to Akshay Mehta
    Guru 12155 points
    Hi akshay-san,

    Thank you for your support.

    I have additional question regarding question 3.
    I understand that the Rboot is a trand-off between switching noise and efficiency. Our concern is that the too much resistor could occur the UVLO at BOOT pin if the BOOT has UVLO value.

    So that, please give us the maximum value of RBOOT if the RBOOT has a limitation.

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Intellectual 2790 points

    Koji san,

    1. Device will skip pulses to maintain the regulation when the ton_min is triggered. So it will stay in regulation but has larger ripple.

    2. Frequency decreases linearly once the toff_min is triggered at low VIN.

    3.We don't have data for Rboot max value, also I believe this may vary with different configuration of the circuit. So from our experience, a resistor lower than 20ohm is preffered.

    BR

    Luke