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BQ24640: Switching Frequency abnormal

user5724885
Prodigy 10 points
Part Number: BQ24640

I use an aux power 30v supply bq24640's VCC to avoid sleep mode, and output voltage design for 28.5v.

When Vin < 28.5, the idea Buck's output should be equal Vin, and the PWM should keep switch frequency with maximum duty.

But When Vin small then 28.5V, the switching frequency down to 0.65KHz (typ is 650KHz)

How do I avoid this situation?

  • 0
    TI__Guru 57551 points
    Hi,

    The behavior is the nature of buck converter when running into maximum duty ratio limitation. The charger is basically a step-down DC-DC (buck) converter. It requires the input voltage to be higher than the output voltage with some headroom for normal operations. Please refer to the paper below for the details. Users should avoid maximum duty ratio operations in their designs.

    Extract maximum power from the supply when charging a battery
    www.ti.com/.../slyt569.pdf

    Thanks!
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    TI__Guru 57551 points
    We haven’t heard back from you for a while, the thread is considered solved and closed. Thanks!
    Please click "This Resolved my issue" button if this post answer your question.