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LM43602 Frequency foldback

Masatoshi Oguri
Expert 4780 points
Other Parts Discussed in Thread: LM43602

Hello,

LM43602 supports low dropout operation by frequency foldback.

The eventual (low end) frequency seems dependent on the current as shown below.

What does determine the eventual frequency? 

Thanks.

Oguri (TIJ automotive FAE)

  • 0
    TI__Genius 11085 points
    The minimum frequency is determined by the internal oscillator. It is designed at about 1/8 of the RT frequency.
    -Yang
  • 0 in reply to YangZhang
    TI__Expert 4780 points
    Hi Yang,

    I am afraid that is not an answer to my question. My question was why that is not always 1/8, and depending on the load current.

    Oguri
  • 0 in reply to Masatoshi Oguri
    TI__Genius 11085 points
    It could come from the operation near dropout condition. With peak current mode control, the HS switch is turned off by (Sensed inductor current + slope compensation) >= current command. At the boundary of dropout, the current command should be at its maximum value, regardless of load. Slope compensation value is propotional to Ton. With higher load current, the sum of (Sensed inductor current + slope compensation) reached current command max value at lower Ton. With lower load current, since the DC current is lower, it allows longer Ton by the peak current mode control loop, thus the minimum frequency is lower than higher current conditions. I hope this answers your question.

    -Yang