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TPS92520-Q1: COFT‘advantages vs. COT

21710070
Prodigy 70 points
Part Number: TPS92520-Q1
Other Parts Discussed in Thread: TPS922052

Hi team，

I find lots of CC/CV BUCK coverters utilizes constant-on-time control (COT), while TPS92520 is featured with constant-off-time control(COT). Would you please help explain why TI chooses COFT? Thanks!

  • 0
    TI__Mastermind 18535 points

    I am not sure where you got your information, but we use an adaptive on-time control topology.  

  • 0 in reply to fhoude
    Prodigy 70 points

    Sorry for my mistake. Just as you said, TPS92520 uses adaptive on-time control, and I'd like to know what's the difference between adaptive on-time and adaptive off-time because I find some BUCK LED drivers such as TPS922052 use adaptive off-time control.

  • 0 in reply to 21710070
    TI__Mastermind 18535 points

    The TPS922052 is a hysteretic controlled converter and it basically regulates the peak current, and not average current like the TPS92520, which is a current mode control device.  The TPS92520 has a closed loop and regulates any error out of the average current, which hysteretic converters don't.  The average current is a function of the output ripple which is controlled by the off time.  That off time doesn't take into account inductor tolerances and thus the average current isn't tightly controlled and has lower regulation current accuracy.  Hysteretic control has a very fast transient response, which is the benefit of this type of control.  If you need very fast transient response then you would likely go to a hysteretic control.  If you need good regulation then you would go to average current mode control, which is important to avoid color shifting.  Shunt fet dimming for high PWM frequencies (>1-2 kHz) and wide range duty cycles needs hysteretic controllers.  The transient response bandwidth of a hysteretic controller is half the switching frequency, vs average mode might be 4-40kHz depending on compensation. 

    -Francis Houde