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TPS92515HVEVM-749: TPS92515-Q1 Coff charge from Vcc

Tomohiro Nagasawa58
Expert 1570 points
Part Number: TPS92515HVEVM-749
Other Parts Discussed in Thread: TPS92515

Dear support team

Seeing at TPS92515EVM,
it is possible to charge Coff from Vcc through R7, but what are the advantages of this configuration?
Is there a merit of charging from Vcc instead of Vout during normal operation?

Regards

Tomohiro Nagasawa

  • 0
    TI__Mastermind 28655 points

    Hello Tomohiro,

    The reason the off-time resistor is connected to the LED stack is to keep the current ripple amplitude constant.  This is because the TPS92515 regulates peak current and if the ripple amplitude varies the average current will also vary.

    The Coff resistor Roff can be connected to Vcc, the only benefit would be the switching frequency would vary less witch changes in output voltage (the LED stack voltage).

    It requires the same amount of parts to do this either way.  If they want current regulation accuracy it's best to use the output voltage.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Expert 1570 points

    Hello Irwin-san

    Thank you for quick response.

    In my understanding , demerit of Coff resistor connected to Vcc  is that the average current will change due to input /output voltage vary. Is it correct?

    And I have one more question.  What is the merit of constant on-time control compared to hysteresis control of a type that simply decides peak and bottom?

    Regards

    Tomohiro Nagasawa

  • 0 in reply to Tomohiro Nagasawa58
    TI__Mastermind 28655 points
    Hello Tomohiro,

    The Coff capacitor will charge at the approximate rate of Vcc/Roff (neglecting the Voff voltage). If Vout changes the di/dt changes in the inductor. Toff is constant because it is referenced to Vcc. that means dt for the inductor discharge stays constant meaning di has to change due to the equation V = L*(di/dt). L is constant, dt is constant due to being tied to Vcc, if Vout changes the di has to change. If it is tied to Vout the ratio of C = I*(dv/dt) and V = L*(di/dt) stay similar and the ripple stays similar.

    This is called constant off time but it really is constant ripple current. Toff changes when Vout changes. The TPS92515 is really detecting the peak and open loop detecting the valley, so similar to peak and bottom, you just have to calculate the di/dt during discharge to find the valley (bottom).

    Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 1570 points

    Hello Irwin-san

    Thank you for detailed explanation.

    Does open loop detection have any advantage/disadvantage?  compared to the method that really detects the peak and bottom.

    Sorry for asking about detail.

    Regards

    Tomohiro Nagasawa

  • 0 in reply to Tomohiro Nagasawa58
    TI__Mastermind 28655 points
    Hello Tomohiro,

    An advantage is it makes it easier to dim into discontinuous mode, it is a bit more simple operation.
    A disadvantage is it includes the accuracy of the inductor value for calculating the bottom.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 1570 points

    Hello Irwin-san

    Thank you for respose. I understand.

    I think that frequency variation due to the variation of L is also small at open loop control.

    Regards

    Tomohiro Nagasawa

  • 0 in reply to Tomohiro Nagasawa58
    TI__Mastermind 28655 points

    Hello Tomohiro,

    If the input voltage and output voltage is fixed the frequency shouldn't change much.  If the input change and/or the output it can change a lot depending on the corners of operation.

    Regards,