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TPS92640: control mode

K.Hamamoto
Guru 12155 points
Part Number: TPS92640

Hi

This device is quasi-hysteric control. (on time control)
However this device has the valley current control loop also and has the loop compensation.

1) I do not understand how is this device defined the start point of off time.

I know this device has CS pin to detect the inductor current. But there is no detail on the datasheet.) On time is defined at RON pin ( Ron and Con).

Also this device has on-time control and valley current loop.

2) How can we measure the phase margin?

This device has on-time control and valley current loop and need a loop compensation.

Which pin should we set for the FRA ? Is that CS pin? (Because CS pin is the current loop pin for the inductor valley current. )

Best Regards,

Koji Hamamoto

  • 0
    TI__Mastermind 28655 points
    Hello,

    1) The control is very similar to peak current mode control. In peak current mode control the error amplifier is comparing the peak to a reference generated by the error amplifier. The error amplifier is comparing the current setting to the actual measured current, the compensation allows the output of the error amplifier to change the reference to the peak current measurement. It increases if the current is too low or decreases if the current is too high until it is regulating the correct current. The part does the same thing but uses the valley instead of the peak, this is the valley current control. The difference between the valley and peak is a DC offset. The error amplifier is an integrator so the DC offset gets integrated out by the average current control loop.

    2) It doesn't have valley current control loop, the valley is the same thing as the peak in a peak current mode controller. I dont' know what FRA means. The CS pin is used both for average current (the compensation loop), and valley current control. For calculating the CS resistor you you average current, the LED current, per the datasheet.

    Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello,

    As for measuring phase margin you can use a loop analyzer and inject into the CS pin.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Guru 12155 points

    Hi Irwin-san,

    I am sorry for the delay in my response. Also thank you for your support. That was very helpful information. 

    I had discussion with our customer about the above question. And we still have some question about this device.

    On the datasheet, it is mentioned as the following description about slope compensation.

    The on-time control(quasi-hysteretic control) reduce the sub-harmonic oscillation as mentioned above.

    We do not understand why the on-time control reduce that oscillation. Why does the on-time reduce that phenomenon?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Mastermind 28655 points

    Hello Koji,

    From the datasheet:  "The control architecture is a combination of valley current control and a one-shot on-timer that varies with input
    and output voltage." 

    Subharmonic oscillation is an alternating of short and long on-time pulses due to how peak current mode control works above 50% duty cycle.  If you control the on-time you cannot have short and long pulses, they are determined by Vin and Vout so they are fixed.

    Regards,