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PMLKBUCKEVM: Questions about Vnode waveforms and switching frequency

Other Parts Discussed in Thread: TPS54160

Hi, support team

When testing PMLKBUCKEVM, it was found that at light loads, the Vnode waveform seems to have some oscillations at higher input voltages.

CH1 represents Vin, CH2 represents current ripple, CH3 represents output voltage ripple, CH4 represents Vnode.

In addition, the manual says that the TPS54160 should be based on the external resistor to set the frequency, in the case of a fixed resistor why the frequency changes in such a large range?

Thanks for your support.

  • HI Aaron

    Can you please make a more detailed description ?  CH3 is current for what? And where do we see the large frequency change? 

    Thank you.

  • Thanks for your reply and sorry for my poor expression.

    CH3 is the input current ripple.

    My question is why in the second and third figure, before the rising edge, the waveform of Vnode(green trace) and output voltage ripple (purple trace) show an oscillation while in figure 1 it didn't, or in other words, why in DCM mode the waveform of Vnode and output voltage ripple show an oscillation?

    From the last figure we can see that in datasheet, if we set the RT as 200Kohm, the switching frequency ranges from 450kHz to 720kHz, but in my understanding, I think if RT is set, there should be an formula for the switching frequency calculation, and it should be a certain value rather than a large range.

  • Hi Aaron

    The first figure is under CCM mode, as Vin rises, the converter enters into DCM mode, the figure2&3 is a typical waveform for DCM switch node. During the time scale when both Mosfet and Diode are off, the inductor, the parasitic capacitor for mosfet& diode and Cin , can form resonance, thus the oscillation.

    Yes you are right about when the RT is fixed, the frequency is set by RT. You only need to refer to the typical value. The minimum& maximum value in the table are the boundary of a normal distribution for a very large amount of devices. 

    Hope that helps. 

    Thank you.