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TPS62125: frequency variation

Part Number: TPS62125
Other Parts Discussed in Thread: TPS62150


Looking at figure 33,34 and 35 shows a frequency variation that doesn't match with theory ("Understanding frequency variation in the
Tm topology" - by Chris Glaser)

Indeed, in DCS control scheme, frequency would rise with load current (as duty cycle would rise with losses and constant Ton, only frequency can rise).

How can you explain this phenomena ?



  • Hi Adrien,

    I simulated this device in WEBENCH and found that the on-time increased as load current increased. The combined RDSon and LDCR affects how quickly the inductor charges, so for higher currents the device sets a higher on-time in order to get enough inductor current to maintain regulation. This causes the frequency to decrease since the off time would be about the same or longer.

    Let me know if this clarifies the issue.



  • Hi Joseph,

    Thank for your response.

    However, I'd like to understand how this is achieved.

    In all literature, DCS control schemes are illustrated by this following figure :

    According to this,On time Timer would only depends on VIN and VOS (mainly DC Component I guess).

    it turns out these assumptions are no longer true.

    It is tricky since TPS62150 or TPS6208x - DCS Control also - behave as expected in theory.

    How can we explain this differences between these three components ?

    Would you have a paper or a more realistic scheme for the DCS control that would allowed me to predict this behavior ?



  • Hi Adrien,

    There are certain cases where the on-timer doesn't strictly depend on Vin and Vos. This is due to the Direct Control and compensation from the modulator (shown by the comparator in the block diagram). Typically, with DCS-control this frequency drop is most noticeable when approaching 100% duty cycle. The modulator will dominate the on-timer at high duty cycle and will decrease the switching frequency because of the increased on-time.

    I will check to see if we have any extra material that would be beneficial in understanding DCS-control.