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TPS548A20: About Lout selection

Tsukui Yusuke
Guru 21930 points
Part Number: TPS548A20

Hi,

Could you tell me about Lout selection?
The following contents are described in the data sheet.

What is the problem of TPS548A20 operation when the design is less than 25% ripple?

When the operating frequency was reset to 1MHz in Webench,
the ripple was less than 25% for the recommended inductor value.
https://webench.ti.com/power-designer/switching-regulator/customize/2299?AppType=None&O1I=12&O1V=1.05&Topology=BUCK&VinMax=12&VinMin=12&base_pn=TPS548A20

I think Webench does not consider the contents of the datasheet page17.




Regards,
Yusuke

  • 0
    TI__Mastermind 23715 points

    The 25% to 35% ripple is a rule of thumb that works well when designing on paper.

      It tends to give a good compromise of inductance vs physical size and output ripple.

    It is important to obtain acceptable size of  inductor and output capacitor with sufficiently low output voltage ripple.

    A large value of inductance would have many windings and will be physical large.   Low value inductance will have large ripple and would need more output capacitance to meet a voltage ripple requirements.     

    In webench, many calculations are made to find a good solution that is a good balance of size, efficiency and transient/ripple performance.

  • 0 in reply to David_Daniels
    Guru 21930 points

    Hi David,

    Thank you for your kind support.
    I understand your explanation.

    Let me ask you a question from the point of view of the control loop.

    TPS548A20 is Dcap3.
    I understand it as a converter controlled by a comparator.
    Does the ripple current too small affect the control system?
    For example,
    Does 16% ripple affect the control system (control loop) as in Webench simulation?

    Best regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Guru 56795 points

     

    The TPS548A20 does use D-CAP3 control, but the injected ripple comes from the internal RC ripple generator.  The larger inductor will slow down the transient response by requiring greater frequency shift to change the inductor current, but it should not sacrifice stability.

    In general, I would recommend decreasing the inductor value to increase the inductor ripple current when increasing the switching frequency, but it is not strictly necessary, it just helps maximize the size and performance benefits of the higher switching frequency.