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TPS53355: Ripple Generation Values Question

Casey McCrea
Guru** 103215 points
Part Number: TPS53355

Hello Team,

I'm looking for some further insight into how the ripple generation values should be chosen based on the design example in the datasheet. These calculations don't match another app note we have on the subject SVLA653. In the datasheet section 8.2.1.2.2, the C2 value is set to 1nF first (how was this value chosen?), and then C1 is set in a range of 10 to 100xC2 based on equation 11. In the app note on page 5 C1 is set first, and then C2 is set to be 3 or 4xC1 (the opposite relationship). 

I'm trying to understand which equation set is correct and why the design example for TPS53355 would have the opposite relationship for the component selection. 

App note: www.ti.com.cn/.../slva653.pdf

Best Regards,

Casey

  • 0
    TI__Guru**** 191445 points
    There is a fair amount of hand waving in the datasheet, but that method was developed based on actual use of TPS53355. I ran the app note values and got 534 pF and 53.6 k for Cr and Rr respectively. Those values look suspect, but that app note was written with respect to a completely different family of converters. Also they did not provide an example. I would stay with the datasheet method.
  • 0 in reply to JohnTucker
    TI__Guru** 103215 points

    Thanks John,

    Can you provide some insight on how the starting value for C2 was chosen in the datasheet and how the values should relate to each other from a control loop perspective?

    Best Regards,
    Casey

  • 0 in reply to Casey McCrea
    TI__Guru**** 191445 points
    The designer probably picked that range from experience. The key point is to add some ripple information. DCAP control is adaptive on time and hysteretic off time. Without some ripple information to end the off time, the frequency and duty cycle would vary wildly. It would essentially be unstable. I know for a fact that the internal ripple injection of DCAP2 provides a high frequency zero. Looking at the loop response of the TPS53355 EVM, it looks like the ripple injection is providing a zero at lower frequency, well below the LC corner frequency. There is probably not a whole lot of technical detail available as this is a TI proprietary topology, not something widely studied like current mode and voltage mode control.