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TPS54525: Inductor value / Phase boost capacitor

tom
Prodigy 210 points
Part Number: TPS54525
Other Parts Discussed in Thread: TPS54625

Hello,

I have two questions regarding the TPS54525.

1. The TPS54525 gives a single value recommended inductor size for each output voltage setting. The TPS54625 datasheet gives a much broader range. Is this just a difference in datasheet or does it really have different operating conditions? If possible i would like to use 4uH inductor value (for 3.3V and 5V supply) since it gives lower voltage ripple and the transient response in Webbench does not make alot of difference. Pricing of the inductor would be the same since it is in the same inductor family.

2. The datasheet also mentions capacitor values for added phase boost. How would i determine the optimal value?

Furthermore, since hysteric control is inherently stable and low phase margin should be no issue (since phase never crosses 180degree phase), what is the use of boosting the phase-margin? Does it improve transients response (I cannot see much difference in webbench simulations) or is it just something for people that want to reach same phase margin values/stability criteria as in current-mode control?

best regards,

Tom

  • 0
    TI__Guru**** 191445 points

    Can you check / correct the part numbers in your post?  Thanks...

  • 0 in reply to JohnTucker
    Prodigy 210 points
    I'm sorry i made a typo in the second part numbers: I meant the 5.5A/6.5A version of the same family: the TPS54525 and TPS54625
  • 0 in reply to tom
    TI__Guru**** 191445 points
    1. Yes that is mainly just a difference in the datasheets. The principle is the same for both devices. You want to keep the LC corner frequency not to close or too far from the ripple injection zero. So there is some leeway with regard to inductor value. See this app note for more details.

    www.ti.com/.../slva546

    2. That app note may also shed some light on feed forward capacitor selection. The feed forward capacitor forms a zero / pole pair. The distance between the zero and pole is fixed by the output voltage. Higher output voltage has wider spacing and more available phase boost. Maximum phase boost at crossover occurs when teh zero and pole are placed symmetrically about the crossover frequency. However when the zero is placed below Fco, Fco moves higher relative to the un-boosted position. So I typically sacrifice a little phase boost and place the zero just above the crossover frequency.