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Cff of TPS56C215

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: TPS56C215

We have questions about TPS56C215.

In datasheet p.21

Circuit of Figure 29 has the Cff(56pF).

In datasheet p.22  Table.5,

Cff is not listed when Vout are 0.6V and 1.2V.

Question 1.

Is Cff unnecessary when Vout is 1.0V?

Question 2.

Vout is 1.0V

Can we use 47pF if we want to improve the transient response?

Best regards,

Takahiro Nishizawa

  • 0
    TI__Expert 6425 points

    Hi Takahiro,

    1, when Vout is 1.0V, CFF isn't necessary.

    2, when Vout is only 1V, although CFF is added, there isn't much more improvements.

    You want to improve load transient performance, right? can you tell me what's the overshoot voltage and undershoot voltage, when IC happens load transient now. or can you upload load transient waveform?

    Best Regards,

    Edwin.

  • 0 in reply to Edwin Zang
    Genius 5575 points
    Thank you for your answer!


    when Vout is only 1V, although CFF is added, there isn't much more improvements.


    Please tell us the reason why there isn't much more improvements when Vout is 1V.
    Why is there no effect when Vout is lower?

    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Expert 6425 points

    Hi Takahiro,

    If output voltage is small, FB divider resister is small compared with 5vout. According to zero and pole calculation, the zero frequency is far away with LC double pole. sometimes the zero frequency is larger than crossover frequency. So the zero that CFF add couldn't increase bandwidth obviously. so the CFF doesn't have much helpful for load transient performance.