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TPS746-Q1: The role of Cff feedforward capacitor.

Alan Xia
Genius 9600 points
Part Number: TPS746-Q1

Hi team，

The customer found that there was a Cff feedforward capacitor in the recommended design of TPS74601. The customer installed a 10nF capacitor, but this affected the power supply rise time.
Want to confirm that this capacitor has any other effect besides affecting the ripple rejection ratio? Are there any adverse effects if not added?

  • 0
    TI__Genius 14245 points

    Hi Alan, 

    Yes, it is expected that adding a feedforwad cap (Cff) will slow down the output rise time. As you mentioned it will also improve the PSRR and Noise in the mid band frequencies (~1kHz to 100kHz) and can help improve transient response as well. 

    As for disadvantages, it can cause a small error of when a PG pin triggers during a transient event. Also since Cff AC couples Vout to FB, a large magnitude+fast ramp rate transient on Vout (such as short to GND) risks damaging the FB pin when a large Cff is used (generally considered >100nF).

    For more details on each of these please see our App Not Pros and Cons of Using a Feedforward Capacitor (note, the screen shot you included above seems to be from the app note).

    https://www.ti.com/lit/an/sbva042/sbva042.pdf