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TPS7A8300: start up issue with feedforward capacitor

justifice zhou
Intellectual 400 points
Part Number: TPS7A8300

I'm reading this artice

Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator (ti.com)

in part 2.1 Start-Up Issue

It's assumed that with Cff, VOUT tracks VFB during tref time. Which means that VOUT=VFB, but not VOUT=VFB*(R1+R2)/R2.

It means that the voltage on Cff=0V during tref time.

But why Cff should be 0V during tref time?

  • 0
    TI__Expert 4680 points

    Hello Justifice,

    Yes, you are correct on the first part. With Cff start-up time is split into two stages tref and tcff. Theoretically during tref, Vout tracks Vfb, which is controlled by the feedback loop forcing it to match Vref. However, once the output starts to rise Cff also starts to charge so Cff won't be 0V during tref nor Vout will be equal to Vref, but will be a close approximation since Vout can take much longer if Cff is large.

    Best,

    Eunize Tobias