SLVSE84D December 2017 – July 2021 TLV767
For the adjustable-voltage version device, a feed-forward capacitor (CFF) can be connected from the OUT pin to the FB pin. CFF improves transient, noise, and PSRR performance, but is not required for regulator stability. Recommended CFF values are listed in the Recommended Operating Conditions table. A higher capacitance CFF can be used; however, the start-up time increases. For a detailed description of CFF tradeoffs, see the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application report.
CFF and R1 form a zero in the loop gain at frequency fZ, while CFF, R1, and R2 form a pole in the loop gain at frequency fP. CFF zero and pole frequencies can be calculated from the following equations:
CFF ≥ 10 pF is required for stability if the feedback divider current is less than 5 µA. Equation 6 calculates the feedback divider current.
To avoid start-up time increases from CFF, limit the product CFF × R1 < 50 µs.
For an output voltage of 0.8 V with the FB pin tied to the OUT pin, no CFF is used.