SLVUAP3A April 2016 – October 2021 TPS54202
1.3.2 Output Capacitor and Feed-Forward Capacitor
Considering the loop stability and the effect of the internal parasitic parameters, choose a crossover frequency less than 40 kHz, without considering the feed-forward capacitor. A simple estimation for the crossover frequency without feed-forward capacitor C8 is shown in Equation 2, assuming COUT has small ESR.
Depending on VOUT, if the output capacitor, COUT, is dominated by low-ESR (ceramic types) capacitors, a low phase margin could result. To improve the phase boost, an external feed-forward capacitor, C8, can be added in parallel with R2. C8 is chosen such that phase margin is boosted at the crossover frequency.
C8 is calculated in Equation 3:
For this design, C8 = 75 pF. C8 is not needed when COUT has high ESR, and C8 calculated from Equation 3 should be reduced with medium ESR. Use Table 1-3 as a starting point.