The LM76002-Q1/LM76003-Q1 is internally compensated and the internal R-C values are 400 kΩ and 50 pF, respectively. Depending on the VOUT and frequency FS, if the output capacitor COUT is dominated by low ESR (ceramic types) capacitors, it could result in low phase margin. To improve the phase boost an external feed-forward capacitor CFF can be added in parallel with RFBT. CFF is chosen such that phase margin is boosted at the crossover frequency without CFF. A simple estimation for the crossover frequency without CFF (fx) is shown in Equation 25, assuming COUT has very small ESR.
The Equation 26 for CFF was tested:
If capacitors with high ESR are used CFF is not required. The CFF capacitor creates a time constant with RFBT that couples the attenuated output voltage ripple to the FB node. Using a value that is too large for CFF may couple too much ripple to FB node and affect output voltage regulation. For capacitors with medium ESR (20 – 200 mΩ) Equation 26 can be used as quick starting point. For the application in this design example, a 47-pF C0G capacitor is used.