SNVSAH3E February 2018 – July 2020 LMR23625
PRODUCTION DATA
Refer to the PDF data sheet for device specific package drawings
The LMR23625 is internally compensated. Depending on the VOUT and frequency fSW, if the output capacitor COUT is dominated by low ESR (ceramic type) 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. Choose CFF so that phase margin is boosted at the crossover frequency without CFF. A simple estimation for the crossover frequency (fX) without CFF is shown in Equation 17, assuming COUT has very small ESR, and COUT value is after derating.
Equation 18 for CFF was tested:
For designs with higher ESR, CFF is not needed when COUT has very high ESR, and CFF calculated from Equation 18 should be reduced with medium ESR. Table 8-1 can be used as a quick starting point.
For the application in this design example, a 18-pF, 50-V, COG capacitor is selected.