Using a ferrite bead for the second stage L-C filter minimizes the external component count because most of the noise sensitive circuits use a RF bead for high frequency attenuation as a default component at their inputs.
It is important to select a ferrite bead with sufficiently high inductance at full load, and with low DC resistance (below 10 mΩ) to keep the converter efficiency as high as possible. The ferrite bead inductance decreases with increased load current. Therefore, the ferrite bead should have a current rating much higher than the desired load current.
The recommendation is to choose a ferrite bead with an impedance of 8 Ω to 20 Ω at 100 MHz. Refer to Table 8-6 for possible ferrite beads.
|PART NUMBER||MANUFACTURER||SIZE||IMPEDANCE AT 100 MHZ||INDUCTANCE AT 100 MHz (CALCULATED)||DC RESISTANCE||CURRENT RATING|
|BLE18PS080SN1||MuRata||0603||8.5 Ω||13.5 nH||4 mΩ||5 A|
|74279221100||Wurth Elektronik||1206||10 Ω||15.9 nH||3 mΩ||10.5 A|
|7427922808||Wurth Electronik||0603||8 Ω||12.7 nH||5 mΩ||9.5 A|
The internal compensation has been designed to be stable with up to 50 nH of inductance in the second stage filter. To achieve low ripple, the second L-C filter requires only 5-nH to 10-nH inductance. The inductance can be estimated from the ferrite bead impedance specification at 100 MHz, with the assumption that the inductance is similar at the selected converter switching frequency of 1 MHz or 2.2 MHz, and can be verified through tools available on some manufacturer websites. The inductance of a ferrite bead is calculated using Equation 7: