3 Method to Choose an LC Combination

When selecting inductor and output capacitor for Buck switching regulator, many factors need to be considered includes output ripple, loop stability and transient response. For example, in general, it is preferable to choose lower inductance in switching power supplies, because it usually corresponds to faster response, smaller DCR, and reduced size for more compact designs. Too low of an inductance can generate too large of an inductor current ripple so that the device can generate more inductor core loss. Larger inductor current ripple also implies larger output voltage ripple with the same output capacitors. For system loop stability, at the output filter pole frequency, the gain rolls off at a -40 dB per decade rate and the phase drops has a 180 degree drop. The internal ripple generation network introduces a high-frequency zero that reduces the gain roll off from -40 dB to -20 dB per decade and leads the 90 degree phase boost. Table 3-1 shows the stability of different LC combinations of TPS56325x that have been tested in the laboratory with an input voltage of 12 V and a load current of 3 A at an output voltage of 3.3 V. These combinations shown in below table can meet the requirement of loop response performance while for other performances like output voltage ripple, load transient response, it still needs to check based on real requirement.

From Table 3-1, the blue cells are within the data sheet recommended LC range and are thus stable. The gray cells are outside the data sheet recommended range, but have been tested and were stable with appropriate Cff. The corner frequencies listed are based on the effective inductance and capacitance which means that the dc bias effect of ceramic capacitors are taken into account. For example, the part number of 22 uF capacitor is GRM21BR61A226ME44L. Considering the DC bias de-rating, its effective value is 14.4 uF at a 3.3-V bias.