SLASEC2B November 2016 – February 2019 TAS2557
The boost converter requires three passive devices that are labeled L1, C1 and C2 in Figure 148 and whose specifications are provided in Table 128. These specifications are based on the design of the TAS2557 and are necessary to meet the performance targets of the device. In particular, L1 should not be allowed to enter in the current saturation region. The saturation current for L1 should be > ILIM to deliver Class-D peak power.
Additionally, the ratio of L1/C2 (the derated value of C2 at 8.5 V should be used in this ratio) has to be lesser than 1/3 for boost stability. This 1/3 ratio should be maintained including the worst case variation of L1 and C2. To satisfy sufficient energy transfer, L1 needs to be >= 1µH at the boost switching frequency (~1.7 MHz). Using a 1µH will have more boost ripple than a 2.2µH but the PSRR should minimize the effect from the additional ripple. Finally, the minimum C2 (derated value at 8.5 V) should be > 3.3µF for Class-D power delivery specification.