SLUU180B January 2022 – February 2022 TPS40070 , TPS40071
The power inductor is selected by calculating the range of peak-to-peak ripple current, IPP, which is obtained with various values of inductance over the total input/output voltage range. In previous generations of buck converters, electrolytic capacitors with significant ESR were the normal, and the inductor ripple current would be selected to be 10% to 20% of IOUT to minimize the output voltage ripple. Now, ceramic output capacitors with ESR in the range of 1 mW to3 mW are readily available, so the ripple current can be allowed to be 20% to 50% of the output current. The following equation was used to calculate the ripple current; and complete results are presented for the selected inductor value of 1.6 μH.
VIN | VOUT | IRIPPLE |
---|---|---|
12 | 3.3 | 4.98 |
12 | 1.8 | 3.19 |
12 | 1.2 | 2.25 |
8 | 3.3 | 4.04 |
8 | 1.8 | 2.91 |
8 | 1.2 | 2.12 |
5 | 3.3 | 2.34 |
5 | 1.8 | 2.4 |
5 | 1.2 | 1.9 |
Ceramic capacitors are selected for the output capacitors. The minimum value is determined by output voltage ripple considerations:
Three 47-mF ceramic capacitors are selected to handle the worst case ripple current of 5 A when VIN = 12 V and VOUT = 3.3 V. As the output voltage gets lower, the corresponding ripple current is reduced, so excessive output voltage ripple should not be an issue.