ZHCSOR1 August 2021 TPS25860-Q1 , TPS25862-Q1
PRODUCTION DATA
The output capacitor or capacitors, COUT, must be chosen with care since it directly affects the steady state output voltage ripple, loop stability, and the voltage overshoot/undershoot during load current transients.
The value of the output capacitor and its ESR, determine the output voltage ripple and load transient performance. The output capacitor is usually limited by the load transient requirements rather than the output voltage ripple if the system requires tight voltage regulation with presence of large current steps and fast slew rate. When a fast large load increase happens, output capacitors provide the required charge before the inductor current can slew up to the appropriate level. The control loop of the regulator usually needs four or more clock cycles to respond to the output voltage droop. The output capacitance must be large enough to supply the current difference for four clock cycles to maintain the output voltage within the specified range. Table 11-3 can be used to find output capacitors for a few common applications. In this example, good transient performance is desired giving 3 × 22-µF ceramic as the output capacitor.
FREQUENCY | COUT | SIZE/COST | TRANSIENT PERFORMANCE |
---|---|---|---|
2.1 MHz | 3 × 22 µF ceramic | Small size | Good |
2.1 MHz | 2 × 47 µF ceramic | Small size | Better |
2.1 MHz | 2 × 22 µF ceramic | Smallest size | Minimum |
400 KHz | 3 × 47 µF ceramic | Small size | Good |
400 KHz | 2 × 47 µF ceramic | Small size | Minimum |
400 KHz | 4 × 22 µF + 1 × 260 µF, < 50-mΩ electrolytic | Larger size, low cost | Good |
400 KHz | 1 × 4.7 µF + 2 × 10 µF + 1 × 260 µF, < 50-mΩ electrolytic | Lowest cost | Minimum |
where