ZHCSL19C May 2019 – June 2021 LM61440
PRODUCTION DATA
The value of the output capacitor and its ESR determine the output voltage ripple and load transient performance. The output capacitor is usually determined by the load transient requirements rather than the output voltage ripple. Table 9-3 can be used to find the output capacitor and CFF selection for a few common applications. Note that a 1-kΩ RFF can be used in series with CFF to further improve noise performance. In this example, improved transient performance is desired giving 2 x 47-μF ceramic as the output capacitor and 22 pF as CFF.
| FREQUENCY | TRANSIENT PERFORMANCE | 3.3-V OUTPUT | 5-V OUTPUT | ||
|---|---|---|---|---|---|
| CERAMIC OUTPUT CAPACITANCE | CFF | CERAMIC OUTPUT CAPACITANCE | CFF | ||
| 2.1 MHz | Minimum | 3 x 22 μF | 10 pF | 2 x 22 μF | 22 pF |
| 2.1 MHz | Better Transient | 2 x 47 μF | 33 pF | 3 x 22 μF | 33 pF |
| 400 kHz | Minimum | 4 x 22 μF | 4.7 pF | 3 x 22 μF | 10 pF |
| 400 kHz | Better Transient | 5 x 22 μF | 33 pF | 4 x 22 μF | 33 pF |
To minimize ceramic capacitance, a low-ESR electrolytic capacitor can be used in parallel with minimal ceramic capacitance. As a starting point for designing with an output electrolytic capacitor, Table 9-4 shows the recommended output ceramic capacitance CFF values when using an electrolytic capacitor.
| FREQUENCY | TRANSIENT PERFORMANCE | 3.3-V OUTPUT | 5-V OUTPUT | ||
|---|---|---|---|---|---|
| COUT | CFF | COUT | CFF | ||
| 400 kHz | Minimum | 2 x 22 μF ceramic + 1 x 470 μF, 100-mΩ electrolytic | 10 pF | 2 x 22 μF ceramic + 1 x 470 μF, 100-mΩ electrolytic | 10 pF |
| 400 kHz | Better Transient | 4 x 22 μF ceramic + 2 x 280 μF,100-mΩ electrolytic | 33 pF | 3 x 22 μF ceramic + 1 x 560 μF, 100-mΩ electrolytic | 22 pF |
Most ceramic capacitors deliver far less capacitance than the rating of the capacitor indicates. Be sure to check any capacitor selected for initial accuracy, temperature derating, and voltage derating. Table 9-3 and Table 9-4 have been generated assuming typical derating for 16-V, X7R, automotive grade capacitors. If lower voltage, non-automotive grade, or lower temperature rated capacitors are used, more capacitors than listed are likely to be needed.