9.2.2.4 Output Capacitor

Output capacitor also should have enough ripple current rating to absorb output switching ripple current. In buck mode the output capacitor RMS current is given:

To get good loop stability, the resonant frequency of the output inductor and output capacitor should be designed between 10 kHz and 20 kHz. The preferred ceramic capacitor is 25-V X7R or X5R for output capacitor. Minimum 10-µF effective capacitance (7 pcs of 10-µF 0805 size capacitor) is suggested to be placed by the inductor, and 50-µF effective distributed capacitance on Vsys output. Place the capacitors after Q4 drain. Place minimum 10 µF after the charge current sense resistor for best stability.

Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a significant capacitance drop, especially for high output voltages and small capacitor packages. See the manufacturer's data sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage rating or nominal capacitance value in order to get the required value at the operating point.