9.2.2.2 Output Capacitor

For the output capacitor, it is recommended to use small ceramic capacitors placed as close as possible to the VOUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which can not be placed close to the IC, using a smaller ceramic capacitor in parallel to the large one is highly recommended. This small capacitor should be placed as close as possible to the V_OUT and GND pins of the IC. To get an estimate of the recommended minimum output capacitance, Equation 6 can be used.

Equation 6.

Where f is the switching frequency which is 3.5MHz (typ.) and ΔV is the maximum allowed output ripple.

With a chosen ripple voltage of 20mV, a minimum effective capacitance of 9μF is needed. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 7

Equation 7.

An MLCC capacitor with twice the value of the calculated minimum should be used due to DC bias effects. This is required to maintain control loop stability. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies. There are no additional requirements regarding minimum ESR. Larger capacitors cause lower output voltage ripple as well as lower output voltage drop during load transients but the total output capacitance value should not exceed ca. 50µF.

DC bias effect: high cap. ceramic capacitors exhibit DC bias effects, which have a strong influence on the device's effective capacitance. Therefore the right capacitor value has to be chosen very carefully. Package size and voltage rating in combination with material are responsible for differences between the rated capacitor value and it's effective capacitance. For instance, a 22µF X5R 6.3V 0805 MLCC capacitor would typically show an effective capacitance of less than 8µF (under 5V bias condition).