5.2 Output Capacitor Selection

The type of the output capacitor is usually determined by its ESR (equivalent series resistance). Voltage and RMS current capability are two other important factors for selecting the output capacitor. Recommended capacitor types are tantalum, low-ESR aluminum electrolytic, OS-CON, and POSCAP. The output capacitor’s ESR is usually the main cause of the output ripple. The output capacitor ESR also affects the control loop from a stability point of view. The maximum value of ESR is calculated:

$ES{R}_{C_{OUT}} \le \frac{∆V_{OUT\left(PP\right)}}{∆I_{L\left(PP\right)}}$

Where:

ΔV_OUT(PP) = Peak-to-peak output voltage ripple.

ΔI_L(PP) = Peak-to-peak inductor current ripple.

The total output ripple is a combination of the ESR and output capacitance. The total ripple is calculated below:

$∆V_{OUT\left(PP\right)} = \sqrt{{\left(\frac{∆I_{L\left(PP\right)}}{C_{OUT} \times f_{SW} \times 8}\right)}^2 + {\left(∆I_{L\left(PP\right)} \times ES{R}_{C_{OUT}}\right)}^2}$

Where:

C_OUT = The output capacitance value.

f_SW = The switching frequency.

As described in the Theory of Operation section, the MIC26400 requires at least 20mV peak-to-peak ripple at the FB pin to make the g_m amplifier and the error comparator behave properly. Also, the output voltage ripple should be in phase with the inductor current. Therefore, the output voltage ripple caused by the output capacitors value should be much smaller than the ripple caused by the output capacitor ESR. If low-ESR capacitors, such as ceramic capacitors, are selected as the output capacitors, a ripple injection method should be applied to provide the enough feedback voltage ripple. Please refer to the Ripple Injection section for more details.

The voltage rating of the capacitor should be twice the output voltage for a tantalum and 20% greater for aluminum electrolytic or OS-CON. The output capacitor RMS current is calculated below:

$I_{C_{OUT}\left(RMS\right)} = \frac{∆I_{L\left(PP\right)}}{\sqrt{12}}$

The power dissipated in the output capacitor is:

$P_{DISS\left(C_{OUT}\right)} = {I_{C_{OUT}\left(RMS\right)}}^2 \times ES{R}_{C_{OUT}}$