The output capacitor is mainly selected to meet the requirements for output ripple and loop stability. The ripple voltage is related to capacitor capacitance and its equivalent series resistance (ESR). Assuming a ceramic capacitor with zero ESR, the minimum capacitance needed for a given ripple voltage can be calculated by Equation 3.

Equation 3.

where

• D_MAX is the maximum switching duty cycle
• V_RIPPLE is the peak-to-peak output ripple voltage
• I_OUT is the maximum output current
• f_SW is the switching frequency

The ESR impact on the output ripple must be considered if tantalum or aluminum electrolytic capacitors are used. The output peak-to-peak ripple voltage caused by the ESR of the output capacitors can be calculated by Equation 4.

Equation 4.

Take care when evaluating the derating of a ceramic capacitor under dc bias voltage, aging, and ac signal. For example, the dc bias voltage can significantly reduce capacitance. A ceramic capacitor can lose more than 50% of its capacitance at its rated voltage. Therefore, always leave margin on the voltage rating to ensure adequate capacitance at the required output voltage. Increasing the output capacitor makes the output ripple voltage smaller in PWM mode.

TI recommends using the X5R or X7R ceramic output capacitor in the range of 4μF to 1000μF effective capacitance when output current is lower than 1A, and 10μF to 1000μF effective capacitance when output current is higher than 1A. The output capacitor affects the small signal control loop stability of the boost regulator. If the output capacitor is below the range, the boost regulator can potentially become unstable. Increasing the output capacitor makes the output ripple voltage smaller in PWM mode.

For large output capacitance more than 40μF application, TI recommends a feedforward capacitor to set the zero frequency (f_FFZ) to 1kHz.