The desired response to a load transient is the first criterion for output capacitor selection. The output capacitor must supply the load with the required current when not immediately provided by the regulator. When the output capacitor supplies load current, the impedance of the capacitor affects the magnitude of the voltage deviation during the transient.

To meet the requirements for control-loop stability, the device requires the addition of compensation components in the design of the error amplifier. While these compensation components provide for a stable control loop, they often also reduce the speed with which the regulator can respond to load transients. The delay in the regulator response to load changes can be two or more clock cycles before the control loop reacts to the change. During that time, the difference (delta) between the old and the new load current must be supplied (or absorbed) by the output capacitance. The output capacitor impedance must be designed to supply or absorb the delta current while maintaining the output voltage within acceptable limits. Equation 13 and Equation 14 show the relationship between the transient response overshoot (V_OVER), the transient response undershoot (V_UNDER), and the required output capacitance (C_OUT).

Equation 13.

Equation 14.

If

• V_IN(min) > 2 × V_OUT, use overshoot to calculate minimum output capacitance.
• V_IN(min) < 2 × V_OUT, use undershoot to calculate minimum output capacitance.

In this case, the minimum designed input voltage, V_IN(min), is greater than 2 × V_OUT, so V_OVER dictates the minimum output capacitance. Therefore, using Equation 15, the minimum output capacitance required to meet the transient requirement is 600 µF.

Equation 15.

The bandwidth of the voltage loop must also be considered when calculating the minimum output capacitance. The voltage loop can typically be compensated to have a bandwidth of 1/10th the f_SW. Equation 16 calculates the minimum output capacitance to be 979 µF.

Equation 16.