9.2.2.8.1 Response to a Load Transient

The output capacitance must supply the load with the required current when current is not immediately provided by the regulator. When the output capacitor supplies load current, the impedance of the capacitor greatly affects the magnitude of voltage deviation (such as undershoot and overshoot) during the transient.

Use Equation 9 and Equation 10 to estimate the amount of capacitance needed for a given dynamic load step and release.

Equation 9.

Equation 10.

where

• C_{OUT(min_under)} is the minimum output capacitance to meet the undershoot requirement
• C_{OUT(min_over)}is the minimum output capacitance to meet the overshoot requirement
• D is the duty cycle
• L is the output inductance value (0.47 µH)
• ∆I_LOAD(max) is the maximum transient step (10 A)
• V_OUT is the output voltage value (900 mV)
• t_SW is the switching period (2.0 µs)
• V_IN is the minimum input voltage for the design (12 V)
• ∆V_LOAD(insert) is the undershoot requirement (50 mV)
• ∆V_{LOAD(release)} is the overshoot requirement (50 mV)

• This example uses a combination of POSCAP and MLCC capacitors to meet the overshoot requirement.
  • POSCAP bank #1: 2 x 330 µF, 2.5 V, 3 mΩ per capacitor
  • MLCC bank #2: 3 × 100 µF, 6.3 V, 1 mΩ per capacitor