SNVA406C May   2010  – January 2022 LM27402

 

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9.4 Output Capacitor, Co1–Co4

The value of the output capacitor in a buck regulator influences the steady state voltage ripple as well as the output voltage response to a load transient. Given the peak-to-peak inductor current ripple (ΔIL), the output voltage ripple can be approximated by:

Equation 7. GUID-6C289E81-92BE-451C-A546-8862457F58F1-low.gif

where

  • ΔVOUT (V) is the amount of peak-to-peak voltage ripple at the power supply output.
  • RESR (Ω) is the series resistance of the output capacitor.
  • fSW (Hz) is the switching frequency.
  • COUT (F) is the output capacitance used in the design and is the sum of Co1 through Co4.

For the evaluation board, four 100-µF 6.3-V X5R ceramic capacitors were selected for the output capacitance to provide adequate transient and DC bias performance in a relatively small package. From the technical specifications of this capacitor, the ESR is approximately 3 mΩ and the effective in-circuit capacitance is approximately 60 µF (reduced from 100 µF due to the 1.5-V DC bias and worst case tolerance). With these values, the peak-to-peak voltage ripple when operating from a VIN of 12 V is:

Equation 8. GUID-4A0A914A-A13E-4A69-8709-BED4F589E601-low.gif