Buck Output Ripple (with ESR)

Formula

Description

The output ripple of a buck converter has two parts: a resistive term from the ripple current flowing through the capacitor’s equivalent series resistance (ESR), and a capacitive term from the charge it stores and releases. At typical frequencies the ESR term usually dominates, which is why low-ESR capacitors matter more than raw capacitance for ripple.

Variables

• ΔVout — Output ripple voltage (V)
• ΔIL — Inductor ripple current (A)
• ESR — Capacitor equivalent series resistance (Ω)
• fs — Switching frequency (Hz)
• Cout — Output capacitance (F)

Practical Notes

For ceramic capacitors the ESR term is tiny and the 1/(8·fs·Cout) capacitive term dominates; for electrolytics the ESR term dominates. Add the two in worst case, or RSS them for a typical estimate.