Output Ripple Example The output capacitor has an ESR of 100m Ω, and the peak-to- peak current through the output inductor is 0.25A. Figures 2 and 3 show examples of the inductor current waveform and the resulting output voltage ripple waveforms respectively.
Input voltage (VIN) = 12V. Allowable input ripple voltage (ΔVIN) < 0.36V. Output voltage (VO) = 1.2V. Output current (IO) = 12A. Inductor peak-to-peak ripple current (ΔIpp) = 3.625A. Switching frequency (FSW) = 600KHz. Temperature-rise limit of the ceramic capacitors < 10°C. Figure 2 shows the input ripple-current waveform. Figure 2.
Ripple current is the AC current that enters and leaves the capacitor during its operation in a circuit. Ripple current generates heat and increase the temperature of the capacitor. This rate of heat generation in a capacitor can be described by using the common power formula:
Select key ceramic capacitors to bypass input ripple current Among the different types of capacitors, the multilayer ceramic capacitor (MLCC) is particularly good regarding allowable ripple current. A starting point is to select the key ceramic capacitors to meet the requirements for ripple voltage and current.
(Figure 1) shows the trapezoidal current flowing through the inductor that creates an I·R drop across the ESR of the output capacitor that creates the output ripple. Output Ripple Example The output capacitor has an ESR of 100m Ω, and the peak-to- peak current through the output inductor is 0.25A.
The resulting ripple voltage and current can be calculated as 210mVp-p/74.23mVrms, and 22.3A respectively. These are significantly greater than the target ripple voltage and maximum allowable ripple current for the capacitor. Figure 1. capacitance loss with frequency.