A power supply input typically has a relatively high source impedance at the switching frequency. Good-quality input capacitors are necessary to limit the input ripple voltage. As mentioned earlier, dual-channel interleaved operation significantly reduces the input ripple amplitude. In general, the ripple current splits between the input capacitors based on the relative impedance of the capacitors at the switching frequency.
- Select the input capacitors with sufficient voltage and RMS ripple current ratings.
- Worst case input ripple for a two-channel buck regulator typically corresponds
to when one channel operates at full load and the other channel is disabled or
operates at no load. Use Equation 41 to calculate
the input capacitor RMS ripple current assuming a worst-case duty-cycle
operating point of 50%.
- Use Equation 42 to find the required input
- ΔVIN is the input peak-to-peak ripple
- RESR is the input capacitor ESR.
- Recognizing the voltage coefficient of ceramic capacitors, select two 10-µF, 50-V, X7R, 1210 ceramic input capacitors for each channel. Place these capacitors adjacent to the relevant power MOSFETs.
- Use four 10-nF, 50-V, X7R, 0603 ceramic capacitors near each high-side MOSFET
to supply the high di/dt current during MOSFET switching transitions. Such
capacitors offer high self-resonant frequency (SRF) and low effective impedance
above 100 MHz. The result is lower power loop parasitic inductance, thus
minimizing switch-node voltage overshoot and ringing for lower EMI signature.
Refer to Figure 11-2 in Layout Guidelines for more details.