SNVSB29C October 2018 – June 2021 LM5143-Q1
Input capacitors are necessary to limit the input ripple voltage to the buck power stage due to switching-frequency AC currents. TI recommends using X7S or X7R dielectric ceramic capacitors to provide low impedance and high RMS current rating over a wide temperature range. To minimize the parasitic inductance in the switching loop, position the input capacitors as close as possible to the drain of the high-side MOSFET and the source of the low-side MOSFET. Use Equation 19 to calculate the input capacitor RMS current for a single-channel buck regulator.
The highest input capacitor RMS current occurs at D = 0.5, at which point the RMS current rating of the input capacitors is greater than half the output current.
Ideally, the DC component of input current is provided by the input voltage source and the AC component by the input filter capacitors. Neglecting inductor ripple current, the input capacitors source current of amplitude (IOUT − IIN) during the D interval and sink IIN during the 1−D interval. Thus, the input capacitors conduct a square-wave current of peak-to-peak amplitude equal to the output current. It follows that the resultant capacitive component of AC ripple voltage is a triangular waveform. Together with the ESR-related ripple component, use Equation 20 to calculate the peak-to-peak ripple voltage amplitude.
Use Equation 21 to calculate the input capacitance required for a particular load current, based on an input voltage ripple specification of ΔVIN.
Low-ESR ceramic capacitors can be placed in parallel with higher valued bulk capacitance to provide optimized input filtering for the regulator and damping to mitigate the effects of input parasitic inductance resonating with high-Q ceramics. One bulk capacitor of sufficiently high current rating and four 10-μF 50-V X7R ceramic decoupling capacitors are usually sufficient for 12-V battery automotive applications. Select the input bulk capacitor based on its ripple current rating and operating temperature range.
Of course, a two-channel buck regulator with 180° out-of-phase interleaved switching provides input ripple current cancellation and reduced input capacitor current stress. The previous equations represent valid calculations when one output is disabled and the other output is fully loaded.