Input capacitors are necessary to limit the input ripple voltage of the converter due to switching-frequency AC currents. TI recommends using ceramic capacitors to provide low impedance and high RMS current rating over a wide temperature range. Equation 11 gives the input capacitor RMS current, where D = V_OUT/V_IN is the converter duty cycle. The highest input capacitor RMS current occurs at D = 0.5, at which point the RMS current rating of the capacitors must be greater than half the output current.

Equation 11.

Ideally, the DC and AC components of input current to the buck stage are provided by the input voltage source and the input capacitors, respectively. Neglecting inductor ripple current, the input capacitors source current of amplitude (I_OUT – I_IN) during the D interval and sink I_IN during the 1 – D interval. Thus, the input capacitors conduct a square-wave current of peak-to-peak amplitude equal to the output current. The resultant capacitive component of AC ripple voltage is a triangular waveform. Together with the ESR-related ripple component, Equation 12 gives the peak-to-peak ripple voltage amplitude:

Equation 12.

Equation 13 gives the input capacitance required for a particular load current:

Equation 13.

where

• ΔV_IN is the input voltage ripple specification.

The Enhanced HotRod QFN package of the LM64440-Q1 and LM64460-Q1 provides two input voltage pins and two power ground pins on opposite sides of the package. This allows the input capacitors to be split and placed optimally with respect to the internal power MOSFETs, thus improving the effectiveness of the input bypassing. The converter requires a minimum of two 4.7µF ceramic input capacitors, preferably with X7R or X7S dielectric and in 1206 or 1210 footprint. In this example, place two 10μF, 50V ceramic capacitors in a symmetrical layout immediately adjacent to the converter – one at each input-to-ground pin pair: [VIN1, PGND1] and [VIN2, PGND2].

Install additional capacitance for automotive applications to meet conducted EMI specifications, such as CISPR 25 Class 5 (that limits EMI over a frequency range from 150kHz to 108MHz). For example, place a 10nF, 0402 ceramic capacitor at each input-to-ground pin pair immediately adjacent to the converter. These capacitors minimize the parasitic inductance in the switching loops and can suppress switch-node voltage overshoot and ringing, which reduces high-frequency EMI. The two 10nF capacitors, designated as C_IN-HF1 and C_IN-HF2 in Figure 8-1 or Figure 8-2, must be rated at 50V with an X7R or better dielectric.

As discussed in Section 8.3, a moderate-ESR electrolytic bulk capacitance (68µF to 100µF) at the input in parallel with the ceramics provides low-frequency filtering and parallel damping to mitigate the effects of input parasitic inductance resonating with the low-ESR, high-Q ceramic input capacitors. This is especially true if long leads or traces are used to connect the input supply to the converter.