The input capacitor selection and placement on the board layout is very important for any buck converter design. This component provides the pulsing high di/dt current every switching cycle and reduces the input voltage ripple seen by the buck converter. Use a good-quality 10-µF, 1210 (3225) case size, X5R or X7R ceramic capacitor with sufficient voltage rating on the input of the device. Alternatively, in applications with strict size constraints and more stable input voltage it is possible to use a 10-µF, 1206 (3216) case size or a parallel combination of 2 × 4.7-µF, 0805 (2012), X5R or X7R capacitors. Ceramic capacitors have a DC bias dependence on their effective capacitance and can de-rate their value significantly when used at higher bias voltage. TI recommends ceramic capacitors with ≥ 50-V rating when using the device with a 24-V input supply. TI recommends ceramic capacitors with ≥ 25-V rating when using the device with a 12-V input supply.

Just like with any buck converter, place the input capacitor as close as possible and next to the LMZM23601. Connect the capacitor directly to the VIN (pin 3) and GND (pin 1) terminals of the device. This placement ensures that the area of the high di/dt current loop in the buck converter is kept to a minimum, resulting in the lowest possible inductance in the switching current path. The proper placement of the input capacitor in any buck converter helps to keep the output noise of the converter to a minimum. See Table 8-2 for several input capacitor choices.

For this design example a single 10-µF, 50-V 1210 X7R capacitor is used.