As with all buck converters, the input current of the TPS6287x devices is discontinuous. The input capacitors provide a low-impedance energy source for the device, and the value, type, and location are critical for correct operation. TI recommends low-ESR multilayer ceramic capacitors for best performance. In practice, the total input capacitance is typically comprised of a combination of different capacitors, in which larger capacitors provide the decoupling at lower frequencies and smaller capacitors provide the decoupling at higher frequencies.

The TPS6287x devices feature a butterfly layout with two pairs of VIN and GND pins on opposite sides of the package. This allows the input capacitors to be placed symmetrically on the PCB so that the electromagnetic fields generated cancel each other out, thereby reducing EMI.

The duty cycle of the converter is given by:

where

• V_IN is the input voltage.
• V_OUT is the output voltage.
• η is the efficiency.

The value of input capacitance needed to meet the input voltage ripple requirements is given by:

where

• D is the duty cycle.
• f_sw is the switching frequency.
• L is the inductance.
• I_OUT is the output current.

100mV is used as the input voltage ripple target.

The value of C_IN calculated with Equation 9 is the effective capacitance after all derating, tolerance, and aging effects have been considered. 5µF effective capacitance per input pin is required. TI recommends multilayer ceramic capacitors with an X7R dielectric (or similar) for C_IN, and these capacitors must be placed as close to the VIN and GND pins as possible to minimize the loop area.

Table 10-4 lists a number of capacitors suitable for this application. This list is not exhaustive and other capacitors from other manufacturers can also be suitable.