In practice, the total output capacitance typically comprises a combination of different capacitors, in which larger capacitors provide the load current at lower frequencies and smaller capacitors provide the load current at higher frequencies. The value, type, and location of the output capacitors are critical for correct operation. TI recommends low-ESR multilayer ceramic capacitors with an X7R dielectric (or similar) for best performance.

The TPS6287Bx devices feature a butterfly layout with two GND pins on opposite sides of the package. This allows the output capacitors to be placed symmetrically on the PCB such that the electromagnetic fields generated cancel each other out, thereby reducing EMI.

The transient response of the converter is defined by one of two criteria:

• The loop bandwidth, which must be at least 4 times smaller than the switching frequency.
• The slew rate of the current through the inductor and the output capacitance.

  In typical low-output-voltage application, this is limited by the value of the output voltage and the inductors.

Which of the above criteria applies in any given application depends on the operating conditions and component values used. We therefore recommend calculating the output capacitance for both cases, and selecting the higher of the two values.

If the converter remains in regulation, the minimum output capacitance required is given by:

If the converter loop saturates, the minimum output capacitance is given by:

In this case, choose C_OUT(min) = 195 μF, as the larger of the two values, for the output capacitance.

When calculating worst-case component values, use the value calculated above as the minimum output capacitance required. For ceramic capacitors, the nominal capacitance when considering tolerance, DC bias, temperature, and aging effects is typically two times the minimum capacitance. In this case the nominal capacitance is thus 390 μF.