The TPS2585x-Q1 device requires a high frequency input decoupling capacitor or capacitors, depending on the application. A high-quality ceramic capacitor type X5R or X7R with sufficient voltage rating is recommended. The ceramic input capacitors provide a low impedance source to the converter in addition to supplying the ripple current and isolating switching noise from other circuits. The typical recommended value for the high frequency decoupling capacitor is 10 μF of ceramic capacitance. This must be rated for at least the maximum input voltage that the application requires; preferably twice the maximum input voltage. This capacitance can be increased to help reduce input voltage ripple, maintain the input voltage during load transients, or both. In addition, a small case size 100-nF ceramic capacitor must be used at IN and PGND, immediately adjacent to the converter. This action provides a high frequency bypass for the control circuits internal to the device. For this example a 10-μF, 50-V, X7R (or better) ceramic capacitor is chosen, and the 100-nF ceramic capacitor must also be rated at 50 V with an X7R or better dielectric.

Additionally, an electrolytic capacitor on the input in parallel with the ceramics can be required, especially if long leads from the automotive battery to the IN pin of the TPS2585x-Q1, cold or warm engine crank requirements, and so forth. The moderate ESR of this capacitor is used to provide damping to the voltage spike due to the lead inductance of the cable or the trace.