SNVS872D August 2012 – August 2018 LMZ21700
The added inductance of long input cables together with the ceramic (and low ESR) input capacitor can result in an underdamped RLC network at the input of the buck converter. This can cause oscillations on the input and instability. If long wires are used, it is recommended to add some electrolytic capacitance in parallel with the ceramic input capacitor. The electrolytic capacitor's ESR will improve the damping.
Use an electrolytic capacitor with CELECTROLYTIC ≥ 4 x CCERAMIC and ESRELECTROLYTIC ≈ √ (LCABLE / CCERAMIC)
For example, two cables (one for VIN and one for GND), each 1 meter (~ 3 ft) long with ~1.0 mm diameter (18 AWG), placed 1 cm (~0.4 in) apart will form a rectangular loop resulting in about 1.2 µH of inductance. The inductance in this example can be decreased to almost half if the input wires are twisted. Based on a 22 µF ceramic input capacitor, the recommended parallel CELECTROLYTIC is ≥ 88 µF. Using a 100 µF capacitor will be sufficient. The recommended ESRELECTROLYTIC≈ 0.23 Ω or larger, based on about 1.2 µH of inductance and 22 µF of ceramic input capacitance.
See application note SNVA489C for more details on input filter design.