8.2.1.2.5 Input and Output Capacitors

Input and output capacitors are not necessary in all applications. Any current that the external MOSFET conducts in the on-state will decrease very quickly as the MOSFET turns off. All trace inductances in the design including wires and printed circuit board traces will cause inductive voltage kicks during the fast termination of a conducting current. On the input side of the LM5060 circuit this inductive kick can cause large positive voltage spikes, while on the output side, negative voltage spikes are generated. To limit such voltage spikes, local capacitance or clamp circuits can be used. The necessary capacitor value depends on the steady state input voltage level, the level of current running through the MOSFET, the inductance of circuit board traces as well as the transition speed of the MOSFET.

Since the exact amount of trace inductance is hard to predict, careful evaluation of the circuit board is the best method to optimize the input or output capacitance or clamp circuits.