To run a successful AC Loop Simulation, a periodic waveform is required. If the waveform being simulated is not periodic, AC Analysis usually results in the following error message: Simulation failed. Check your simulation settings. The most common cause of a waveform not being periodic is use of PFM (Pulse Frequency Modulation) Mode or Skip Mode. In the image below, note the non-periodic nature of the ICout waveform during light load conditions: In fact, the waveform envelope and the change in Vout regulation suggest that PFM mode engages at about 7mA. Efforts to perform AC Analysis on this circuit with a load of less than 7mA do indeed result in the above error message. Many of Maxim’s DC-DC converters offer a PFM or Skip mode. When this mode can be enabled/disabled, an option is available on the Design Requirements tab to set this preference. Some (but not all) parts that include a pulse skipping or PFM mode include MAX15038, MAX15053, MAX15062, MAX15066, MAX15462, MAX16935, MAX17230, MAX17242, MAX17505, MAX17506, MAX17521, MAX17532, MAX17545, MAX17546, MAX17550, MAX17620, MAX17632, MAX17761, MAX20002, MAX20075, MAX77874, MAXM17505.