I am evaluating the LM53601-Q1 fixed 3.3V part in an application which requires the part to work from 6V to 21V operational with a load of 280mA. From the data sheet I see that the recommended output filter of 4.7uH and 20uF. Simulation in WEB BENCH indicates about the same values as well. The application circuit I am analyzing (actually two separate ideas) has two different configurations: 1) 10uH (1 amp rated) with 10uF; 2) 10uH (again 1 amp) with a split 10uF in parallel with a ferrite bead shunted by an additional 10uF capacitance. All of this is on a very small PCB so trace length is not a factor.
When simulating Design 2, I cannot include directly of course the ferrite bead so I use an equivalent of 22uF. In this case, @ 6V the simulation indicates that the phase margin is 31 degrees and this does not meet the "requirement" of a minimum phase margin of 35 degrees. Design 1 is even worse, the phase margin being very close to 16 degrees.
My question(s) are as follows: (Let me preface all of this: WEB BENCH is a nice convenient tool for sure, but I take all simulations with a grain of salt until I can measure something for real and confirm data) How tolerant is this part to using a 10uH inductor? Where is the 35 degree minimum phase margin specified, or is this a recommendation based on general stability analysis of typical current mode control SMPS? How seriously should the output filter values be taken?
