Simple switcher

My suggestion is to open the pdf datasheet of any prospective switcher IC and carry out a text search (Ctrl + F) for the world foldback. If you find it, question the vendor about its full impact before you select the part for your application. Foldback is, in general, a good idea in terms of protecting the converter under abnormal conditions, but it should be used very judiciously so as not to impact normal behavior. For example, the Simple Switcher family has a hidden second-level current limit protection at which frequency foldback (or skipped pulses) occurs. But that trip level can only be encountered under very severe conditions—namely, a sudden overload with a completely incorrectly sized inductor that hard-saturates in the process. At other times it is not encountered and doesn t therefore interfere. It is considered transparent to all but the most novice engineers. And that is what I consider the right type of foldback. 

Figure 4.14 SPICE equivalent schematic for 78S40 simple switcher IC.

Test your switcher s phase margins with something very close to the final load as early as possible. As mentioned previously, even simple startup problems of switchers do not usually show up with resistive loads. For these, you really need to test using an electronic load, placed at least in constant current (CC) mode. 

In a particular case I handled, a major Japanese customer was managing to blow up his simple Buck switcher under short-circuits on the output. I knew that these parts had not only the usual cycle-by-cycle current limiting (their first line of defense), but in fact, a hidden second level of current limit protection, which if ever encountered, caused protective... 

Figure 12-8 A Simple Check of the Results Provided by an Online Tool for an Emulated Current Mode Control Switcher...

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