In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Peter,

I absolutaly agree (maybe, I did not make this clear enough in my last post) - internal switching times and propagation delays inside the silicon are more-or-less constant, and independent of the clock oscillator.

Thomas,

when I mentioned the 5 ns transition time, this was just an estimate. Assuming that the SX silicon was designed for a maximum of 100 MHz clock, i.e. for a 10 ns clock period, I made the assumption that the internal transition time must be at least half the clock period, i.e. 5 ns but I guess that the 2 ns you had mentioned are much more realistic.

Concerning the DC/DC converter: For one of my commercial projects, I needed a 24/5 step-down DC/DC converter and I was lucky enough to acquire 300 Rohm BP5220 converters at a very reasonable price on eBay, assuming that this supply of converters would be more than I ever needed. (Un)fortunately, it soon turned out that I had under-estimated the number of units that could be sold. Instead of buying  BP5220 at a remarkably higher price, I designed this little PCB around the LM2574, pin-compatible to the BP5220 and my costs for this replacement were lower than the regular price for the BP5220. Another big advantage of the LM2574-based design is that it has short-circuit protection where the BP5220 does not. Shorting the BP5220 usually immediately shorts the internal series transistor. This means that the full input voltage will be fed into the output pin after removing the short, causing a "nice" chain reaction - did you ever see how an SX looks like after 24V have been fed into the Vdd/Vss pins?

In the meantime, I have re-designed most of my SX-based boards to have an LM2574 on-board instead of using the small add-on board.

In the end, I can only report very positive results with the LM2574 - you are right - no large heatsinks required, although there are some "hidden" ones - the ground planes on both sides of the PCB.

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