Implied expectation

Why should they There is no guarantee that a part must switch without excessive jitter, for example. These are only implied expectations you may have when you buy an IC. Further, what exactly constitutes excessive Every switcher has some jitter ... 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

Another milder example of this is a standard Non-Synchronous Buck switcher IC. Every single typical applications diagram on the datasheet shows a Schottky diode, without perhaps explicitly stating as much. This is an example of an implied expectation on the part of the vendor—that you, the customer, won t miss the truly obvious. Yet there are many who think they have achieved some slender advantage in substituting an ultra-fast diode in its place. First read the Abs Max section of the EC tables carefully. Most vendors specify that the SW node should never be taken more than 0.4V below IC ground. That is because... 

Author s Comments The customer may have thought or assumed that if it was spec d to this value, it would work fine for the current application, but he or she actually needed to confirm that from the vendor. We should always be wary of implied expectations, as pointed out previously on several occasions. 

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