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RE: Speaking of Linear Power Supplies....

Hi,

Curious... I cannot make any active regulation circuit to have a greater Delta I / Delta T ability than a correctly implemented passive decoupling combo. Nor have I seen any regulator IC or discrete circuit that could make such a claim. At least not directly at the IC Pin's, where an 0402 or 0603 cap usually fits well and where it matters.

The trick is not to make the regulator as fast as possible (it is ALWAYS going to be too slow), but to tune the whole supply rail so that for each element in the decoupling circuit any current "too fast" is taken over by an element closer to the IC Pin supplied, of a value sufficient to deal with any current steps.

In this kind of set-up there is no benefit making the regulator fast, but low noise is always good. On the contrary, often it is better slow the regulator down (most brutally using a choke in series) so it does not fight the decoupling caps. A network analyser can help here. There is no absolute "patent recipe", one must analyse the demands a given circuit node imposes on the power supply and then design accordingly.

In the end it is easy to see if you succeeded even without network analyser. Just play a low frequency, high level square-wave and look at the supply pins with a differential probe. If there is no appreciable noise, whatever is decoupling/regulating the pin has enough Delta I / Delta T. If you see a lot of noise, usually there are insufficiently damped resonances or insufficient decoupling.

Oh and watch ground bounce, because sadly common logic ties the negative supply and ground together, silly thing that. That is why looking at PSU decoupling differential probes are a must.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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