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18 expect problems when noise couples to a sensitive signal, but here it was coupling with a power pin. The resulting strange functionality made the problem difficult to diagnose. The power converter still worked, but it put out the wrong DC voltage by a few millivolts. In some circuits, a couple of millivolts would not matter, but they caused serious problems in our precisely sized circuit. As designs become denser, surprises like this may pop up more frequently. Resonant frequencies and ringing at higher switching frequencies can sometimes cause problems, depending on the circuit and application. Increasing switching frequency helps make physically smaller circuits. It's interesting to note that as chip designs have become denser, their resonant frequencies between chip and package have not changed much, largely because as silicon capacitance went up, technology evolved to reduce inductance. To make switching most efficient for power applications, however, you must reduce capacitance and parasitic inductance simultaneously. This reduction makes all the ringing frequencies go up as switching frequencies increase, which does not create a problem as long as no other signaling is in that resonant frequency range. The switching frequencies of DC sources are now often in the few-megahertz range, while the clock frequencies of the major chips are in the gigahertz range. Overcoming power-distribution challenges requires being educated about applications and circuit designs as well as checking everything as much as possible. Simulation is not always the answer. The DC-DC converter problem cited earlier would never have been discovered in simulation before it actually happened. When modifying an existing design, look carefully at the geometry. If you have a working design with high-current circuit loops of a certain size and distance and you make the design denser, the ratio between the distance and the size should stay the same. If distances shrink faster than loop shrinkage, you will increase the coupling coefficient, which can cause unexpected problems. '' Overcoming power-distribution challenges requires being educated about applications and circuit designs as well as checking everything as much as possible."

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