With system voltages dropping, load currents rising, and clock rates increasing, effective power management is now crucial to system performance. Consider, for example, the integration of dc-dc converters that mount on pc boards along with their associated system circuits. Converter manufacturers cannot know every application beforehand, so they design converter circuits for typical loads. To optimize an application, the designer must add capacitors between the converter and the specific load (Fig. 1).
Optimizing the converter-load interface requires knowledge of both the converter and its load. Critical converter characteristics include its bandwidth, output current slew rate, and output impedance. These parameters determine its ability to respond to rapid load changes. Important load characteristics include the system clock rate, the type of load changes to expect, and the allowable voltage deviation due to load transients. Also, system layout affects power delivery because the pc-board's parasitic resistance and inductance result from the power and ground planes located between the converter and its load.
One solution to this system problem is placing capacitors across the load that act as reservoirs to supply charge during a step load, and absorb charge during a step unload until the converter can respond. The optimum solution is to determine the capacitor configuration that provides the required performance at the lowest cost and the smallest physical size.
Great explanation of why you need a mix of capacitor values and types for circuit board decoupling.
Eliot Glaser -November 10, 2004
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