I recently read a statement somewhere along the lines of “you always want low equivalent series resistance (ESR) capacitors in your design” which got me thinking about ESR and capacitors in general. The truth of the matter is that you do often want capacitors with moderate ESR (>10 Ohms) for bypass applications, especially in voltage regulation circuits. The main reason is that the capacitor introduces filtering (a zero) into the circuit just through its own capacitance and series resistance, and this filtering can be used to stabelize the feedback of a voltage regulator and prevent ringing/oscillations. AN-1148 is a great application note from National semiconductor that gives an overview of feedback in voltage regulation and frequency domain analysis.
Once the concept of adding a zero to the frequency response of a circuit is clear (see this app note if it is not), it should be apparent that this is exactly the reason why designers put several bypass capacitors in parallel when decoupling a complicated integrated circuit. The IC injects a known set of frequencies into the power system and the designer tries to use each of the capacitors, with a unique zero, to try to filter out each frequency individually. This works well for circuits with precise clocks, however, an RC oscillator can have plenty of drift so it would be advantageous to use capacitors with higher ESR which lead to lower Q values and broaden the range of frequencies which they filter out most effectively. This follows the notion that you should not use high quality components to filter a low quality circuit, all components should be of the same quality.
( an-1148 )
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