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Talking about the seventh capacitor: the equivalent series resistance ESR of the capacitor

Published on:2021-05-28

     The general view is: a relatively large external capacitor with a small equivalent series resistance (ESR) can well absorb the peak (ripple) current during fast switching. However, sometimes such a choice may easily cause the voltage regulator (especially the linear regulator LDO) to be unbalanced, so the capacitance value of the small and large capacity capacitors must be selected reasonably. Always remember that a voltage regulator is an amplifier, and it will appear in every situation that an amplifier may have.
     Because the response speed of the DC/DC converter is relatively slow, the output decoupling capacitor plays a leading role in the load step phase, so additional large-capacity capacitors are needed to slow down compared to DC/DC The rapid conversion of the converter, while using high-frequency capacitors to slow down the rapid conversion relative to large capacitors. Generally, the equivalent series resistance of the large-capacity capacitor should be selected to an appropriate value so that the peak value and glitch of the output voltage are within the Dasheet specification of the device.

     In high-frequency conversion, small-capacity capacitors in the order of 0.01uF to 0.1uF can well meet the requirements. Surface mount ceramic capacitors or multilayer ceramic capacitors (MLCC) have a smaller ESR. In addition, under these capacitance values, their volume and BOM cost are relatively reasonable. If the local low-frequency decoupling is insufficient, the input voltage will decrease when switching from low-frequency to high-frequency. The voltage drop process may last for several milliseconds, and the length of time mainly depends on the time for the regulator to adjust the gain and provide a larger load current.

    It is certainly more cost-effective to use a capacitor with a large ESR in parallel than a single capacitor with just as low ESR. However, this requires you to find a compromise between PCB area, number of components and cost.

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