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I using a TI buck regulator (LM21212) to power a bunch of components on my board at a certain voltage. When calculating stability, do I need to take into account all the decoupling capacitors (e.g. 0.1 uF and 0.01 uF) that I have placed next to all the components that is powered by the regulator?

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Practically speaking, it's safe to ignore the decoupling capacitors around your circuit when designing a switching power supply. Pedantically, those capacitors will have some affect on the power supply, but they're negligible. Here's a few reasons why:

  1. Most of the decoupling capacitors are electrically "far away". Your power supply is switching fast (probably 100's of kHz or MHz). The natural RC filter that is created by the length of copper trace and the distant 0.1uF capacitor makes it virtually unnoticed to the local LC filter happening right at the output of your power supply.

  2. The output capacitance of switching regulators are typically large. Probably in the 10's or 100's of uF. The few tenths and hundredths of uF peppered around your circuit is lost in the noise of that bulk capacitance. In fact, the manufacturing tolerances on the big capacitors (5% - 20%, usually) will be larger than all the decoupling capacitors combined.

  3. Your design should be stable enough where that little bit of extra capacitance doesn't affect stability margins. In other words, if your power supply is so close to instability that it is relying on a few tenths of uF, you've designed a poor power supply.

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It depends what your specs are for step load response for transient delay and amplitude as well as ringing. Since every contributing dynamic and/or reactive part affects voltage ripple, it cannot be ignored.

Depending on your noisy immunity spec, you must specify worst case Ripple Voltage with a budget from each source. Then test and verify your design and qualified parts with step load response and ripple according to the design needs and phase margin if ringing is excessive.

The good part is the IC spec includes a PID compensation filter design.

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