I am looking over a datasheet, and I saw something that puzzled me. Why is there a 1 ohm resistor (R2) in series with the capacitor?

This is a power rail, so I'll assume it is a decoupling capacitor.

Pin 15 is VREG_OUT - Power Regulator output (1.8 V while awake, 0 V during deep sleep).

After a few Google search attempts I was not satisfied with the responses or lack thereof. What is a plausible purpose of "R2"?

Snip of Schematic


Some linear regulators are not stable when the ESR of the output capacitor is too low. This can result in oscillations when high-Q ceramic capcitors are used at the output.

The designer may have been concerned this issue applied to the regulator used, and added the series resistor as a solution.

Edit After reviewing the linked datasheet, this is almost certainly the reason. Note this line on page 207 of the datasheet:

enter image description here

The added one ohm resistor makes up the difference in ESR between the X5R ceramic capacitor chosen for the application circuit (pg. 7) and the 2 ohm minimum for stable operation.


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