decouplingdecoupling-capacitorpower supply
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"?
The most likely reason why that is done is because, in real life, capacitors do not have infinite bandwidth. Generally, the higher the capacitance of the capacitor, the less it will be able to react to high frequencies, while small-valued capacitors react better to higher frequencies, as seen in the graph below. Using two different-valued capacitors together is just done to improve the response of the filtering.
Large decoupling capacitors such as those are meant to compensate for larger sags in the supply of the chip. Using a larger value will not hurt.
Don't skimp out on the smaller value decoupling capacitor though, since it's used to filter high-frequency noise from/on the supply; use as close to the prescribed value as possible, otherwise use a 100nF.
Best Answer
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:
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.