Board of Education Shield -- capacitors chosen in schematic

ptel

Hello,

I bought the "Robotics Shield Kit (for Arduino)"/"Board of Education Shield(BoE)" a while ago and was interested in understanding the schematic.
https://www.parallax.com/downloads/board-education-shield-schematic

So I dug deeper and looked at the data sheet for the voltage regulator LM2940L. Under "typical applications" the data sheet suggests a single 22µF capacitor to be put in parallel after the voltage output of the regulator. However the BoE uses two capacitors rated at 33µF and 100nF, instead.

I understand the point of up-sizing the capacitor to have more buffer in case of sudden load increases (by the motors). But why exactly 33µF and why 100nF in parallel? Why not just a single bigger capacitor?


Another point that puzzles me is why in the schematic the 5V out is attached to C2, the 100nF capacitor, but for the 3V out it is attached to C3, the 33uF capacitor. Is there a special reason for not connecting the 3V out to C4 instead?

I am also somewhat surprised the capacitors for the 5V and 3.3V regulators are the same.

I would greatly appreciate any clarifications on those subjects. Reading up general information about capacitors and voltage regulators got me only thus far.

Phil Pilgrim (PhiPi)

There's more to an output capacitor than just the capacitance. There's also equivalent series resistance (ESR). Some regulators, like the LM2940, are very particular about the ESR of their output capacitors. If an aluminum electrolytic is used instead of a tantalum, it may have too high of an ESR to keep the regulator stable. To remedy this, a smaller-value ceramic cap can be added in parallel. Ceramic caps have very low ESR values, and the combination will bring the net capacitance and ESR into spec.

As to which caps the 5V and 3V outputs are connected to, they're both connected to both of their respective caps. Unless it's important, a schematic will not show how things are connected together, only that they are. Even bypass caps, which are spread all over a board, might be shown clumped together in the schematic next to the regulator. In the BOE Shield's case the connection order is probably not important anyway, as the caps in each pair are likely very close to each other on the PCB.

-Phil

Genetix

ptel,

The 33 uF will filter out the low frequency surges such as from the servos while the 100 nF, or 0.1 uF, will filter out the high frequency spikes such as the microcontroller.

If you look at other schematics you will often see 0.1 uF capacitors across the power pins of ICs. These are called decoupling capacitors since they remove the noise caused by other high-speed switching devices on the power pins.