I am hoping to use a microSD card in an AVR project and am just trying to find values for all my components.
However, I am not sure what value of ferrite to use here. The few microSD schematics that I've seen with a bead just said "FERRITE" and didn't specify an inductance, or an impedance at a given frequency.
I have an Olimex development board with a full-size SD card using a 470 nH inductor here (PX128A1).
simulate this circuit – Schematic created using CircuitLab
Is this an appropriate inductor for a microSD card's supply voltage as well?
Edit: the accepted answer is a good general answer, but I also have specific example of another Olimex board using a microSD. The OLiniXino board (schematics) uses a microSD with a 470 nH/1.76 R/250 mA inductor for Vcc filtering. They use the same inductors for SD/MCC and microSD. I suspect this is due to being able to reuse an inventory part, but at least it shows a known-working value.
Best Answer
The bead is meant to absorb high-frequency noise currents (and dissipate it as heat) to maintain adequate high-frequency separation between shared power rails. There is a common type of ferrite beads that are designed to do exactly that. These beads have high impedance over wide high-frequency bands (low-pass filters).
Typical parameters are
- DC resistance -> should be as low as possible
- rated current -> requirements obviously depend on the load
- impedance characteristics curve -> depends on what kind of noise you primarily want to suppress
Look at the parameters of this ferrite bead for example:
This would actually be a good choice for your design. It has a low DC resistance of 0.8 Ohm (minor losses) and is rated for 200mA. The average microSD shouldn't require more than 100mA (but look at the data sheet to be sure).
This shows impedance VS frequency. High-frequency noise, especially in the range from about 7MHz-3000MHz, will be absorbed.