TL;DR: RTFD. Always. More detail at the bottom.

Resistors

This is the easiest of the bunch.

• When specifying a tolerance, use 1% for most applications.

  • 5% and higher can be cheaper and easier to find, but the extra penny or two that it takes to get the better tolerance package is often worth it
  • Too wide of a tolerance, and your devices may be out of spec!

• For very small resistances, like current sense, you should use < 1%

  • You’ll know if you need this amount of tolerance

• Each package has different power ratings: make sure to pick the right rating for the job!

  • UTAT uses packages no smaller than 0603 for your soldering convenience

Look at the imperial side only. Yes, it’s annoying. Yes, there are duplicate codes. I know.

Inductors

DCR

All inductors have a DC winding resistance — it’s basically a resistor in series with an ideal inductor.

Higher DCR means more efficiency loss, since power is lost in the windings.

Higher also DCR means that there is a lower max current, due to heat constraints

For SMPS, you want lower DCR to improve the efficiency of the converter

Current Rating vs Saturation Current

Saturation current is the point where the inductor can no longer store any more energy

Saturation current is defined as the current at which the inductance drops by a certain percent: different manufacturers use different thresholds, so double check at the datasheet!

Since the inductance changes rather quickly at this point, it means that the inductor current waveform becomes distorted

The current rating is the maximum continuous DC current that the windings of an inductor can handle given a certain temperature rise. This relates to DCR.

At higher currents, the inductor gets hot, so the inductance decreases, so peak current increases, which results in a positive feedback loop that results in thermal runaway!

For example, in a 5A buck converter, you know that the max DC current is 5A, so you should find an inductor that has a current rating of 5A. However, depending on the design, the current ripple is around 50% of the DC current. So, you need an inductor that has a saturation current of 5A DC + 2.5A ripple = 7.5A