Inductors and ferrite beads
For an inductor you look at saturation current and DCR; for a ferrite bead, at impedance against frequency. They look alike but serve different purposes.
Choosing an inductor
| Specification | Meaning |
|---|---|
| Inductance | The converter design value (sets the ripple current) |
| Saturation current Isat | Above this the inductance falls sharply |
| Rated current Irms | The current at which self-heating raises the temperature by 40 °C |
| DCR | DC resistance, conduction loss |
In a buck converter, Isat must exceed the maximum load current + half the ripple. On saturation the current shoots up in an instant and can destroy the switch. A magnetically shielded type leaks less field into nearby circuits.
Ferrite bead
A bead is nearly 0 Ω at low frequency and hundreds of ohms at tens to hundreds of MHz, dissipating high-frequency noise as heat. Datasheets state it as, for example, "600 Ω at 100 MHz".
- Between the digital supply and the analog supply (AVDD)
- EMI measures on USB and cable signal lines
Cautions
A bead's impedance also falls when DC current flows through it. A bead and a capacitor can resonate and make noise worse at a particular frequency, so when a large capacitor follows a bead, check by simulation or measurement.