What the 680 µH rating means for your power rail
The Coilcraft LPS4018-684MRC is a shielded drum-core inductor from the LPS4018 series, rated 680 µH ±20% at 100 kHz test frequency. The 120 mA current rating matches the saturation current (Isat) — this is a design where the core saturates at the same current the winding can carry, so the inductor's usable range is the full 120 mA without a separate derating step for saturation.
Shielding and DCR — the trade-off in a 3.9 mm footprint
The ferrite core is fully shielded, which keeps the magnetic field contained — critical when the inductor sits next to a sensitive ADC, a CAN transceiver, or a clock oscillator on a dense automotive PCB. The 11.5 Ohm max DCR is the penalty for 680 µH in a 3.9 x 3.9 mm package: at 120 mA, the copper loss is about 165 mW, which the 1.8 mm seated height dissipates into the board plane without a hot spot. Self-resonant frequency is 3.5 MHz — above that, the inductor behaves capacitively. For a switching regulator switching at 500 kHz to 2 MHz, the SRF provides enough headroom that the impedance stays inductive across the loop bandwidth.
The nonstandard package means the footprint is specific to this series — no direct drop-in from a different manufacturer without a layout change.