Before ideal diode controllers became cheap, engineers used Schottky diode OR-ing to combine two power sources. It's still the go-to solution when you need dead-simple, no-firmware, cost-optimized redundancy — think RTC backup, memory retention supplies, or small IoT devices switching between wall power and a coin cell.

The circuit is trivial: two Schottky diodes, anodes tied to their respective sources (main supply and backup battery), cathodes joined at the load. Whichever source is higher wins — its diode conducts, the other reverse-biases. If main power drops below the battery voltage minus a diode drop, the battery seamlessly takes over. No glitches, no switching transients, no controller IC.

Why Schottky, not standard silicon? Forward drop. A 1N4148 costs you 0.7V; a BAT54 costs about 0.3V at low current. For a 3.3V rail fed from a 3.6V lithium primary, that difference is the gap between "works" and "browns out." Schottkys also switch fast enough that the load capacitance holds the rail steady during the handoff.

Key design considerations:

• Reverse leakage: Schottkys leak — often 1-100 µA at rated reverse voltage, and it doubles roughly every 10°C. For a coin cell backup, this leakage drains your battery even when main power is present. Pick a low-leakage part (BAT54 series is ~2 µA typical) or you'll murder a CR2032 in months.
• Reverse charging: Never OR a rechargeable NiMH with a primary lithium — if the diode fails shorted, you can force-charge a non-rechargeable cell and cause venting or fire.
• Voltage headroom: Whatever downstream regulator you feed must tolerate V_source − V_F. An LDO with 200 mV dropout fed from 3.3V through a Schottky sees ~3.0V — cutting it close for a 2.8V rail.

Real-world example: A datalogger runs from a 5V USB supply and falls back to a 3V CR2032 for the RTC and SRAM during outages. Two BAT54s OR the sources; the RTC sees ~4.7V during USB operation and ~2.7V on battery. The RTC's spec sheet lists 1.8V minimum, so we're fine. Estimated battery leakage: 2 µA × 24 h × 365 d ≈ 17.5 mAh/year — a CR2032 (220 mAh) lasts over a decade.

Rule of thumb: Load current × V_F = power dissipated in the diode. Above roughly 100 mA continuous, the Schottky loss (30-50 mW) becomes wasteful and you should switch to an ideal diode controller with a MOSFET pass element.