5.1 V Zener in a hermetic SQ-MELF
The 1N6490US is a 5.1 V Zener diode from Microchip Technology in a hermetic SQ-MELF (D-5A) surface-mount package, rated for 1.5 W continuous power dissipation. The ±5% tolerance on Vz is the standard precision band for general-purpose voltage reference or clamping — tight enough for most rail-clamp and bias circuits, though a 2% or 1% part would be needed for a precision reference where the load draws significant current through the Zzt impedance. With a maximum Zzt of 7 Ohms at the test current, regulation stiffens as the current increases — the dynamic impedance drops further above the knee, so a bias current of 10–20 mA yields a cleaner reference than running at the 1 mA minimum. Reverse leakage is specified at 1 µA max at 1 V reverse, well below the knee, confirming the junction quality for low-leakage applications.
Hermetic package and temperature range
The SQ-MELF (D-5A) is a hermetic, ceramic-based surface-mount package with a metal lid — the same construction used in JAN and MIL-S-19500 qualified devices. It handles the full -65°C to 175°C operating temperature range without the moisture sensitivity or thermal-cycle fatigue of plastic packages. This makes the 1N6490US a natural fit for avionics, satellite, downhole instrumentation, and other high-reliability environments where a plastic SOT-23 or SMA would not survive the temperature extremes or outgassing requirements. The forward voltage spec — 1.5 V max at 1 A — also means the diode can be used as a high-temperature rectifier in a pinch, though the 1.5 W power limit constrains continuous forward current to about 1 A at 25°C ambient, derating to zero at 175°C case temperature.