Analog Angle Article

Depletion-mode MOSFET emulates mechanical switch

The old-fashioned mechanical switch has many desirable attributes: near-infinite off-resistance, near-zero on-resistance, operation down to zero volts, and can be normally open or normally closed, among other features. A new series of quad and dual N-channel matched-pair depletion-mode MOSFETs from Advanced Linear Devices (ALD) comes close. These ultraprecise devices use the company's EPAD (electrically programmable Analog Device) process technology to yield MOSEFT arrays which operate in the normally-off mode, with near-zero power consumption.

The devices are well-suited to sub-1 V circuits (0.7 V), including fail-safe circuits, battery backup systems, micropower energy harvesting, and similar situation where a normally closed relay function is needed. They can be used for controlling secondary power backup systems, but without drawing power from the secondary source, critical in applications where that source is very limited in capacity. Unlike electromechanical relays, with their millisecond switchover time, these MOSFETs switch over in the low-nanosecond range. Key specifications include matched gate threshold voltage of -0.4 V with 0.020 V tolerance for the A grade, and -0.4 V with 0.04 V tolerance for the B grade. The drain provides sink current up to 20 microamps with an applied input voltage of 0 V at the gate.

The depletion-mode MOSFET are designated ALD114804 and ALD114904, and are available as dual and quad devices in plastic DIP and SOC packages. Prices begin at $0.91 (1000 pieces)..

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