12V fixed output with 0.8V dropout at 1A — the headroom trap
The LM2940CSX-12 is a fixed 12V positive-output LDO from Texas Instruments, rated for 1A continuous output with a maximum dropout of 0.8V at that full load. That dropout figure is the spec that catches designers: a 12V rail fed from a 13V source leaves only 0.2V headroom, which is below the dropout floor — the regulator drops out and the output follows the input minus the pass element voltage. For reliable regulation, budget at least 1V above 12V at the input under worst-case load and lowest line.
66dB PSRR at 120Hz — post-rectifier filtering without a second stage
That attenuation knocks down 1Vpp of 120Hz ripple to about 0.5mVpp at the output — enough for most analog and logic loads without a second LC filter. The PSRR rolls off above a few kHz, so switching-regulator feed-through at 100kHz or higher passes with less rejection; a post-filter or a separate low-noise LDO is needed for sensitive RF or audio rails.
That means TI is not actively promoting it for new builds, and the eventual EOL notice is a matter of when, not if. For existing production, the part is still available through independent distribution on a quoted-to-order basis — no stock-holding claim, sourced per RFQ.
Protection set and operating range
The LM2940CSX-12 integrates over-current, over-temperature, reverse polarity, and short-circuit protection — a full guard band that reduces external clamp and foldback circuitry. Input voltage range extends to 26V maximum, giving a 14V headroom above the 12V output. Operating temperature is rated from 0°C to 125°C, which covers commercial and most industrial ambient environments but not extended automotive or military ranges.