Dual 1-of-4 decoder/demultiplexer in 16-SOIC
The SN74HCT139DR is a dual 1-of-4 decoder/demultiplexer from TI's 74HCT logic family. Each of the two independent circuits selects one of four active-low outputs based on a 2-bit binary address input, with a common active-low enable per decoder.
The 1.27mm lead pitch and gull-wing leads are pick-and-place friendly with good solder-joint inspection access. Surface-mount assembly on a two-layer board is straightforward — the 4mA output drive per channel does not demand heavy copper pour for thermal management. Standard reflow profiles for 1.6mm FR-4 apply.
Output drive and fan-out
Each output sources or sinks 4mA at the rated supply voltage. This is sufficient to drive up to 10 standard 74HCT gate inputs per output (fan-out of 10), or directly switch a low-power LED indicator through a current-limiting resistor. The 4mA drive is typical for HCT family parts — adequate for on-board logic fan-out but not intended for driving relays or high-capacitance bus lines without a buffer.