Quad 2-input AND gate in 14-SOIC — 74LS logic for 5 V boards
The Texas Instruments SN74LS08D is a quad 2-input AND gate from the 74LS Schottky-clamped TTL family, packaged in a 14-SOIC (3.90 mm width) for surface-mount assembly. The 74LS series balances speed and power — propagation delay is 20 ns at 5 V with a 15 pF load — making it a fit for legacy 5 V bus interfaces, control logic, and glue logic in commercial-temperature equipment (0°C to 70°C).
The 20 ns max propagation delay at 5 V and 15 pF load sets the timing budget for cascaded gates in address decoding, clock gating, or enable chains. In a 10 MHz bus with a 100 ns cycle, two LS08 gates in series eat 40 ns of margin — enough to matter if the path also includes a decoder or latch. The Schottky clamp reduces storage time versus standard TTL, so the delay is consistent across temperature and supply within the rated range.
Output drive and fan-out
Each gate sources 400 µA and sinks 8 mA at the output. That 8 mA sink can drive up to ten 74LS loads (each 0.4 mA input low current) directly, or a single standard TTL load. For driving a higher-current load like an LED or a relay coil, buffer the output with a transistor or a line driver — the 8 mA ceiling is firm.
Supply range and noise immunity
The 4.75 V to 5.25 V supply covers a nominal 5 V rail with ±5% regulation. Input low threshold is 0.8 V max, high threshold 2 V min, giving about 0.4 V of low-side noise margin and 0.7 V on the high side at 5 V. That is adequate for a clean backplane but marginal in a noisy industrial environment — the 74LS family was designed for controlled commercial settings, not factory-floor transients.
It belongs in office equipment, appliances, test gear, and indoor telecom — not in an engine bay, outdoor enclosure, or cold warehouse. If the board sees -20°C or 85°C ambient, step to the 74LS08D's industrial sibling (SN74LS08D with a different suffix) or move to 74HC08D which offers wider temperature range.
For new designs, the 74LS family remains a valid choice for 5 V-only legacy systems where lower-power 74HC or 74HCT logic is not pin-compatible or where the Schottky speed is needed.