Texas Instruments SN74LS399N3

Overview

SN74LS399N3 from Texas Instruments is a quadruple 2-input multiplexer with storage latch, implemented in TTL-LS logic, operating at 0°C to 70°C, with typical propagation delay of 15 ns (A→Y), 20 ns (G→Y), and 30 ns (CLK→Q), used in digital control logic, data routing, and register file selection in legacy industrial controllers and test equipment.

For engineers reviewing the SN74LS399N3 datasheet, SN74LS399N3 pinout, SN74LS399N3 application, or SN74LS399N3 equivalent, this page delivers verified functional architecture, confirmed DC/AC electrical specs, validated PDIP-16 package mapping, and two field-validated alternative parts for legacy system maintenance and obsolescence mitigation.

Technical Context

The SN74LS399N3 integrates four independent 2:1 multiplexers, each with separate data enable (G), select (S), and clocked storage (CLK) inputs, allowing synchronous latching of selected outputs. Each channel drives a registered output (Q) with complementary unregistered output (Y).

It uses bipolar TTL-LS process technology, requiring V_CC = 5 V ±5%, with guaranteed fan-out of 20 LS-TTL loads, input thresholds compatible with standard TTL, and output drive capability of I_OH = –0.4 mA / I_OL = 8 mA at V_CC = 5 V.

Key Specifications

Pinout & Package

SN74LS399N3 is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.3-inch body width, with standard through-hole mounting and JEDEC MO-016AA outline. Pin 1 is located at the left end of the top row, identified by a notch or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar multiplexer-with-storage applications.

Compared with SN74LS399N3, SN74LS253N offers higher input density but no latching, while SN74LS157N matches pinout and basic MUX function but omits storage-both require external synchronization logic to replicate SN74LS399N3's integrated latch behavior.

Availability

SN74LS399N3 is available at Aetrix Electronics and suitable for industrial control upgrades, legacy test equipment repair, and aerospace maintenance programs requiring stable component supply and long-term traceability.

Supply support for SN74LS399N3 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial and defense heritage.

The SN74LS399N3 belongs to TI's legacy 74LS TTL logic family, designed specifically for reliable, noise-tolerant digital control in commercial-grade instrumentation, automation, and computing systems of the 1970s–1990s.

FAQ

What is the function of the G (enable) input on SN74LS399N3?

The G input on SN74LS399N3 is an active-low enable for each individual multiplexer channel. When G = 0, the selected input (A or B, per S) appears at the unregistered Y output; when G = 1, Y is forced into high-impedance state. This allows per-channel output disabling without affecting other MUX sections. SN74LS399N3 uses four independent G inputs (pins 7, 8, 15, 16), enabling selective gating in multi-signal routing applications.

Does SN74LS399N3 support 3.3 V operation?

No, SN74LS399N3 is strictly a 5 V TTL-LS device and does not support 3.3 V operation. Its input thresholds (V_IL = 0.8 V, V_IH = 2.0 V) and output drive characteristics are specified only at V_CC = 5 V ±5%. Operation below 4.75 V may cause marginal noise immunity or timing violations. SN74LS399N3 must be powered from a regulated 5 V supply and is incompatible with mixed-voltage 3.3 V/5 V systems without level translation.

How does the storage latch in SN74LS399N3 operate?

The storage latch in SN74LS399N3 is edge-triggered: a rising edge on the CLK input captures the current value at the Y output and transfers it to the Q output. Each channel has its own CLK input (pins 1, 4, 9, 12), allowing independent latching. The latch holds Q steady until the next CLK edge, decoupling output stability from select (S) or enable (G) changes. This behavior is intrinsic to SN74LS399N3 and requires no external clock conditioning.

Is SN74LS399N3 pin-compatible with SN74LS157N?

SN74LS399N3 is not pin-compatible with SN74LS157N. Although both are quad 2:1 multiplexers in PDIP-16 packages, SN74LS399N3 repurposes pins 1, 4, 9, 12 as CLK inputs (and Q outputs), whereas SN74LS157N uses those same pins as Y outputs only. SN74LS399N3 also adds dedicated G inputs (pins 7, 8, 15, 16) absent in SN74LS157N. Direct substitution would cause functional failure and potential bus contention.

What is the maximum clock frequency supported by SN74LS399N3?

SN74LS399N3 does not specify a maximum clock frequency in its datasheet; instead, it defines minimum pulse width and setup/hold timing. The CLK input requires t_W ≥ 20 ns (minimum high/low pulse width) and t_su(S) = 20 ns before CLK edge. Based on propagation delays (CLK→Q = 30 ns max), practical maximum toggle rate is ~12 MHz for reliable latch operation. SN74LS399N3 is intended for synchronous control-not high-speed data streaming-and performs best below 5 MHz in robust designs.