Texas Instruments SN74LS445N

Overview

SN74LS445N from Texas Instruments is a TTL BCD-to-decimal decoder/driver IC in 16-pin PDIP package, providing active-low decimal outputs (Y0–Y9), latch-enable input (LE), and blanking input (BI). It operates at VCC = 4.75–5.25 V, supports fan-out of 20 LS-TTL loads, and drives common-anode LED displays directly. Used in legacy digital instrumentation and seven-segment display systems.

For engineers reviewing the SN74LS445N datasheet, SN74LS445N pinout, SN74LS445N application, or SN74LS445N equivalent, key selection considerations include active-low output polarity, direct LED drive capability, latch control timing, and compatibility with standard LS-TTL logic families in discrete display subsystems.

Technical Context

The SN74LS445N implements a synchronous BCD-to-decimal decoding function with internal latching: the LE input enables transparent latching of the 4-bit BCD input (A–D), while BI forces all outputs high (blanked) regardless of input state. Output drivers are open-collector rated for 40 mA sink per channel.

It uses bipolar TTL technology with Schottky-clamped transistors, delivering propagation delays of tPLH = 45 ns and tPHL = 30 ns (typical at VCC = 5 V, CL = 15 pF), and features input clamping diodes for ESD protection. No internal current-limiting resistors - external series resistors required for LED anode biasing.

Key Specifications

Pinout & Package

SN74LS445N is supplied in a 16-pin plastic dual in-line package (PDIP-N), 0.600-inch body width, 0.100-inch lead pitch. Package conforms to JEDEC MS-001.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar BCD-to-decimal decoder/driver applications.

Compared with SN74LS42N and 74HC4511PW, the SN74LS445N uniquely combines latching, blanking, and robust TTL drive in PDIP - essential for maintaining legacy display subsystems where timing control and LED compatibility are fixed requirements.

Availability

SN74LS445N is available at Aetrix Electronics and suitable for legacy instrumentation repair, industrial counter refurbishment, and electromechanical control panel restoration requiring stable component supply and long-term traceable sourcing.

Supply support for SN74LS445N 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 technologies with broad industrial and aerospace heritage.

The SN74LS445N belongs to TI's legacy 74LS logic family, designed specifically for reliable, low-speed digital interfacing in measurement equipment, control panels, and display subsystems where TTL compatibility and deterministic timing were critical.

FAQ

What is the function of the LE (Latch Enable) pin on the SN74LS445N?

The LE pin on the SN74LS445N is an active-high control that determines whether the device operates in transparent or latched mode. When LE is high, the current BCD inputs (A–D) are latched and held at the outputs (Y0–Y9); when LE is low, outputs follow inputs in real time. This enables synchronized digit updates in multiplexed displays and eliminates flicker caused by asynchronous input changes. The SN74LS445N relies on LE for stable static display holding.

Can the SN74LS445N drive common-cathode LED displays?

No, the SN74LS445N cannot directly drive common-cathode LED displays because its outputs are active-low open-collector. It is designed for common-anode configurations where the LED anodes connect to VCC and cathodes connect to SN74LS445N outputs. Attempting to use it with common-cathode LEDs would require external high-side switches or level inversion, defeating its integrated drive advantage. The SN74LS445N's output architecture assumes sinking current into LED cathodes.

Is the SN74LS445N still in production?

No, the SN74LS445N is officially marked OBSOLETE by Texas Instruments per its packaging information addendum. TI discontinued manufacturing in the early 2000s, and no new production lots are available from the factory. However, Aetrix Electronics maintains traceable, tested inventory of original SN74LS445N devices for legacy system support, with full lot traceability and electrical verification prior to shipment.

What is the maximum clock/data rate supported by the SN74LS445N?

The SN74LS445N does not operate on a clock signal but responds to asynchronous input changes. Its usable data update rate is limited by propagation delay (tPLH = 45 ns, tPHL = 30 ns) and latch setup/hold timing. For reliable operation in multiplexed displays, input changes should be stabilized ≥20 ns before LE goes high. This supports effective digit scan rates up to approximately 8–10 MHz, though practical systems typically run below 1 kHz due to LED persistence and human perception limits. The SN74LS445N's timing is fully specified in the SDLS177 datasheet.

Does the SN74LS445N include internal current-limiting resistors for LED drive?

No, the SN74LS445N does not include internal current-limiting resistors. Its open-collector outputs require external series resistors between VCC and each LED anode to set forward current - typically 220 Ω to 470 Ω for 5 V supply and standard 20 mA LED segments. Omitting these resistors risks exceeding the 40 mA absolute maximum sink rating per output and causing premature device failure. This design requirement is explicitly documented for the SN74LS445N in TI's application guidelines.