Texas Instruments SN74HC138DB

Overview

SN74HC138DB from NXP Semiconductors is a 3-to-8 line inverting decoder/demultiplexer with three binary address inputs (A0–A2), eight active-low mutually exclusive outputs (Y0–Y7), and three enable inputs (E1, E2 active LOW; E3 active HIGH). It operates from 2.0 V to 6.0 V, supports −40 °C to +125 °C, and delivers propagation delay as low as 12 ns at VCC = 6.0 V - ideal for memory chip select decoding in industrial microcontroller systems.

For engineers reviewing the SN74HC138DB datasheet, SN74HC138DB pinout, SN74HC138DB application, or SN74HC138DB equivalent, key selection considerations include its CMOS-level input compatibility, active-low output architecture, SSOP16 package footprint, and parallel expansion capability to 1-of-32 decoding using four units and one inverter.

Technical Context

The SN74HC138DB implements combinational logic decoding with strict mutual exclusivity: only one output goes LOW when all enables are satisfied (E1 = LOW, E2 = LOW, E3 = HIGH) and a unique 3-bit address is applied. Its enable structure allows cascading without external logic - E1/E2/E3 collectively gate all outputs, enabling hierarchical address decoding in multi-bank memory systems.

Input clamp diodes permit interface to voltages exceeding VCC when used with current-limiting resistors, and the device complies with JEDEC standard no. 7A. Static characteristics confirm VIH = 4.2 V (min) and VOL = 0.26 V (max) at IO = 5.2 mA and VCC = 6.0 V, ensuring robust noise margin and drive capability in mixed-voltage digital systems.

Key Specifications

Pinout & Package

SN74HC138DB is housed in a plastic shrink small outline package (SSOP16) per SOT338-1, with 16 leads, 5.3 mm body width, and 0.65 mm lead pitch. The package is RoHS-compliant and designed for fine-pitch surface-mount assembly.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with SN74HC138DB, SN74HCT138DB offers superior noise immunity in mixed-signal 5 V systems, while MC74HC138ADR2G trades package compactness for broader manufacturing compatibility - both require validation of enable timing and load capacitance in high-speed address buses.

Availability

SN74HC138DB is available at Aetrix Electronics and suitable for memory decoding, peripheral selection, LED multiplexing, and industrial signal routing requiring stable component supply across extended temperature ranges.

Supply support for SN74HC138DB 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

NXP Semiconductors is a global semiconductor company headquartered in Eindhoven, Netherlands, specializing in secure connectivity solutions for automotive, industrial, and IoT applications.

The 74HC138 family was designed for high-reliability, low-power address decoding in resource-constrained embedded systems - emphasizing noise immunity, wide supply range, and seamless integration with microcontroller buses.

FAQ

What is the maximum operating frequency supported by the SN74HC138DB?

The SN74HC138DB does not have a clock input and is asynchronous, so it has no defined "operating frequency." However, its propagation delay (tpd) is 12 ns typical at VCC = 6.0 V, supporting reliable address decoding in systems with address bus toggle rates up to approximately 40 MHz. System-level timing depends on input transition rates, load capacitance, and enable signal synchronization - always verify setup/hold margins in the target application.

Can the SN74HC138DB be used as a demultiplexer, and how is that configured?

Yes, the SN74HC138DB can function as a 1-to-8 demultiplexer. To configure it, apply the data signal to one enable input (e.g., E1), hold the other enables at fixed valid states (E2 = LOW, E3 = HIGH), and use A0–A2 as the 3-bit channel select. Each output Y0–Y7 then mirrors the data input state (inverted), enabling time-division or address-directed signal routing - confirmed in the functional description and Figure 1 of the NXP datasheet.

What are the absolute maximum ratings for SN74HC138DB supply voltage and output current?

The absolute maximum supply voltage (VCC) for SN74HC138DB is −0.5 V to +7.0 V, and the maximum output current (IO) is ±25 mA per pin. Exceeding these limits risks permanent damage. For reliable operation, use VCC within 2.0–6.0 V and limit steady-state output current to ≤5.2 mA (as specified in static characteristics), with proper PCB thermal relief for sustained loads.

Does SN74HC138DB support cascading to decode more than eight outputs, and what is required?

Yes, SN74HC138DB supports cascading to build larger decoders - e.g., a 1-of-32 decoder uses four SN74HC138DB units and one inverter. The enable structure (E1, E2, E3) allows hierarchical control: higher-order address bits select which SN74HC138DB is active, while lower bits select the output within that unit. No additional logic is needed beyond the inverter for E3 polarity inversion between stages.

Is the SN74HC138DB pin-compatible with older 74LS138 or 74F138 devices?

No, SN74HC138DB is not pin-compatible with bipolar 74LS138 or 74F138. While pin numbering and basic function align (A0–A2, E1–E3, Y0–Y7, VCC, GND), the SN74HC138DB uses CMOS technology with different DC characteristics, input thresholds, and drive strength. Interchange requires verification of VIH/VIL margins, fan-out capability, and timing - especially propagation delay and enable skew - in the target circuit.