STMicroelectronics M74HC138TTR

Overview

M74HC138TTR from STMicroelectronics is a high-speed CMOS 3-to-8 line decoder with inverting outputs, used for address decoding and signal routing in memory systems and digital logic control. It operates from 2V to 6V, delivers 13ns typical propagation delay at 6V, supports ±4mA symmetrical output drive, and features three enable inputs (G1, G2A, G2B) for cascading. It is commonly deployed in microcontroller peripheral decoding and FPGA I/O expansion.

For engineers reviewing the M74HC138TTR datasheet, M74HC138TTR pinout, M74HC138TTR application, or M74HC138TTR equivalent, key selection criteria include its inverting 3-to-8 decode function, TSSOP-16 package compatibility, wide VCC range (2–6V), low ICC (4µA max), and guaranteed noise immunity (28% VCC min).

Technical Context

The M74HC138TTR implements a fully static, silicon-gate C2MOS logic architecture with Schmitt-trigger input hysteresis absent-inputs respond directly to TTL- and CMOS-compatible voltage thresholds. Its decoding logic is purely combinational: when enabled (G1=H, G2A=L, G2B=L), one of eight outputs (Y0–Y7) goes low based on the binary value of A, B, C inputs; all outputs remain high otherwise.

All inputs include ESD protection diodes rated for ±20mA DC current, and outputs are specified for symmetrical sourcing/sinking (|IOH| = IOL ≥ 4mA at VCC = 4.5V/6V). Propagation delays tPLH and tPHL are balanced (tPLH ≅ tPHL), ensuring minimal skew between rising and falling transitions across temperature ranges from –55°C to +125°C.

Key Specifications

Pinout & Package

TSSOP-16 package: 4.9 × 6.4 mm body, 0.65 mm pitch, exposed pad not present, JEDEC MO-153 compliant.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138PWR and 74AHC138PW,118, the M74HC138TTR uniquely balances extended temperature support (–55°C to +125°C), 6V tolerance, and 13ns speed-making it optimal for industrial and automotive subsystems requiring long-term reliability without derating.

Availability

M74HC138TTR is available at Aetrix Electronics and suitable for memory address decoding, peripheral select logic, FPGA I/O expansion, and industrial control logic requiring stable component supply across extended temperature and voltage ranges.

Supply support for M74HC138TTR 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, digital, and mixed-signal ICs for automotive, industrial, and consumer markets.

The M74HC138 belongs to ST's high-speed CMOS logic family, engineered for pin-compatible upgrades of legacy 74LS devices while delivering lower power, wider voltage range, and enhanced noise immunity.

FAQ

What is the maximum clock frequency supported by M74HC138TTR in address decode applications?

The M74HC138TTR is a combinational logic device with no internal clock; its usable frequency depends on propagation delay and system timing margins. With 13ns typical tPD at 6V, it supports reliable decode cycles up to approximately 30 MHz in well-designed PCB layouts with controlled trace lengths and proper decoupling.

Can M74HC138TTR drive LEDs directly?

Yes, but only low-current indicator LEDs. With IOL ≥ 4mA (min) at VCC = 4.5V, it can sink up to ~3mA through a 1kΩ current-limiting resistor at 3.3V supply. For higher-brightness or multiplexed LED arrays, external transistor buffering is recommended to avoid exceeding output current limits or degrading VOL.

Is the M74HC138TTR pinout identical to the 74LS138 in TSSOP-16?

Yes-the M74HC138TTR uses identical pin numbering and function assignment as the 74LS138 in TSSOP-16: A/B/C on pins 1–3, G1/G2A/G2B on 6/4/5, Y0–Y7 on 7/9–15, GND on 8, and VCC on 16. No layout redesign is needed for LS-to-HC migration.

Does M74HC138TTR require external pull-up resistors on its outputs?

No-its outputs are push-pull CMOS, not open-drain. Y0–Y7 actively drive both high and low states. Pull-ups are unnecessary and would increase power consumption and slow rising edges due to added RC time constant.