SN74HCT138NS Texas Instruments Integrated Circuit (Small Outline Packages) In Stock
SN74HCT138 is a 3-to-8 line decoder and demultiplexer with three enable inputs and active-low outputs. Operating from a 4.5 V to 5.5 V supply with TTL-compatible input levels, it decodes a 3-bit binary address to activate one of 8 output lines. Packaged in a 16-pin SOIC for use in address decoding and memory selection circuits.
- Manufacturer
- Texas Instruments
- Package
- Small Outline Packages
- Pin Count
- 16
- Lifecycle
- ACTIVE
- Datasheet
- SN74HCT138NS Datasheet PDF
- Category
- Integrated Circuit
- RoHS
- Compliant
- Lead Time
- 3–7 business days
- Shipping
- DHL Express · Worldwide
Key Features
- 3-to-8 line decoder with active-low outputs for address and memory decoding
- TTL-compatible input thresholds operating from 4.5 V to 5.5 V supply
- Three enable inputs (two active-low, one active-high) for cascading
- HCT CMOS technology combining low power with TTL interface compatibility
- 16-pin SOIC package for space-efficient board layouts
- Propagation delay typically under 15 ns at 5 V
Applications
The SN74HCT138 is widely used in microprocessor and microcontroller systems for memory address decoding, enabling selection of individual memory banks or peripheral chips from a common address bus. It serves as a chip-select generator in embedded systems with multiple SPI, SRAM, or I/O expansion devices sharing a 3-bit address line. In programmable logic and FPGA prototyping boards, it decodes outputs to drive up to 8 separate control signals from a compact 3-line input.
Compliance & Regulatory
| RoHS Status | Compliant |
| Lead-Free | Yes (Pb-Free) |
Alternate & Equivalent Parts
Compatible alternatives and drop-in replacements for SN74HCT138NS:
Frequently Asked Questions
How does the SN74HCT138 decode a 3-bit address into 8 output lines?
The SN74HCT138 takes a 3-bit binary input on address lines A0, A1, and A2, and drives one of 8 active-low outputs (Y0 through Y7) low while all others remain high. This one-of-eight decoding is gated by three enable inputs, allowing the chip to remain inactive until all enables are asserted. Typical propagation delay from input to output is under 15 ns at 5 V, making it suitable for fast bus cycles in SRAM and I/O decoding applications.
In what situation would you cascade two SN74HCT138 devices to create a 4-to-16 decoder?
Two SN74HCT138 devices can be cascaded to create a 4-to-16 line decoder by using the fourth address bit to enable one chip while disabling the other through the enable input pins. The first device handles outputs Y0 through Y7 when address bit A3 is low, and the second handles Y8 through Y15 when A3 is high. This cascading approach is used in systems with 16 or more peripheral chips needing individual chip selects, such as modular I/O expansion boards in industrial controllers.
Can the SN74HCT138 interface directly with 3.3 V microcontroller outputs?
The SN74HCT138 has TTL-compatible input thresholds with a logic-high threshold of 2 V, which means 3.3 V CMOS microcontroller outputs (typically swinging to 3.0 V or higher) can reliably drive it. However, the SN74HCT138 operates from a 4.5 V to 5.5 V supply, so its outputs will swing to approximately 5 V logic levels. Level translation on the output side is required when interfacing to downstream 3.3 V logic, or a 74LVC or 74ALVC variant should be substituted instead.
What package options are available for the SN74HCT138 and how do they differ for PCB layout?
The SN74HCT138 is available in a 16-pin SOIC package (SN74HCT138DR or NS suffix) with a 1.27 mm lead pitch for surface-mount assembly, and in a 16-pin DIP package (SN74HCT138N) with a 2.54 mm through-hole pitch for breadboard prototyping and socketed designs. The SOIC package occupies roughly 100 mm² less board area than a DIP, making it preferred for space-constrained production boards, while the DIP version is convenient for hand assembly and lab evaluation.
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About Texas Instruments
Texas Instruments (TI) is a global semiconductor company headquartered in Dallas, Texas. TI designs and manufactures analog and embedded processing chips used in industrial, automotive, consumer, communications, and enterprise systems.
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