What are the three types of logic level translators (LLTs)?

Logic-level translators (LLTs) are electronic circuits that enable communication between devices operating at different voltage levels, such as 3.3V and 5V. Also known as voltage-level translators or level shifters, LLTs convert signals between incompatible logic standards. This article outlines key LLT specifications and features, reviewing bidirectional, high-to-low, and low-to-high LLTs. It also explores the role of logic families such as transistor-transistor logic (TTL), standard CMOS, and emitter-coupled logic (ECL) in LLT selection and system design.

Key LLT specifications and features

Electronic engineers typically use LLTs in systems where components such as microcontrollers (MCUs) and sensors operate at varying voltage levels. This is common in devices integrating older, higher-voltage components with newer, low-power technologies. Key LLT specifications include:

Input and output voltages: Defines the voltage level conversion range.
Load capacitance: Specifies the electrical load capacitance the LLT can drive.
Number of channels: Indicates how many voltage signals an LLT can process and convert simultaneously.
Data rates: quantify the maximum data throughput in megabits per second (Mbits/s), which depends on supply voltage, load capacitance, and circuit-specific factors.
Propagation delay: measures the time required for a change at the input to produce a corresponding change at the output.
Power dissipation: evaluates the energy lost as heat during operation, with lower dissipation improving efficiency.

Most LLTs improve core functionality with additional key features. For example, auto-direction sensing eliminates the need for direction control pins, simplifying connectivity between different processors and devices. Electrostatic discharge (ESD) protection shields circuits from sudden voltage surges, while Schmitt triggers add hysteresis to counteract noise and stabilize signals in noisy environments.

Understanding bidirectional LLTs

Based on translation capability, LLTs are categorized into three primary types: bidirectional, high-to-low, and low-to-high. As shown in Figure 1, bidirectional logic level translators, also known as dual-supply translators, operate with two supply voltages of different ranges, enabling both high-to-low and low-to-high voltage conversions.

Figure 1. A SparkFun bidirectional logic level converter is designed to safely step down 5V signals to 3.3V and simultaneously step up 3.3V signals to 5V. (Image: SparkFun)

These versatile devices handle signal conversion in both directions on the same data line and are essential for complex systems and protocols such as I²C and SPI, which require bidirectional communication. Bidirectional LLTs with automatic direction sensing further simplify integration into complex designs by eliminating the need for external direction control signals. This capability ensures reliable signal conversion in dynamic voltage domains.

While high-to-low LLTs are common in integrated circuits (ICs), low-to-high LLTs are more challenging to implement due to the additional circuitry required to pull low signals to higher voltage levels reliably. Bidirectional LLTs address both requirements in a single chip, although their design is generally more complex. Bidirectional LLT applications include:

Microprocessor (MPU) and IC integration: facilitates communication between MPUs and ICs operating at different voltage levels, such as 1.8V ICs interfacing with 3.3V flash memory or display panels.
Industrial computing: enables connectivity in industrial PCs and servers by bridging lower-voltage I/Os with peripherals operating at varying voltage levels.
Interface protocols: supports common protocols such as SPI, UART, JTAG, I²C, GPIO, and I²S. In automotive systems, bidirectional LLTs optimize communication between lower-power processors and peripheral devices in advanced driver assistance systems (ADAS).
Solar panel inverters: aligns the lower I/O voltage levels of MCUs with higher I/O voltage levels in peripheral devices, such as inverters and monitoring systems, ensuring compatibility in solar inverter setups.
Mixed-voltage systems: allow seamless communication between devices operating at different voltage levels, such as 1.8V, 3.3V, and 5V, while maintaining signal integrity and speed.

Exploring high-to-low and low-to-high LLTs

High-to-low LLTs are used in systems where the driving device’s output voltage exceeds the receiving device’s input voltage. These translators typically incorporate overvoltage-tolerant inputs to prevent damaging current flow from the higher-voltage driver to the lower-voltage input. System designers may also use current-limiting resistors to support input voltages above specified levels, provided the maximum current rating isn’t exceeded.

Commonly used to convert 5V signals to 3.3V signals, high-to-low LLTs are essential for a wide range of applications, including:

Mobile and portable devices: enables advanced CMOS processors operating at 1.8V or lower to communicate with peripherals using higher voltages, ensuring safe operation between components.
Overvoltage protection: safeguards systems from higher input voltages while maintaining signal integrity and preventing damage.
Legacy system integration facilitates communication between modern low-voltage devices and older hardware operating at higher voltage levels, which is particularly crucial for industrial and embedded systems.

In contrast to their high-to-low counterparts, low-to-high LLTs are designed for systems where the output voltage of the driving device is lower than the input voltage of the receiving device. These translators achieve conversion through two common methods: CMOS logic devices with low-threshold inputs or open-drain transistors paired with pull-up resistors. The pull-up resistor ensures a true high state during translation, as in converting TTL to CMOS logic. Low-to-high LLTs are less common than high-to-low LLTs, largely due to the limited availability of ICs specifically designed for this purpose.

Key low-to-high LLT applications include:

Peripheral integration: facilitates communication between low-voltage processors and higher-voltage peripherals, such as memory devices, image sensors, RF transceivers, and relays.
Mixed-signal systems: connects components with different voltage levels, such as 1.8V MCUs interfacing with 3.3V flash memory or display panels.
Communication protocols: supports various protocols spanning SPI, I²C, UART, and GPIO interfaces. Choosing between high-to-low and low-to-high LLTs depends on system requirements like specific voltage levels, speed demands, and overall architecture.

As shown in Figure 2, LLTs, whether high-to-low or low-to-high, are crucial in systems where components operate at different voltage levels.

Figure 2. An IC-level translator with an SPI/QSPI™/MICROWIRE® interface, demonstrating both high-to-low and low-to-high signal translation for devices operating at different voltage levels. (Image: Analog Devices)

Key LLT logic families

System designers select LLTs based on key operating characteristics, such as maximum temperature range and logic family. The logic family of an LLT determines its underlying architecture, impacting performance, power consumption, and compatibility with specific applications. Some of the most common logic families include:

TTL: Uses bipolar junction transistors (BJTs), diodes, and resistors to create digital circuits.
Standard CMOS: Combines p-type and n-type metal-oxide-semiconductor field-effect transistors (MOSFETs) to build logic gates and other digital circuits. CMOS dominates modern ICs due to its low power consumption and versatility in digital and analog applications, including computing and telecommunications.
BiCMOS: This technology integrates the high speed of TTL with the low power consumption of CMOS, offering a balance of performance and efficiency. It uses silicon-germanium (SiGe) bipolar technology.
ECL: Directs current through gates to compute logical functions. Unlike TTL, which uses transistors as digital switches, ECL keeps transistors in the active region, enabling high-speed operation at the cost of increased power consumption and thermal buildup.

Summary

Bidirectional, high-to-low, and low-to-high LLTs enable communication between devices operating at different voltage levels by converting signals between incompatible logic standards. Key LLT specifications span input and output voltages, load capacitance, data rates, power dissipation, number of channels, and propagation delay. Logic families play a critical role in LLT system integration, with electronic engineers selecting from options such as TTL, standard CMOS, and ECL to optimize system performance and compatibility.