Last revised: January 7, 2025

Logic level translators (LLTs), also called voltage level translators, are used to adapt or convert one voltage or logic level to another. They are used to enable communication between two devices in a system with incompatible input/output voltages. Often these mismatched voltages are found in circuits containing both old and new devices, since newer technologies are continuing to lower power requirements.

This diagram provides a simple depiction of 5V to 3V level translation.

Types of Logic Level Translators

LLTs can be classified into three basic forms based on translation: bidirectional, high-to-low, and low-to-high.

Bidirectional LLTs, also named dual-supply LLTs, have two supply voltages with different voltage ranges. They can be used for both low-to-high and high-to-low voltage translations.

High-to-low LLTs are incorporated in systems where the output voltage of the driver device is higher than the input voltage of the receiving device.

To ensure that there is no damaging current flow from the high voltage driver to the low voltage inputs, products should have overvoltage tolerant inputs. A logic device is defined as input-overvoltage protected if it can withstand (without damage) an input voltage higher than its supply voltage.

Current limiting resistors can also be incorporated at the inputs of CMOS (complementary metal-oxide-semiconductors) to allow input voltages which exceed maximum specified values, provided the maximum current rating is observed.

Low-to-high LLTs are incorporated in systems where the output voltage of the driver device is lower than the input voltage of the receiving device. These devices include two subtypes.

CMOS logic devices with low threshold inputs can be used to translate the low voltage levels into high voltage levels.

Open drain outputs of transistors can be used to translate the output voltage to a specific/desired operating voltage level. A pull-up resistor is required to achieve a true high state in level translation, and is used to translate transistor-transistor logic (TTL) to CMOS logic.

Specifications

LLT performance can be described by a number of parameters. These include input and output voltages, number of channels, data rate, load capacitance, propagation delay, and power dissipation.

• Input and output voltages are the specified voltages which the LLT translates between. This is the primary specification for voltage or logic level translators.
• The number of channels determines the number of voltage signals that can be received, processed, and translated.
• Maximum data rate is the maximum amount of information which can be processed through an LLT, usually measured in Mbits/s (megabits per second). This rate varies with supply voltage, load capacitance, and several other circuit-dependent factors.
• Load capacitance is the capacitance of the electrical load that can be driven to the LLT device.
• Propagation delay is thetime delay between the occurrence of a change at the output (eitherhigh-to-low or low-to-high) and the application of a change at the inputs.
• Power dissipation is the power expended in the form of heat from the translator during operation. The lower this rating, the higher its operating efficiency.

Operating Characteristics

Industrial buyers may also select LLTs based on a number of operating characteristics, such as its logic family, operating temperature, and features.

The logic family of an LLT determines the mechanism by which the device operates. Common logic families include:

Transistor-transistor logic (TTL) — a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors. It is notable, as it was the base for the first widespread semiconductor integrated circuit (IC) technology.

Standard CMOS — Complementary metal-oxidesemiconductor (CMOS) logic uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFET) to implement logic gates and other digital circuits found in computers, telecommunications and signal processing equipment. It is the technology of choice for many present-day digital integrated circuits.

BiCMOS — a SiGe Bipolar technology that combines the high speed of bipolar TTL with the low power consumption of CMOS.

Emitter coupled logic (ECL) — uses transistors to steer current through gates that compute logical functions. By comparison, TTL and related families use transistors as digital switches, where the transistors are either cut off or saturated, depending on the state of the circuit. This distinction explains ECL's chief advantage: that because the transistors are always in the active region, they can change state very rapidly, so ECL circuits can operate at very high speed; and also its major disadvantage: the transistors are continually drawing current, which means the circuits require high power, and thus generate large amounts of waste heat.

Operating temperature is the range of temperatures at which the device is designed to operate. The standard temperature of the circuit or circuit environment should fall within the range specified by the LLT.

Features

LLT device features include functions such as auto-direction sensing and the inclusion of ESD protection, and Schmitt triggers.

• Auto-direction sensing eliminates the need for direction control logic pins and signals, improving connectivity between next generation processors and peripheral devices. They offer low power consumption, VCC isolation, and partial power-down-mode operation.
• ESD protection is circuit or device protection from electrostatic discharge (ESD) or radiation.
• Schmitt triggers are a type of circuitry added to gates to introduce hysteresis (analysis of output history) to counteract noise.

Applications

Devices that often use logic level translators include microprocessors and integrated circuits that have inputs and outputs functioning at 1.8 volts and logic levels for flash memory or panel display requiring 3.3 volts. These mismatched voltages can be mitigated between the integrated circuit (IC) and the device by using the logic level translator.

Logic Level Translators FAQs

What are the operating characteristics of logic level translators?

The operating characteristics of logic level translators (LLTs) can be described by several parameters and features. Here are some key points based on the information available:

Specifications

Input and Output Voltages: These are the specified voltages between which the LLT translates. This is a primary specification for voltage or logic level translators.

Number of Channels: Refers to how many separate signals can be translated simultaneously.

Data Rate: The speed at which data can be transferred through the translator.

Load Capacitance: The capacitance that the translator can drive.

Propagation Delay: The time it takes for a signal to pass through the translator.

Power Dissipation: The amount of power consumed by the translator during operation.

Features

Auto-Direction Sensing: This feature eliminates the need for direction control logic pins and signals, improving connectivity between processors and peripheral devices.

ESD Protection: Provides protection from electrostatic discharge.

Schmitt Triggers: These introduce hysteresis to counteract noise in the signal.

Performance

The LSF family of translators, for example, can translate signals up to 100 MHz and down at 200 MHz with good signal integrity. Signal integrity depends on factors like line capacitance, output load, and pull-up resistor sizes.

These characteristics help ensure that logic level translators can effectively manage the translation of voltage levels between different components in electronic systems, maintaining signal integrity and protecting against potential electrical issues.

How do auto-direction sensing features work in logic level translators?

Based on the information available, here is an explanation of how auto-direction sensing features work in logic level translators:

Elimination of Direction Control Logic

Auto-direction sensing in logic level translators removes the need for additional direction control logic pins and signals. This simplifies the design and connectivity between processors and peripheral devices, as the translator can automatically determine the direction of data flow without external intervention.

Improved Connectivity

By eliminating the need for manual direction control, auto-direction sensing enhances the ease of integration between different components, particularly in systems with next-generation processors and peripherals that may operate at varying voltage levels.

Low Power Consumption and VCC Isolation

These features contribute to the efficiency of the system by reducing power usage and providing isolation between different voltage domains. This is particularly beneficial in applications where power efficiency is critical.

Partial Power-Down-Mode Operation

Auto-direction sensing logic level translators can support partial power-down modes, allowing parts of the system to be powered down without affecting the operation of the translator. This feature is useful in power-sensitive applications.

How do operating characteristics like voltage range and propagation delay impact circuit design?

Operating characteristics like voltage range and propagation delay significantly impact circuit design in several ways:

Voltage Range

Power Consumption and Speed: Lowering the supply voltage can reduce power consumption, which is critical for battery-powered and high-performance devices. However, this reduction in voltage can lead to increased circuit delay, which might require design adjustments such as parallelization or pipelining to maintain performance without increasing power consumption significantly.

Signal Integrity and Noise: A low supply voltage can degrade the signal-to-noise ratio (SNR) and limit the frequency tuning range in synthesizer designs. This can affect the performance of high-speed digital circuits and require compensatory design techniques to maintain functionality.

Propagation Delay

Timing and Synchronization: Propagation delay is the time interval between the application of an input signal and the occurrence of the corresponding output. It affects the timing and synchronization of signals within a circuit, which is crucial for ensuring reliable operation, especially in high-speed digital circuits.

Performance Bottlenecks: Increased propagation delay can lead to performance bottlenecks, particularly as node sizes shrink and interconnects become shorter and narrower, increasing resistance and capacitance. This necessitates the use of advanced design techniques to mitigate these effects and maintain signal integrity.

These characteristics must be carefully considered during the design phase to ensure that the circuit meets its performance, power, and reliability requirements.

What are some methods to improve signal integrity in circuits with reduced node sizes?

To improve signal integrity in circuits with reduced node sizes, several methods can be employed. These methods address the challenges posed by smaller semiconductor nodes, such as increased electrical interference, crosstalk, and signal degradation. Here are some strategies:

Improved Interconnect Materials

Use materials with lower resistance and capacitance to reduce signal delay and maintain signal integrity as node sizes shrink.

Advanced Design Techniques

Optimize the layout of interconnects and employ shielding methods to mitigate the effects of crosstalk and maintain signal integrity.

Ground Holes Around Vias

In multi-layer PCBs, placing several ground holes around vias can restrict the electric field and improve signal quality, especially for high-frequency signals.

Use of Reference Planes

Incorporate ground and/or power reference planes in high-speed circuits to control impedance, noise, and crosstalk, although this may affect the flexibility of the circuit.

Simulation and Conservative Techniques

Apply accurate simulation during the design phase and use conservative techniques like stretched sizing/spacing and shielding to minimize integrity loss.

These methods help ensure that semiconductor devices operate reliably and efficiently despite the challenges associated with reduced node sizes.

Logic Level Translators Media Gallery

References

Electronics360—As node sizes shrink, manufacturing challenges grow

Electronics360—The influence of vias on signal transmission in multi-layer PCBS

GlobalSpec—System-on-Chip Test Architectures: Nanometer Design for Testability

Image credit:

Texas Instruments