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.

Image Credit: Daycounter, Inc.

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.

References

ALD, Inc. - Voltage Level Translator

Maxim - Logic-Level Translation

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Texas Instruments