Voltage-Level Translators: Connecting a World of Increasingly Varying Voltages

November 30, 2020 by Nicholas St. John

Voltage-level translators are essential in devices like SD/micro SD card readers, wireless access points, and 5G femtocells. How exactly do these devices work?

Nowadays, microprocessors, SoCs, and ASICs are being fabricated in different technology nodes, resulting in different voltage supply levels. These varying voltage levels can prevent devices from talking to one another—that is unless we use voltage-level translators.


A Growing Market of BDVL Translators

The market for voltage-level translators, especially bi-directional voltage-level (BDVL) translators, is steadily growing. For instance, just recently Diodes Incorporated released a bi-directional voltage translator, the PI4ULS3V4857, that can convert a 1.2 V to a 1.8 V signal to/from a 1.8 V to a 3.0 V signal.


Block diagram of the PI4ULS3V4857

Block diagram of the PI4ULS3V4857. Image used courtesy of Diodes Incorporated

The device is SD 3.0 compliant and is designed for applications like SD/micro SD card readers, wireless access points, and 5G femtocells, according to the press release. It can also operate at clock rates up to 208 MHz which, along with the low voltage capabilities, allow this device to be used with some of the newest SoC and ASIC technology.


What Is a Voltage Translator?

In order for different devices to communicate digitally with one another, they both must be able to interpret digital signals the same in terms of logic levels. Logic level values depend on the supply voltage levels, as some common digital devices have voltage levels of +5V, +3.3V, +/-5V, and others.

If devices are just hooked right up to one another, they can only communicate properly if they share supply voltage levels. However, if they use voltage-level translators, any device can communicate with another. This can occur because these devices convert input signals into ones that are compatible to the power supply rail on the output side.

This process is shown in the below figure:


Example of a uni-directional translator

Figure 1. Example of a uni-directional translator, the FXLP34 from ON Semiconductor. Image used courtesy of ON Semiconductor


The only issue with this setup is the fact that the translator only has one pre-defined input and one pre-defined output. This is not very efficient, since many digital communication protocols require communication to and from both devices.


Two Types of Bi-Directional Translators

A more robust solution to this is a voltage translator that can change its direction in terms of which device is the input and which is the output. There are two methods that can achieve this, one of which is shown below:


Example of a bi-directional translator

Figure 2. Example of a bi-directional translator, the FXLH1T45 from ON Semiconductor. Image used courtesy of ON Semiconductor


From Figure 2, we see a bi-directional translator with a third input that comes from one of the connected devices. Based on that input, the translator will determine which device is the input and which is the output. While this method does work, ON Semiconductor notes that it's not an ideal method of performing the translation.

The burden of direction control is now still on either Device A or B, and thus extra steps must be taken by the designer to properly operate with this translator.

Figure 3 presents a much easier option because it is the most robust kind of bi-directional translator—one that includes an auto-direction feature.


Example of an auto-direction bus-hold translator

Figure 3. Example of an auto-direction bus-hold translator, the FXLA101 from ON Semiconductor. Image used courtesy of ON Semiconductor


This is done through a circuit that performs a “bus-hold” function. A sample of this circuit is shown below:


Block diagram of a circuit with a bus-hold function

Block diagram of a circuit with a bus-hold function. Image used courtesy of ON Semiconductor


This circuit will detect a transition of HIGH-LOW or LOW-HIGH transitions and time out for a short time. During this time out, the previous state is stored. When the next transition is found, the circuit will—based on the previous state that is stored—drive the circuit into its new state, where the state determines the direction of input and output.


Simple in Theory, Complex in Application 

Voltage-level translators, including the subset of bi-directional voltage-level translators, are crucial components to accommodate a wide variety of operating voltages. While these devices may seem simple in principle, they are in fact complex devices that have a great deal of functionality and need to be placed within a circuit properly to ensure the best operation