Voltage-Level Translation

Overview

Voltage-level translation refers to the conversion to digital logic signals from one voltage level (e.g. +3.3V) to another (e.g. +5.0). It is commonly used to provide communication capability between two ICs which are operating of a different voltage rail.

Do not confuse voltage-level translation with voltage converters (e.g. linear regulators or SMPS) which are designed to provide power.

Discrete MOSFETs

The below schematic shows a simple circuit for voltage-level translation using a single MOSFET and pull-up resistors. It supports bi-directional digital signal translation.

What happens if (V_{low}) is driven?

If \(V_{low}\) is driven high, then the gate-source voltage of the N-channel MOSFET (\(V_{GS}\)) is \(0V\), and the MOSFET is OFF. This means that \(V_{high}\) is pulled high by its \(10k\Omega\) resistor.

If \(V_{low}\) is driven low, then the gate-source voltage of the N-channel MOSFET (\(V_{GS}\)) is now \(+3.3V\), and the MOSFET is ON. This means that \(V_{high}\) is driven LOW through the MOSFET.

What happens if (V_{high}) is driven?

If \(V_{high}\) is driven high, the body-diode of the MOSFET will be reverse-biased, and OFF. This means that the source of the MOSFET will be pulled to \(+3.3V\) by the \(10k\Omega\) resistor, (\(V_{GS}\)) will be \(0V\), the MOSFET OFF, and \(V_{low}\) also high because of it’s \(10k\Omega\) resistor.

If \(V_{high}\) is driven low, the body-diode of the MOSFET will be forward-biased, and switch ON. This will start pulling \(V_{low}\) to ground plus the forward voltage drop of the diode (\(0V + 0.7V = 0.7V\)). As the voltage on \(V_{low}\) drops, the (\(V_{GS}\)) of the MOSFET will start to increase, and the MOSFET will soon turn ON. At this point \(V_{low}\) will be driven fully to ground (0V).