# 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).

Posted: May 8th, 2015 at 5:53 pm
Last Updated on: May 31st, 2016 at 1:09 pm